Post on 19-Mar-2020
University “SS Cyril and Methodius”
Institute of Earthquake Engineering and
Engineering Seismology
Skopje, Republic of Macedonia
"DEVELOPMENT OF CONSISTENT METHODOLOGY FOR SEISMIC UPGRADING OF EXISTING BUILDINGS (CM-SUEB) BASED ON REFINED STRUCTURAL
DIAGNOSIS WITH OPTIMIZED NONLINEAR MODELS”
“РАЗВОЈ НА КОНЗИСТЕНТНА МЕТОДОЛОГОЈА ЗА СЕИЗМИЧКО ЗАЈАКНУВАЊЕ НА ПОСТОЈНИ ЗГРАДИ ВРЗ ОСНОВА НА
ДЕТАЛНА ДИЈАГНОСТИКА НА КОНСТРУКЦИЈАТА СО ОПТИМАЛНИ НЕЛИНЕАРНИ МОДЕЛИ”
Candidate:
Erald KERLUKU, graduate civil engineer,
- Master Thesis -
Mentor:
Prof.Dr. Danilo RISTIC
Skopje, 2014
2 Master thesis
SUMMARY This thesis presents an investigation of different codes, methodology and techniques used in
different countries with the aim to define an appropriate concept for rehabilitation of
existing buildings.
Chapter 2 is focused on the seismicity of Albania as a country with a high rate of seismicity
in which earthquake risk reduction has been an important, on-going socioeconomic
concern. A revised catalogue of Albanian earthquakes, from 58 A.D. to 2000, with
magnitude Ms>4.5 in the region between 39/N and 43/N and 18.5/E and 21.5/E was used
in this study. Ten seismic source zones were used to define the seismicity. The four spectral
parameter maps allowed the construction of site-specific Uniform Hazard Spectra for all of
Albania and were suggested as the basis of the next version of the KTP-N.2-89 Technical
Seismic Regulations to improve earthquake-resistant design code in Albania.
In Chapter 3 seismic design requirements or levels are the intended post-earthquake
condition of a building; a well-defined point on a scale measuring how much loss is caused
by earthquake damage. In addition to casualties, loss may be expressed in terms of property
and operational capability.
The seismic performance requirement must be achieved through system selection, detailing
requirements, design force levels, and permissible drift, based on the Seismic Design Code,
considering also the use of the building and the seismicity of the region containing the
building site together with the effect of the site conditions.
Once the energy demand for a structure is estimated from the earthquake ground motion,
the damage potential must be quantified by a combination of response and energy
parameters according to Park and Ang, 1985.
Within the scope of a specific project the investigation was basically devoted to
development of a practical and consistent methodology for structural state diagnosis.
Basically, the development procedure is regarded as a specific tool which will provide
successful identification of the basic parameters needed for elaboration of an optimal project
for revitalization of the structural system, and it is predominantly based on application of
the experimental non-destructive tests studied in chapter 5.
Chapter 6 is focused on measures for improvement of structural systems in order that they
will be capable to withstanding the expected earthquake effects. Decay of the building is
3 Master thesis
usually consequence of weather condition, load effects and foundation settlement, so,
building should be safe under normal load and resist the lateral loads without collapse. The
types of intervention necessary to enhance the performance of the building can be broadly
grouped under the following three categories - Repair, Restoration and Strengthening.
Chapter 7 explores a method for damage assessment, in which the mathematical model of
an existing building gives the level of damage index. The fragility curves developed inhere
represent one of the possible forms of the earthquake intensity – damage to structures
relationship. A random point on the fragility curve shows the conditional probability that
the damage under an earthquake of a given intensity will exceed a certain damage state.
4 Master thesis
РЕЗИМЕ
Во овој магистерски труд е прикажано истражување на разни кодови, методологии и техники кои се користат во различни земји со цел да се дефинира соодветен концепт за рехабилитација на постојни згради.
Глава 2 е фокусирана врз сеизмичноста на Албанија како земја со висок степен на сеизмичност во која намалувањето на ризикот од земјотреси претставува важно актуелно социолошко-економско прашање. За целите на оваа студија, користен е ревидиран каталог на земјотреси со магнитуда Ms>4.5 кои се случиле во Албанија, меѓу 39/N (северно) и 43/N (северно) и 18.5/E (источно) и 21.5/E (источно) во периодот од 58 година пред наша ера до 2000. За дефинирање на сеизмичноста, користени се десет сеизмички извори. Четирите мапи на спектрални параметри овозможија конструирање на подеднакви спектри на хазард специфични за локација за цела Албанија и за истите е сугерирано да бидат основа за следната верзија на KTP-N.2-89 Технички сеизмички регулативи за подобрување на кодовите за проектирање на сеизмички отпорни конструкции во Албанија.
Во Глава 3, сеизмичките проектни барања или нивоа се целните услови на зграда после земјотрес – добро дефинирана точка на скала со која се мерат загубите предизвикани со штетите од земјотрес. Освен жртви, загубите можат да бидат изразени и преку загуба на имот и функционалност. Барањето за сеизмички перформанси мора да се задоволи преку селекција на систем, проектирање на детали, проектни нивоа на сила и дозволено поместување врз основа на Кодот за асеизмичко проектирање при што исто така се зема предвид намената на зградата и сеизмичноста на регионот во кој се наоѓа локацијата на зградата заедно со ефектот од локалните услови.
Откако ќе се оценат енергетските барања од сеизмичкото движење на тлото, потенцијалот за штети мора да биде квантифициран со комбинација на параметри на одговор и енергија според Парк и Анг, 1985.
Во рамките на еден специфичен проект, истражувањето беше воглавно посветено на развој на практична и конзистентна методологија за дијагноза на состојба на конструкција. Во основа, дефинираната постапка се смета за специфична алатка која ќе обезбеди успешна идентификација на основните параметри кои се неопходни за изработка на оптимален проект за ревитализација на конструктивниот систем и истата е воглавно заснована врз примена на експериментални недеструктивни тестови киои се прикажани во Глава 5.
Глава 6 е фокусирана на мерки за подобрување на конструктивни системи кои ќе бидат во состојба да се спротивстават на очекувани земјотресни дејства, деградирање на зградите од временски услови, влијанија од разни товари и фундирањето.
5 Master thesis
ACKNOWLEDGEMENT
This thesis is part of the author`s larger education which started in 2005 at Institute of
Earthquake Engineering and Engineering Seismology in Skopje, Macedonia.
It is my pleasure to note that the investigations within the thesis were carried out in the
Institute of Earthquake Engineering and Engineering Seismology (IZIIS) where I found a
great understanding and extraordinary conditions.
I am taking this opportunity to express my deep gratitude to the entire IZIIS’ staff, the
director of IZIIS–Prof. Dr. Mihail Garevski, as well as to all professors and colleagues who
gave me their friendly support.
It is particularly my honour and pleasure to express my most sincere gratitude to my
mentor-Prof. Dr. Danilo Ristic for his great assistance, continuous support and very useful
advices during the elaboration of my thesis. I am particularly grateful for his assistance
during testing of experimental models and analysis of the obtained results as well as for his
advices and proposals during the elaboration of the thesis.
Erald Kerluku, grad.Civil Eng.
6 Master thesis
TABLE OF CONTENTS
Summary 2
Acknowledgement 5
Table of contents 6
Objective of work 8
Introduction 9
1. Chapter 1: Review of evaluation and strengthening documents 14
1.1. United States codes for strengthening of building 14 1.1.1. FEMA 356: Building information, evaluation and retrofit objectives 15 1.1.2. FEMA 310: Building evaluation 15 1.1.3. Rehabilitation methods and analysis 16 1.2. European Code for strengthening of buildings 17 1.3. New Zealand Code for strengthening of buildings 18 1.4. Indian Standard for strengthening of buildings 19
2. Chapter 2:
Probabilistic seismic hazards for Albania 20 2.1. Introduction 20 2.2. Seismicity model 21 2.3. Strong ground motion relations 25 2.4. Seismic risk in Albania 27
3. Chapter 3: Aseismic design requirements . 29
3.1 Basic requirements and design criteria 29 3.2 Structural configuration and material properties 33 3.3 Stiffness, Strength and Ductility 35
4. Chapter 4: Overview on construction of existing buildings 39
4.1 Information on damaged buildings during past earthquakes 39 4.2 Inspection of existing and new building structures in Albania 43 4.3 Common deficiencies in existing buildings 46
5. Chapter 5: Structural diagnosis and damage assessment 56
5.1 In-situ non-destructive tests and destructive laboratory test 56
7 Master thesis
5.1.1 Ambient vibration test method 56 5.1.2 Forced vibration test method 57 5.1.3 Seismo-acoustics Based Methods 58 5.2 Adopting of nonlinear failure models for state analysis 58 5.3 Fragility models and expected damage 59 5.3.1 Basic components of Risk assessment 59 5.3.2 Pre-Disaster Event Activities 62 5.3.2.1 Criteria for Defining Acceptable Level of Seismic Risk 62 5.3.2.2 Planning for Earthquake Disaster Mitigation 64
6. Chapter 6: Measures aimed to reduce the impact of environmental hazards 71
6.1 Repair and strengthening basic concepts 71 6.2 Intervention Techniques 75 6.2.1 Bonding Materials 77 6.2.2 Strengthening techniques 80
7. Chapter 7: Efficient Method for Repair and Strengthening 92
7.1 Technical Characteristics/ Description of the Building 94 7.1.1 Analysis of vertical loads 97 7.1.2 Building`s Geometry 100 7.2 Building Capacity Curve and ADRS Models 106 7.2.1 Pushover and capacity model in both orthogonal directions 106 7.2.2. Building bilinear capacity model in ADRS format 110 7.3 Seismic Response of existing building 112 7.3.1 Capacity spectrum method (CSM) 112 7.3.2 Demand spectra 114 7.3.3 Performance point estimation 116 7.4. Building Seismic Response in Repaired Model 118 7.4.1 Building Geometry 119 7.4.2 Analysis of vertical loads 121 7.4.3 Building Capacity Curve and ADRS Models 123 7.4.4 Building bilinear capacity model in ADRS format 126 7.4.5 Performance point for Repaired Model 127 7.4.6 Building Fragility models and Expected Damage 128 7.4.7 Conclusion 131
Appendix 135 References 136
8 Master thesis
OBJECTIVE OF THE WORK
It is well known that the aging of civil engineering structures, the applied construction materials, and
consequently the structural elements, change their mechanical properties and bearing capacity due to
existing unfavorable environmental conditions as well as due to other degrading effects including
permanent and excessive earthquake or other damaging loads. Considering also the fact that, during
serviceability period, a large number of civil engineering structures are inadequately maintained, it is
clear that the resulting structural deterioration may be in some cases very serious. Under such
circumstances retrofitting, repair and/or strengthening of critical structures may be usually needed. To
optimize structural repair and strengthening concept actual serviceability and seismic safety level of
structures which are in use for a longer period of time may be initially determined in all necessary
details. The need for repair and strengthening generally refer to all types of structures, which during the
serviceability period are characterized by various changes as:
♦ Changes in live load;
♦ Changes in serviceability loads;
♦ Changes in design seismic parameters based on the latest observations and knowledge;
♦ Modifications of service and seismic safety criteria;
♦ Aging and other destructive effects;
♦ Produced damages due to earthquake or other disastrous effects, etc.
To optimize structural revitalization measures of degraded or earthquake damaged structures,
application of practical and consistent methodology is essential.
Regarding this, this work presents important considerations for upgrading the seismic resistance of
existing structures including investigation of existing structural characteristics, identification of
significant deficiencies, and selection of appropriate upgrade criteria and retrofit systems. In addition to
all of the tasks required in design of a new structure, successful seismic upgrade of an existing structure
requires development of a thorough understanding of the existing construction, research into its limiting
strength and deformation characteristics, quantification of the owner's economic and performance
objectives, and selection of an appropriate design criteria to meet these objectives. It also includes
selection of retrofit systems and detailing which can be installed within the existing structure at a
practical cost and with minimum impact on building appearance, function and historic features.
9 Master thesis
INTRODUCTION
Many buildings are informally constructed in a traditional manner without formal design by qualified
Engineers or Architects. Such buildings involve stone, brick, concrete blocks, rammed earth, wood posts
and thatch roof or combination of some or all the above materials. They are built with mud, lime or
cement mortar.
Some times combination of mortars having a mix is also used. The safety of these non-engineered
buildings against earthquakes is of great concern especially because most losses of lives during past-
earthquake have occurred in such buildings. The term non-engineered building is defined rather vaguely
to include those which are not specifically designed against seismic forces. In fact such buildings are
built mostly with load bearing masonry wall, stud wall and wooden and other construction using
combination of load bearing walls, piers in masonry, and columns in RC, steel or wood.
Buildings decay due to weather, load effects and foundation settlement etc. The building, if it has to
resist an earthquake shock it should be safe under normal load and resist the lateral load without
collapse. The types of intervention necessary to enhance the performance of the building can be broadly
grouped under the following three categories - Repair, Restoration and Strengthening.
Many of the early structures were built of adobe. Adobe has many favorable characteristics for
construction of buildings in arid regions: it provides effective thermal insulation, the clayey soil from
which adobe bricks are made is ubiquitous, the skill and experience required for building adobe
structures is minimal, and construction does not require the use of scarce fuel. As a consequence of their
age, design, and the functions they performed, surviving historic adobe structures are among the most
historically and culturally significant structures in their communities.
However, earthquakes pose a very real threat to the continued existence of adobe buildings because the
seismic behavior of mud brick structures, as well as that of stone and other forms of unreinforced
masonry, is usually characterized by sudden and dramatic collapse. There is also the threat to
occupants and the public of serious physical injury or loss of life during and following seismic events.
Generally speaking, it is the evaluation of the engineering community that adobe buildings, as a class,
are more highly susceptible to earthquake damage than are the various other types of buildings.
Nevertheless, it has been observed that some unmodified adobe buildings have withstood repeated severe
earthquake ground motions without total collapse. On this point, a prominent seismic structural
engineer remarked, “The common belief that a building is strong because it has already survived several
10 Master thesis
earthquakes is as mistaken as assuming that a patient is healthy because he has survived several heart
attacks” (Vargas-Neumann 1984).
The seismic upgrading of historic buildings embraces two distinct and apparently conflicting goals:
• Seismic retrofitting to provide adequate life-safety protection
• Preservation of the historic (architectural) fabric of the building
These goals are often perceived as being fundamentally opposed.
If conventional seismic retrofitting practices are followed, extensive alterations of structures are usually
required. These alterations can involve the installation of new structural systems and often substantial
removal and replacement of existing building materials. However, historic structures so strengthened
and fundamentally altered may lose much of their authenticity. They are virtually destroyed by the
effort to protect against earthquake damage, before an earthquake even occurs.
Thus, the conflict is seen to be between retrofitting an adobe building to make it safe during seismic
events, at the cost of destroying much of its historic fabric in the process, and keeping the historic fabric
of the building intact but risking structural failure and collapse during future seismic events
As compared to seismic upgrade of existing structures, design of a new structure for proper seismic
performance is a relatively simple and straight-forward task. Modern building codes for new
construction rigorously prescribe the design procedures to be employed based on intended building
occupancy and performance and extensive research and data on seismic performance of the materials
and detailing specified. The engineer designing a new structure has the opportunity to select the basic
structural system and specify the materials and detailing incorporated. The engineer can participate in
developing the structure's configuration and the placement of structural elements. Finally, the engineer
for a new building has the opportunity to require inspection of important aspects of the construction and
to confirm the quality of materials and workmanship incorporated. As a result, most structural
characteristics important to seismic performance including ductility, strength, deformability, continuity,
configuration and construction quality, can be controlled.
Seismic rehabilitation of existing structures presents a completely different problem. First, for most types
of structures, up to very recently, there was no clear professional consensus on appropriate design
criteria. The building codes for new construction are based on the use of modern materials and
detailing, and are not directly applicable. Further, they incorporate levels of conservatism and
performance objectives which may not be appropriate for use on existing structures due to economic
11 Master thesis
limitations. The material strengths and ductility characteristics of an existing structure will in general
not be well defined.
The configuration and materials of construction are predetermined. The details and quality of
construction are frequently unknown and because the structure has been in service for some time,
deterioration and damage are often a concern.
In addition to all of the tasks required in design of a new structure, successful seismic upgrade of an
existing structure requires development of a thorough understanding of the existing construction,
research into its limiting strength and deformation characteristics, quantification of the owner's
economic and performance objectives, and selection of an appropriate design criteria to meet these
objectives, which is also acceptable to the building official. It also includes selection of retrofit systems
and detailing which can be installed within the existing structure (which may have to remain open
during the upgrade) at a practical cost and with minimum impact on building appearance, function
and historic features.
Damage to buildings was caused by a combination of affects:
• Old decaying buildings predating modern construction practices
• New Buildings not being designed to earthquake codes
• Lack of knowledge, understanding or training in the use of these codes by local engineers
• Unawareness that Albania is a moderate seismic region
• Buildings erected without owners seeking proper engineering advice
• Improper detailing of masonry and reinforced structures
• Poor materials, construction and workmanship used, particularly in commercial buildings
• Alterations and extensions being carried out without proper regard for effects on structure
during an earthquake
• Buildings having poor quality foundations or foundations built on poor soils
• Little or no regularity authority administering or policing the codes
Generally, commercial buildings were worst affected by the earthquake because of poor workmanship,
use of materials and inadequate attention to detailing.
Low-rise rubble masonry buildings were totally destroyed near to the epicenter, but some survived
(though badly damaged) when further away. These were also older forms of construction. Cutstone
masonry and more modern reinforced concrete framed buildings faired much better, although damaged
to varying extents. These later building types are largely built by owner-occupiers and hence better care
12 Master thesis
was taken in the materials used and their workmanship. Many lessons can be learnt from those non-
engineered low rise buildings which survived.
The vast majority of owner-builders are also the ones who have spent their life savings in constructing
their homes, and who wish to ensure their homes are properly repaired to resist a possible future
earthquake, but who are unable to always obtain proper advice. This Guide is intended to help those
people. These are also the most in need of this advice, as they carry no home insurance.
Even though this Guide provides lots of advice on how to repair and strengthen buildings, each building
will respond uniquely in an earthquake, and therefore it is difficult to generalize in a Guide such as this.
Therefore, it is important for the property owner to seek professional advice from an experienced
structural engineer and builder to check whether repairs can be carried out. Also, any repairs must
always consider the safety of the people involved.
The need to improve the ability of an existing building to withstand seismic forces arises usually from
the evidence of damage and poor behavior during a recent earthquake. It can arise also from
calculations or by comparisons with similar buildings that have been damaged in other places. While in
the first case the owner can be rather easily convinced to take measures to improve the strength of his
building, in the second case dwellers that have much more stringent day-to-day needs are usually
reluctant to invest money in the improvement of seismic safety. The problems of repairs, restoration and
seismic strengthening of buildings are briefly stated below:
• Before the occurrence of the probable earthquake, the required strengthening of seismically weak
buildings is to be determined by a survey and analysis of the structures.
• Just after a damaging earthquake, temporary supports and emergency repairs are to be carried
so that precariously standing buildings may not collapse during aftershocks and the less
damaged ones could be quickly brought back into use.
• The real repair and strengthening problems are faced at the stage after the earthquake when
things start settling down. At this stage distinction has to be made in the type of action required,
that is, repairs, restoration and strengthening, since the cost, time and skill required in the three
may be quite different.
The decision as to whether a given building needs to be strengthened and to what degree, must be based
on calculations that show if the levels of safety demanded by present codes and recommendations are
met. Difficulties in establishing actual strength arise from the considerable uncertainties related with
material properties and with the amount of strength deterioration due to age or to damage suffered from
13 Master thesis
previous earthquakes. Thus, decisions are frequently based on gross conservative assumptions about
actual strength.
The method of repair and strengthening would naturally depend very largely on the structural scheme
and materials used for the construction of the building in the first instance, the technology that is
feasible to adopt quickly and on the amount of funds that can be assigned to the task, usually very
limited. Some methods like splints and bandages, wire mesh with gunite, epoxy injection, etc., have
already been tried and applied in a few countries for repairing as well as strengthening earthquake
damaged buildings.
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
14 Master thesis
1
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
1.1 UNITED STATES CODES FOR STRENGTHENING OF BUILDINGS
In the 1990s, triggered by several damaging Californian earthquakes, a vigorous program of
seismic code development was undertaken. Two outcomes are the publication of FEMA
310 and FEMA 356.
FEMA 356, based on extensive theoretical and practical research advocating a displacement
based method and non linear push over analysis, is intended to become a nationally
recognized standard. It will lead and shape current and future earthquake retrofit practice
due to its technical and procedural rigor and breadth.
1.1.1 FEMA 356: Building information, evaluation and retrofit objectives
Although FEMA 356 can be used as an evaluation tool, the more traditional evaluation
resulting from the application of FEMA 310 may be more appropriate for a developing
country. Both documents take a rigorous approach to determining existing structural
conditions by specifying the as-built information required, including exposure of primary
reinforced concrete connections to ascertain the standard of reinforcement detailing.
Uncertainties associated with minimum data collection are accounted for in the analysis by
application of a Knowledge Factor. This approach might be redundant for buildings whose
existing structures are discounted completely, due to serious constructional deficiencies, and
in whom additional structures are inserted to resist lateral loading. The documents also
outline potential geological hazards and provide guidance on assessment and mitigation.
Assuming the evaluation process recommends retrofitting, FEMA 356 requires that the
“design professional” discuss with his/her client the retrofit objectives. For an appropriate
earthquake hazard level, such as a 450 year return period event, the target building
performance level is agreed upon. For typical Turkish apartment buildings, a performance
level between the Life Safety Performance Level and the Collapse Prevention level might be
appropriate. This implies a design standard somewhat less than the current Turkish code
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
15 Master thesis
with associated cost savings and may need approval by Turkish authorities before being
accepted. The design professional also has to explain that in order to achieve the agreed
performance, the quality of design, detailing, construction and supervision will have to be
very significantly better than current practice. The contractual and cost implications of a far
more rigorous approach will be considerable.
1.1.2 FEMA 310: Building Evaluations
FEMA 310 is aimed at identifying vulnerabilities and deficiencies in non-damaged
buildings. It is structured in three tiers of increasing analytical detail and decreasing
conservatives towards safety. Building assessment is performed with respect to compliance
to certain criteria that are deemed sufficient to resist earthquakes. In the first tier criteria are
mainly qualitative, while the few quantitative ones are based on fully elastic performance of
the structure. Compliance of the system is considered first, followed by each of its structural
and non structural components. If the building does not comply with one or more criteria
then the professional has the choice to either perform a more sophisticated (Tier 2)
assessment, or if the building is deemed to fail the assessment, to propose a strengthening
scheme, thence reducing the cost of the assessment stage.
Tier 2 first requires a quantitative elastic analysis of the entire structural system, either
dynamic or static equivalent; compulsory if the building is in the highest risk zone.
Compliance criteria are again laid out by structural element with reference to the rules of
capacity design and assumed ductile behavior. If there is no compliance at this level then a
full non linear analysis, for instance the push-over method, should be performed. Reference
is made to FEMA 356 for the detailed application of Tier 3.
The limitation of FEMA 310 is the a priori assumption of ductility levels and hierarchical
performance of structural elements, which may not necessarily occur in reality, and for
which no alternative provisions are considered. Also, in the event of non compliance, no
suggestion is provided for strengthening strategies to be pursued in order to realize such
compliance.
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
16 Master thesis
1.1.3 Rehabilitation methods and analysis
For concrete moment frames, with or without masonry infill, the Systematic Rehabilitation
Method, involving consideration of non-linear response, is required. In developing countries
it is likely that designers will prefer the Non-linear Static Procedure. This procedure requires
all primary, secondary (defined in the document) and non-structural components (if their
lateral stiffness exceeds 10% of the total storey initial lateral stiffness) to be modeled
mathematically.
Stiffness and strength degradation is also to be included. Unreinforced masonry infill walls
may not be neglected. In fact, analysis may indicate these elements are beneficial, reducing
the extent of new rehabilitation construction.
FEMA 356 requires this procedure to be “reviewed and approved by an independent third-
party engineer with experience in seismic design and non-linear procedures.” This peer
review requirement is crucial as it acknowledges important factors: the innovative nature of
the analytical approach, its technical complexity, the need for consistency, and the
maintenance of construction and professional standards. Some consultants and clients will
react negatively. The document also outlines a Construction Quality Assurance Plan,
involving the contractor, design professional and building officials. The emphasis is on the
importance of detail, so often relegated to non-professionals in developing countries, and on
the structural consequences of poor detailing. For example, “For beams and columns in
which perimeter hoops are either lap spliced or have hooks that are not adequately anchored
in the concrete core, transverse reinforcement shall be assumed not more than 50% effective
in regions of moderate ductility demand and shall be assumed ineffective in regions of high
ductility demand.” ATC-40 [6] comments on an equivalent clause: “This severe
recommendation is made with the understanding that shear failure of poorly confined
columns commonly is a cause of column failure and subsequent structural collapse.”
Observed damage to Turkish apartment buildings emphasizes the relevance of these clauses.
The FEMA approach is thorough and its correct application can be expected to achieve
building performance as close as possible to that desired. Within and outside the US these
documents represent a significant step forward. Design professionals have to consider many
new aspects, especially displacement based design. For developing countries the documents’
content represents a giant leap. Strategies need to be developed to bridge the gap between
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
17 Master thesis
current practice and the high standards necessary for successful rehabilitation. In this respect
local engineering bodies have a role to introduce and support the gradual uptake of the
documents by undertaking a number of measures including:
1. Testing of local materials, foundation conditions and masonry infill wall
properties, to provide ranges of engineering properties and appropriate default
values.
2. Sensitivity analyses that result in simplification of some of the provisions and
provide guidance on overcoming common mathematical modeling difficulties.
3. Case studies of several typical building retrofit illustrating analytical methods and
different retrofit strategies.
4. Disseminating lessons learned from peer reviews to build up local technical
expertise.
1.2 EUROPEAN CODE FOR STRENGTHENING OF BUILDINGS
Eurocode 8: Design of Structures for Earthquake Resistance – Part 3: Strengthening and
Repair of Buildings is currently being developed. Comments made in this paper are based
upon Draft No. 1 (June 2001) [7] and an earlier Draft for Development [8] which includes
Annex G, Details for Concrete Structures.
While comment is not offered on technical details due to the draft nature of these
documents, some general discussion is warranted. Overall, it seems that the code may not
be of great usefulness to designers in developing countries. The main difficulty is a
widespread lack of explicit guidance. Principles are given, but without any specifics.
Designers are left without specific quantitative guidance on many issues. For example,
when undertaking a simplified estimation of stiffness and resistance “model correction
factors” may be used, but no values are given and designers are told that values should be
“conservatively chosen, taking into account available technical literature and local
experience.” Other difficulties include the need to keep referring frequently to other code
documents and an unclear document structure. This general lack of explicit guidance
limits the usefulness of the document.
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
18 Master thesis
1.3 NEW ZEALAND CODE FOR STRENGTHENING OF BUILDINGS
The most up-to-date document [9] has been in draft form since June 1996. It assumes a life
safety performance level and begins with a rapid evaluation procedure based on the visual
screening approach of ATC 21 [10]. Approximately fourteen structural criteria are assessed
and demerit points are awarded for features likely to impair seismic performance. The three
most significant parameters to determine evaluation outcome are the level of site seismicity,
the presence of significant torsion and a weak storey. The “score”, intended to relate to the
building’s damage ratio under a current code earthquake, is then combined with the gross
building area (to reflect the number of occupants and potential casualties) to confirm
whether a more detailed analysis is warranted. This evaluation procedure, if adapted to
typical building types in developing countries, may be very suitable given its simplicity.
For reinforced concrete moment resisting frames, with or without masonry infill, designers
can choose either a force or displacement based approach. Both procedures are outlined in
flow charts and elaborated upon in step-by-step explanations. Less experienced engineers
will prefer the force method. For an identified collapse mechanism and probable member
and joint strengths, assuming no degradation of shear strength, the lateral force capacity is
compared to the code response spectrum to ascertain the minimum acceptable level of
structural ductility. Checks are then made to ensure member ductility capacity exceeds
demand, and that, given member curvature ductility, degraded shear strength capacities
allow the development of member flexural capacities. If previous check outcomes and
ductility and shear capacities are adequate, seismic improvement is not required.
This procedure is suitable for application in developing countries: it is based on first
principles approach, with a clearly set out methodology and very few coefficients necessary
to account for hidden complexities. The designer understands the process and therefore has
better control, while advice is provided by commentary on assumptions and limitations. An
evaluation can be undertaken without reference to other documents, hence making the
process as straight forward as possible. Analytical procedures are expressed in terms of
current design standards: this assists in achieving uniformity of approach and enables better
control by building officials. It also enables easy comparison between the evaluation and
design of retrofitted buildings with new buildings.
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
19 Master thesis
For example, the document suggests that it is generally appropriate for the strength of ‘at-
risk buildings’ to be just 67% the strength of an equivalent new building. Although this
reduced strength level effectively increases the risk to existing buildings between two and
three times, the lower standard makes earthquake improvement more financially viable.
The displacement based approach is also presented. Considerable analytical simplifications
are possible once the collapse mechanism is determined. Member plastic hinge rotation and
(joint) shear capacities are checked against the structure displacement demand which is
determined from displacement spectra, easily generated from typical code spectra, and
effective natural period and damping appropriate to the assumed inelastic mechanism.
Again, this is a first-principle approach and although a pushover curve may be used, it is
feasible to use hand methods. If either of the analytical approaches indicates serious
structural deficiencies that might compromise the life safety performance objective,
guidance is then provided on how structural performance may be improved.
1.4 INDIAN STANDARD FOR STRENGTHENING OF BUILDINGS
The Indian Standard focuses on providing guidance on reinstating damaged or weak
elements by rebuilding or strengthening. General principles as well as some common
strengthening techniques are discussed. For example, details of encasing reinforced concrete
members to improve strength and methods of improving floor and roof diaphragm action
are provided. The Standard appears to concentrate on reinstating or upgrading gravity load
paths of reinforced concrete members, rather than improving seismic resistance. Its value
therefore lies in effecting rapid repairs, probably in most cases to non-engineered structures.
While the lack of emphasis on the need for engineering evaluation, analysis and design of
seismically deficient structures may be addressed in a future revision, it might be argued that
much repair and retrofitting will be undertaken without professional engineering advice.
This highlights the value of practical and detailed documentation in the form of manuals for
contractors who can then retrofit a limited range of common building types.
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
20 Master thesis
2
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
2.1 INTRODUCTION
Aseismic building regulations have been applied in Albania since 1952. The static method
adopted in the first regulations for the seismic calculation of structures, was replaced in 1963
by a version of the dynamic method. Currently, Aseismic Regulation KTP-N.2-89 is in force
in Albania [2, 3].
As a first step to our new evaluation of the seismic hazard of Albania we reviewed the
seismic zonation of Albania at a scale 1:500,000 published in 1980 [4] and more recent
studies of the seismotectonics, seismic source zones and seismic hazard of Albania [5, 6, 7,
8, 9, 10, 11].
Albania is geologically and seismotectonically a rather complicated region. The country is
characterized by obvious microseismicity (a high number of small earthquakes), sparse
medium-sized earthquakes (magnitude M 5.5 - 5.9), and rare large earthquakes (magnitude
M>6.5). Most strong Albanian earthquakes have occurred along three well-defined seismic
belts [12].
• The Ionian-Adriatic coastal belt extending northwest to north-northwest and coinciding
with the boundary between the European plate and the Adria microplate.
• The Peshkopia-Korca belt, extending north-south in the eastern part of the country, and
• The Elbasani-Dibra-Tetova transverse belt, extending southwest-northeast across the
former two belts.
Neotectonic Structure
The Albanian orogen lies on the south-westernmost part of the Eurasian plate, and is a
convergent zone due to northeastward movement of the Adriatic plate (= Adria microplate).
The orogen is divided into two domains of the present-day tectonic regime: a coastal
domain of compression dominated by northwest to north-northwest striking thrusts and
folds, and an interior domain of extension dominated by north-striking normal faults
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
21 Master thesis
(Figure 2.1). Two offshore regions, the South Adriatic Basin and the Periadriatic Foredeep,
have not been further considered because the
first has few earthquakes and the activity rate
of the second is too low to make a significant
contribution to the hazard.
The Pliocene-Quaternary embraced strong and
progressive uplift in the Mediterranean region,
particularly in Albania. The commencement in
the
Pliocene was distinguished by extensional
tectonics, which affected the interior domain of
the country and created its horst-graben
structures. The faults have been statistically
analyzed and their importance assessed for
each seismogenic zone.
Figure 2. 1 Map of active faults in Albania
2.2 SEISMICITY MODEL
Earthquake Catalogue
The revised catalogue of Albanian earthquakes [6] forms the basis of our study (Figure
2). The subset of the catalogue used for hazard calculation includes earthquakes with
magnitude Ms>4.5 that occurred in the region between 39.0EN and 43.0EN and 18.5EE
and 21.5EE (see Fig. 2) between 58 and 2000 A.D. The magnitudes for historical
earthquakes are evaluated from intensity information (Io, or epicentral intensity on the
MSK-64 Scale) using the conversion formula Io = 1.5 Ms - 0.986 [18]. For historical
earthquakes in Greece the Albanian catalogue uses the coordinates and magnitude
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
22 Master thesis
values given by Papazachos [19], and for earthquakes in the former Yugoslavia it uses
the “Balkan Region - Catalogue of Earthquakes” [20].
Earthquake Source Zones
Seismic source zones were determined from consideration of the present-day tectonic
regime of the region, as discussed above, the subset of the Albanian catalogue, and the
full catalogue for smaller earthquakes from 1964-2000 (Fig. 2)[21]. From these
considerations, the regional seismicity of concern to Albania was divided into 10 seismic
sources (see Figures 1 and 3; zone coordinates are available from the authors), which
includes some redefinition of eight zones previously discussed for Albania together with
an interior background zone and a source zone to model earthquakes in the Skopje
region [22]. Parameters used for the probabilistic seismicity model are given in Table 1
and the zones based on work summarized by Aliaj [10] are:
1. Lezha-Ulqini (LU) a coastal zone containing pre-Pliocene WNW-striking pure-
compression thrust faults that parallel the Dalmatian coastal offshore line. The thrust
faults are cut by rare ENE-trending strikeslip faults.
2. Periadriatic Lowland (PL) a coastal zone containing post-Pliocene oblique-
compression thrust faults, Nto NNW-striking, which are cut by rare ENE-trending
strike-slip faults.
3. Ionian Coast (IC) a coastal zone containing pre-Pliocene NW-striking pure
compression thrust faults, which are cut by rare strike-slip faults.
4. Peja-Prizreni (PP) an interior zone in Kosova comprising three normal fault systems,
N- ENE- and WNW-trending, along the boundaries of Dukagjini Pliocene-Quaternary
Depression.
5. Kukesi-Peshkopia (KP) an interior zone comprising Pliocene-Quaternary N-trending
normal-fault controlled grabens.
6. Ohrid-Korca (KO) an interior zone comprising the Pliocene-Quaternary normal-fault
controlled Ohrid graben, and Korça and Erseka half-grabens, which are generally N-
trending.
7. Shkodra-Tropoja (ST) a transverse interior zone comprising NE-striking normal
faults, mainly along the boundary of Mirdita ophiolite zone.
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
23 Master thesis
Figure 2.2 Seismicity of Albania. Red dots show earthquakes used for the estimation of hazard, gray dots
represent other earthquakes
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
24 Master thesis
8. Elbasani-Dibra-Tetova (EDT) a transverse interior zone comprising fragmentary
NE-striking normal faults.
9. Skopje (SK) is a zone adopted together with its magnitude recurrence parameters
from Talaganov [22] to describe the seismicity near Skopje.
10. Eastern Albanian Background (EAB) a background zone comprising the interior
part of Albania and neighboring regions that lies to the east of the coastal zones and is
not included in any of the zones named above.
Table 2.1 Parameters for the ten seismic source zones
Figure 2.3 Seismic source zones used for the hazard maps. EAB encompasses all interior regions not in a
named source.
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
25 Master thesis
2.3 STRONG GROUND MOTION RELATIONS
After examining some recent ground motion relations applicable to the Balkan region
we chose Ambraseys [25] to compute the ground motions for our rock site condition
(average velocity >750 m/s). The Ambraseys relations were determined for MS
magnitudes and are hence consistent with magnitudes of M>4.5 events in the Albanian
earthquake catalogue. Ambraseys [25] follow the work of Boore [26] by using “pseudo
depths”.
Each pseudo depth represents an effective depth that is derived for a particular period
from the regression analysis used to determine the ground motion relations, and it is
used instead of the actual earthquake depth distribution.
Reference Ground Condition for Albania
For the preparation of national hazard maps it is essential to choose a reference ground
condition for which to map hazard. KTP-N.2-89 used rock or firm soil (Soil category I).
Because rock was adopted for Eurocode
8, it is the appropriate choice for the next hazard maps for Albania. Choices in North
America are usually in the mid-range between very hard and very soft ground (thus
minimizing uncertainty in the amplification or deamplification factors) and typically
represent “stiff soil”, rather than being rock. For example Canada’s next building code
has adopted "Site Class C", defined by a 360 to 750 m/s average shear wave velocity in
the uppermost 30 m [27].
RESULTS
Seismic hazard values were calculated for a grid extending over Albania and
neighboring regions and used to create national contour maps for the five ground motion
parameters chosen (Figures 5 and 6). The four spectral values (together with spectral
values at a few additional periods) were used to construct Uniform Hazard Spectra
(UHS) for some important cities to illustrate the range and period dependence of seismic
hazard across Albania (Figure 7). Approximate UHS can be constructed for other
localities by reading the four values off Figures 5-9. We tabulate hazard values for some
selected cities and towns in Table 2.
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
26 Master thesis
Table 2.2 Hazard at 10%/50 year probability for selected Albanian cities and towns %g).
The change of seismic hazard as a function of probability (“hazard curve”) for Tirana is
illustrated in Figure 4. The slope of the curve between probabilities of 10%/50 years and
2%/50 years is of especial interest because national multiplicative factors have
sometimes been applied to values at one probability level to estimate hazard at a
different level considered more appropriate for design [16]. The curve also gives an
indication of the ground motions that might be used for high-reliability designs, though
as the current hazard model was intended for 10%/50 year hazard its estimates for low
probability hazard may be inaccurate [16].
Figure 2.4 Hazard curves for Tirana
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
27 Master thesis
Figure 2.5 UHS for Albanian cities.
Figure 2.6 PGA hazard on rock for a
probability of 10%/50 years (units =%g).
2.4 Seismic risk in Albania
A new suite of national seismic hazard maps provides an opportunity to assess seismic
risk in Albania, where seismic risk is defined in terms of seismic hazard * vulnerability. A
full assessment of seismic risk in Albania involves much non-seismological data,
knowledge and skills to translate the effects of seismic hazard shaking into likely losses.
It is thus beyond the scope of this paper, and beyond the current mandate of the
Seismological Institute. However, a first approximation is extremely useful for allocating
resources to those places where the benefits will be largest. Adams [32] assessed the
distribution of urban seismic risk in Canadafrom 3 probability of damaging ground motion *
city population. For the probability, we have used the probability of exceeding a short-
period damage threshold (18%g), which might be consistent with damage in the recent
Gjilan Earthquake [33]. Choosing different thresholds or ground motion parameters
PROBABILISTIC SEISMIC HAZARDS FOR ALBANIA
28 Master thesis
would produce results that differ in detail, but substantially mimic the risk distribution
shown in Table 4 and Figure 7.
Figure 2.7 Urban seismic risk in Albania
Tirana accounts for at least one quarter of the urban seismic risk, perhaps considerably
more if the official population figure is an underestimate. Albania’s six largest cities at
risk account for over two-thirds of the urban risk.
Table 2.3 Estimate of relative seismic risk for Albanian cities
ASEISMIC DESIGN REQUIREMENTS
29 Master thesis
3
ASEISMIC DESIGN REQUIREMENTS
3.1 BASIC REQUIREMENTS AND DESIGN CRITERIA
The level of damage will vary from the strength and energy dissipation capacity of
the structure according to the desired performance objective based on the dynamic
response of the building assumed by the Structural civil engineer in the Seismic
Design Analysis, according to the figure below.
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Fig. 3.1 Scheme, Limit States related to Seismic Performance Requirements
According to existing practice, the seismic effects upon structures are classified into
three levels:
SLS
DLS ULS Elastic Analysis
Elasto‐plastic Analysis
Kinematic Analysis
Collapse Analysis
ASEISMIC DESIGN REQUIREMENTS
30 Master thesis
Level 1
Under the earthquake of low intensity, with short return periods, the dynamic
behavior of the structure should be such that there will be no vibrations that will
induce damage to the structural and nonstructural elements.
Level 2
Under an earthquake of higher intensity, referred to as the design earthquake, with a
return period of 50 to 100 years, the structure should remain to behave in linear
range, with possible limited nonlinear deformation to individual elements of the
system, which means a limited decrease in stiffness and dissipation of seismic energy
with nonlinear deformation. The maximum top storey displacements are limited to
H/600.
Level 3
Under maximum expected seismic effect, referred to as maximum earthquake, with
return period of 200 to 300 years, the structure elements behave deep into nonlinear
range, the stiffness and the strength of the structure being considerably decrease with
local crushing of material. However, such strong earthquake should also not lead to
failure of elements particularly due to shear forces. The maximum relative
displacements are limited to h/150.
According to Paulay & Priestley 2.2.4.
The seismic design limit states are classified into the next three categories:
Serviceability limit state
o Minor intensity of ground shaking
o No damage to structural or nonstructural components
o Component of the structure remaining essentially elastic
o Concrete and masonry may develop considerable cracking but no significant
yielding of reinforcement
o No crushing of concrete or masonry
o Normally seismic forces are calculated for 50 year return period
o Annual probability of exceedance P=0.02/year
ASEISMIC DESIGN REQUIREMENTS
31 Master thesis
Damage control limit state
o Moderate intensity of ground shaking
o Yielding of reinforcement may result in wide cracks
o Repair requirement
o Crushing or spalling of concrete may occur
o Boundary between economically repairable damage and damage that is
irreparable
o Annual probability of exceedance P=0.002/year
Survival limit state
o Strongest ground shaking
o Loss of life should be prevented
o Survival is the most important design criteria
o Extensive damage to both structural and building contents
o Attention on structural qualities
o Large displacement without significant loss in lateral force resistance
o Annual probability of exceedance P=0.0002/year
According to Eurocode 8: Design of structures for earthquake resistance
Part 1: General rules, seismic actions and rules for buildings.
The structures in seismic region shall be designed and constructed in such a way, that the
following two requirements are met, each with an adequate degree of reliability:
Non-collapse requirement
o To withstand the design seismic action without local or global collapse.
o Retaining its structural integrity and a residual load bearing capacity after the
seismic events.
o The design seismic action is expressed in terms of PNCR or TNCR
Probability of exceedance, PNCR = 10%
Return period, TNCR = 475 years
o The ultimate limit states which are associated with collapse, shall be checked
ASEISMIC DESIGN REQUIREMENTS
32 Master thesis
Damage limitation requirement
o To withstand a seismic action having a larger probability of occurrence than
the design seismic action.
o Without the occurrence of damage and the associated limitation of use.
o The seismic action is expressed in terms of PNCR or TNCR
Probability of exceedance, PDLR = 10%
Return period, TDLR = 95 years
o The damage limitation states which are associated with damage occurrence,
shall be checked
From evaluation of level of seismic hazard, we will follow the design procedure based
on Non-collapse requirement (EC-8 2.1)
For non-collapse requirement Ultimate limit State is used as concept:
o Structural system having resistance and energy dissipation capacity
o Non-linear response
o Energy dissipation by q behavior factor
o Ductility classification
o Checking the stability of the structure (Overturning, Sliding)
o Foundation resist without permanent deformation
o Influence of second order effects
o Verifying the behavior of non-structural elements
ASEISMIC DESIGN REQUIREMENTS
33 Master thesis
WALL SYSTEM
SYSTEM TYPE VERTICAL LOAD
HORIZONTAL FORCE
DUAL SYSTEM
FRAME SYSTEM
EQUIVALENT DUAL SYSTEM
3.2 STRUCTURAL CONFIGURATION AND MATERIAL PROPERTIES
Earthquake resistant concrete buildings are classified into structural types according to
their behavior under horizontal and vertical loading, as EC-8 (5.2)
In o
Table 3.1 Types of system and load distribution
Structural Frame System:
Structural system in which both the vertical and lateral loads are mainly resisted by
spatial frames whose shear resistance at the building base exceeds 65% of the total
shear resistance of the whole structural system.
Dual system:
Structural system in which support for the vertical loads is mainly provided by a
spatial frame and resistance to lateral loads is contributed in part by the frame
system and in part by structural walls, single or coupled.
Both structures are symmetrically in respect to x and y axis in plan.
The mass distribution at the floor level is modeled by diaphragm in order to
generate: EC-8 (4.2.1.5)
WALL SYSTEM
EQUIVALENT DUAL
SYSTEM
FRAME SYSTEM
DUAL SYSTEM
STRUCTURAL SYSTEM VERTICAL LOAD HORIZONTAL FORCE
ASEISMIC DESIGN REQUIREMENTS
34 Master thesis
Story Shear Overturning Moment
Structural Frame Systems
Vi
o Rigid Interconnection
o Large plane stiffness
o Translation and Rotation of the floor behave as rigid body
o Transmitting the inertia forces generated from earthquake acceleration at all
horizontal force resisting elements
o Design to respond elastically, as they are not suitable to dissipate energy
• Criteria of regularity due to Response of the Structure
Fig 3.2 Response in Elevation
The building as a Vertical Cantilever
Center of Mass
During an earthquake, acceleration induced inertia forces will be generated at each
floor level, where the mass of entire story may be assumed to be concentrated. Hence the
location of a force a particular level will be determined by the center of accelerated mass
at that level.
Center of Rigidity
Due to displacement of the story plan �x, the forces induced will be proportional to the
respective stiffness uFK = . The resultant total force F induced by translational
displacement will pass through the center of Rigidity.
ASEISMIC DESIGN REQUIREMENTS
35 Master thesis
Load
or
Stre
ngth
Δy
Sy=Si
Δu
Idealized response
Displacement
Similar will be the force induced in the other translational direction. The point of
center of rigidity or center of stiffness will cause only relative floor translations. Since
the story shear force F acts through center of mass stronger than the center of rigidity, it
cause floor rotation as well as floor translation.
Displacement due to story twist, when combined with those resulting from floor
translations, can result in total element interstory displacements that may be difficult to
accommodate. For this reason we should attempt to minimize the magnitude of story
torsion.
Fig. 3.3 Response in Plan
3.3 STIFFNESS, STRENGTH AND DUCTILITY
• Stiffness
Stiffness is the quantity that relates loads or forces to the ensuing structural deformations.
Familiar relationships are established from first principles of structural mechanics, using
geometric properties of members and the modulus of elasticity for the material. If
serviceability criteria are to be satisfied with a reasonable degree of confidence, the extent
and influence of cracking in members and contribution of concrete in tension must be
considered.
Fig. 3.4 Bilinear Response
ey
Eccentricities
y
Center of Mass
xCenter of Rigidity
FF
M
ex
This may be achieved by a deliberate assignment
of stiffness to lateral force-resisting components,
such as frames or walls, in such a way as to
minimize the distance between center of rigidity
and the line of action of the story shear force in
center on mass.
ASEISMIC DESIGN REQUIREMENTS
36 Master thesis
According to fig.3.4 two bilinear responses are used, where Sy defines the yielding or ideal
strength Si of the member. The slope of idealized linear elastic response Y
ySK
Δ= is used to
quantify the stiffness.
• Strength
A structure must have adequate strength to resist internal actions generated during the
elastic response of the structure. The strength required to resist earthquake-induced forces,
and structural displacements Δ at the development at different levels of strength are related
to each other. This relation will be shown in definition of ductility given below.
• Ductility and Energy Dissipation
To minimize major damage and to ensure the survival of buildings with moderate resistance
with respect to lateral forces, structures must be capable of sustaining a high portion of their
Initial strengths when earthquake imposes large deformations. These deformations may be
beyond the elastic limit. This ability of the structure or its components, or the materials used
to offer resistance in the inelastic domain of response, is described by general term ductility.
In fig. above, is represented by dashed line brittle failure which means, loss of resistance,
often complete disintegration, with absence of adequate warning. For obvious reason, brittle
failure, which may be said to be the cause for collapse of the building in earthquake, and
consequent loss of lives, must be avoid. But fortunately using again the fig. inelastic
deformation may be still possible without structural collapse.
Ductility is defined by the ratio of the total displacement at any instant time to that at the
onset of yielding 1>ΔΔ
=y
μ . The displacement � and �y may represent strain, curvature,
rotation or deflection and displacement. The design of earthquake resistant concrete
buildings shall provide an adequate energy dissipation capacity to the structure without
substantial reduction of its overall resistance against horizontal and vertical loading.
Therefore, three ductility classes are defined as low, moderate and high ductility, which
corresponding to different level of energy dissipation capacity (Hysteretic Loops) of the
element in analysis.
ASEISMIC DESIGN REQUIREMENTS
37 Master thesis
Fig. 3.5 Level of ductility
The level of energy dissipation capacity is account in term of behavior factor, qo, which
depend on the structural type defined previously. The designing of the structure based on
energy dissipation capacity level must consider also seismic hazard analysis of a given site,
allowing or no some levels of energy dissipation applied in the analysis.
Table 3.2 Level of energy dissipation for structural design
ENERGY
DISSIPATION
CAPACITY
SEISMIC HAZARD LEVEL
LOW MEDIUM HIGH
LOW yes NO NO
MODERATED yes yes NO
HIGH yes yes yes
Therefore, for concrete structure are distinguished two ductility classes, according to EC-8
(5.2.2.2 table 5.1).
ASEISMIC DESIGN REQUIREMENTS
38 Master thesis
Table 3.3 Types of structural system regarding ductility capacity
Structural Type DCM DCH
Frame system, dual system, coupled wall system 3.0αu /α1 4.5αu /αu
Wall system 3.0 4.0αu /α1
Torsionally flexible system 2.0 3.0
Inverted pendulum system 1.5 2.0
Both, moderated and high ductility capacity, correspond to structure which is enabling to
develop stable mechanisms associated with large dissipation of hysteretic energy under
repeated reversed loading, without suffering brittle failure.
According to this classification Albania is characterized to moderated ductility capacity.
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
39 Master thesis
4 OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
4.1 INFORMATION ON DAMAGED BUILDINGS DURING PAST EARTHQUAKES
The identification of active faults can be the consequence of a direct approach (retrieval of
surface faulting evidence) or can start from considering in which European areas
earthquakes with M>6 occurred in the past (Fig. 4.1 and 4.2) and then identifying the active
fault zone to which each earthquake could be associated.
Fig. 4.1 - Earthquakes with Io ≥ 9 or M>6 included in BEECD Working File, 1500-1899
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
40 Master thesis
Fig. 4.2 - Earthquakes with Io ≥ 9 or M>6 included in BEECD Working File, 1900-1990
The importance of going backward in time by means of historical earthquake records is
supported by the fact that for some cases surface faulting or the presence of an active fault
could not have been predicted just taking into account this century seismicity, and vice
versa.
Fig.4.3 - Epicenters of earthquakes associated with faulting evidence (from Ambraseys and Jackson, 1997)
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
41 Master thesis
Today knowledge on active faults in Europe derived from historical data is mainly scattered
among studies dealing with large earthquakes. An important methodological approach is
available for the the Eastern Mediterranean region (Ambraseys and Jackson, 1997); the
contribution covers the area shown in Fig. 2a and 2b.
HISTORICAL DATA
Historical data on earthquakes and faults are today available in formats which are not
homogeneous, as it is the case for macroseismic information. Obviously, the best data are
those related to recent, strong earthquakes, investigated with respect to macroseismic effects,
field surveys, etc.
An intermediate case history is chosen to show the type of information available on surface
faulting evidence. It is a strong earthquake occurred in northern Albania on June 1, 1905,
which is listed by Ambraseys and Jackson also. The map with the intensity distribution (Fig.
4.4) shows that the damage was concentrated in the area of the Scutari Lake, and that the
earthquake was strongly felt in Apulia (south-eastern Italy) and north in Croatia and on the
Dalmatian coast.
Fig. 4.4 - Intensity distribution for the earthquake of 1905, June 1, Scutari (Albania)
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
42 Master thesis
Kociu and Sulstarova (1980) present a detailed description of earthquake effects, in about 20
localities located in the epicentral area, and a summary of natural phenomena associated
with the earthquake.
On November 30, 1967, a strong earthquake of magnitude M = 6.6 struck the Dibra region,
eastern Albania, causing considerable loss of human life and grave material damage both in
the territory of Albania and that of ex-Yugoslavia.
From the study of the macroseismic field of this earthquake and its fault, which extends
over 10 km in a 40° northeasterly direction, from the distribution of aftershocks in space and
the focal-mechanism solution of this earthquake, the conclusion has been reached that this
event is connected with the Vlora—Dibra seismogenic belt.
The authors have mentioned the existence of this traverse belt as early as 1969 (Sulstarova
and Koçiaj, 1969). The existence of this belt is also shown by the chronological and
geographical distribution of some strong earthquakes in Albania in the period 1800–1967
(their macroseismic field and the position of their epicentres), and by the focal-mechanism
solutions of some of these earthquakes. The Vlora—Elbasan—Dibra transverse seismogenic
belt continues for several hundred kilometers northeast and southwest beyond the territory
of Albania.
The earthquake seasonality of Albania is observed for the period 1901–1990. Of the
included 211 earthquakes with magnitude Ms > 4.5, 70% occurred in the year's period
November–April. One-third of all earthquakes occur in November and December. The
Schuster test is employed to asses the significance of the apparent earthquake seasonality.
For five zones of the country, an attempt is made to cross-correlate rainfall and seismic
activity. It is found that some zones show a better cross-correlation coefficient than others.
Detailed observation has been carried out for all the ‘felt’ earthquakes in Albania with
inland epicenters for the period 1969–1990. Some of them followed heavy precipitation. A
suggestion that changing underground waters can act as a valve allowing accumulated
seismic energy to be released earlier than without groundwater change is proposed for the
case of Albania.
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
43 Master thesis
4.2 INSPECTION OF EXISTING AND NEW BUILDING STRUCTURES IN ALBANIA
Table 4.1 Buildings by time of construction of mean cities in Albania
Cities
Time of construction Total Before
1945 1945-1960
1961-1980
1981-1990 1991-1995 1996 +
Durrës 1,778 3,485 7,016 5,188 4,312 8,417 30,196 Elbasan 1,955 4,004 10,165 8,827 4,473 4,722 34,146 Fier 618 3,255 10,819 9,693 4,420 5,649 34,454 Gjirokastër 1,847 2,080 3,768 2,648 534 669 11,546 Korçë 2,852 4,267 9,960 5,534 1,585 1,663 25,861 Krujë 884 1,369 3,035 2,909 1,210 1,656 11,063 Lezhe 864 1,362 2,775 2,352 1,359 2,136 10,848 Librazhd 1,081 1,815 3,405 2,454 853 864 10,472 Lushnjë 771 2,267 7,065 6,632 2,908 3,913 23,556 Pogradec 888 1,673 5,032 3,156 754 1,331 12,834 Sarandë 1,169 1,274 2,340 1,505 604 862 7,754 Shkodër 3,505 4,012 8,127 5,846 2,744 4,541 28,775 Tiranë 5,713 5,905 11,698 10,585 11,406 21,231 66,538 Vlorë 2,747 3,657 8,577 6,836 3,530 3,855 29,202
Table 4.2 Characteristic of the buildings
Cities
Type of building
Total Single
dwelling Masonry
Structures
Multiple dwelling Masonry
&RC Structures
Partially for other purposes Masonry &RC
Structures
Institutional household Masonry
&RC Structures
Other Masonry
&RC Structures
Durrës 29,007 1,189 168 261 62 30,687 Elbasan 33,099 1,047 46 145 13 34,350 Fier 33,525 929 66 84 7 34,611 Gjirokastër 11,152 394 23 110 1 11,680 Korçë 25,000 861 80 68 7 26,016 Krujë 10,797 266 14 44 1 11,122 Lezhë 10,251 597 106 94 219 11,267 Librazhd 10,229 243 20 66 10 10,568 Lushnjë 23,051 505 75 144 13 23,788 Pogradec 12,485 349 6 53 1 12,894 Sarandë 7,343 411 40 12 12 7,818 Shkodër 27,660 1,115 60 294 12 29,141 Tiranë 62,971 3,567 405 547 259 67,749 Vlorë 28,161 1,041 91 153 3 29,449
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
44 Master thesis
According to this table more than 50 % of the buildings were constructed before seismic
regulation KTP-89, which indicate that damage assessment and development of efficient
technology for seismic upgrade is necessary.
Two types of structures are met mostly in Albania:
1. Masonry Structures
• Construction material
- Stone - Adobe - Brick - Ceramic blocks - Cement blocks
• Place of construction
- Rural areas - Urban areas
• Structural systems
- Plain masonry - Confined masonry
Fig. 4.5 Historical building in Albania (City of Gjirokaster)
Fig. 4.6 Typical 5 story masonry
building, build in 1961-1980
(Settled in City of Tirana)
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
45 Master thesis
2. Reinforced Concrete structures
• Construction material
- Concrete + Steel
• Place of construction
- Urban areas
• Structural systems
- Frame system
- Dual system
- Prefabricated RC wall system
Fig. 4.7 New building structure, RC dual system
Fig. 4.8 Different types of
building structure (centre Tirana)
Fig. 4.9 Prefabricated type of building structure
(centre Tirana)
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
46 Master thesis
4.3 COMMON DEFICIENCIES IN EXISTING BUILDINGS
This section describes typical deficiencies found in existing construction which can lead to
poor earthquake performance. For the purposes of this section, poor earthquake
performance is defined as endangerment of life safety through either partial or total collapse.
As previously discussed, for some types of structures and occupancies it may be desirable to
obtain better performance as merely protection of life safety. To obtain such performance, it
is necessary to mitigate each of the deficiencies discussed in this section, as well as to ensure
that expected earthquake induced deformations are kept small enough to prevent significant
damage to key elements of the structure.
Until recently, there has been little consensus in the engineering profession as to appropriate
methods for determining if an existing structure is seismically hazardous.
Some engineers have attempted to apply the current building codes as evaluation tools for
existing structures. The problem with this approach is that since the codes are revised every
few years, most existing buildings will not meet the current code to some extent, a few years
down the road. This would result in a finding that nearly every building is hazardous and
requires upgrade. Such a finding is obviously both technically incorrect and economically
not feasible to manage.
One of the most seismically hazardous classes of buildings common throughout the world
are structures constructed with load bearing walls of unreinforced masonry. A significant
amount of research has been performed in recent years on the performance of these
buildings and effective methods of improving their seismic performance.
Incomplete Lateral Force Resisting System:
One of the most common causes of earthquake induced collapse is the lack of a complete
lateral force resisting system. In order to successfully resist collapse, each element of a
structure must be positively connected to the whole in such a manner that inertial loads
generated by the element from motion in any direction can be transmitted back to the
ground in a stable manner.
As a minimum, a complete lateral force resisting system will include at least three no
concurrent vertical lines of lateral force resisting elements (moment frames, braced frames or
shear walls) and at each level of significant mass a horizontal diaphragm to interconnect
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
47 Master thesis
these vertical elements. Together, this assemblage of elements must provide adequate
rigidity to control structural deformations to tolerable levels.
There are a number of common building configuration and design features which often
result in a building without a complete lateral force resisting system. These include open store
fronts/house over garage, clerestory conditions, and expansion joint conditions.
The open store front or house over garage condition, common in urban construction and for
older buildings, has often lead to building collapse during strong ground motion. In older
mid- and high-rise construction, the primary vertical elements of the lateral force resisting
system are often the perimeter concrete or masonry walls which act as perforated shear
walls. A similar condition to the open storefront is the building or house over garage.
When such buildings have store-front systems or open garage fronts at the lower story, the
vertical shear resistance provided by the walls of the upper stories is not present. This results
in a discontinuous lateral force resisting system.
Such a condition is most severe for buildings with openings on two of four sides, as the
building becomes torsionally or laterally unstable at the lower story
The clerestory condition is common in many low- and mid-rise buildings in either
commercial or residential occupancy. The problem is that the clerestory is a major
discontinuity in the horizontal roof diaphragm, which requires the structure on either side of
the clerestory plus the clerestory roof to behave as independent elements. If the structure on
opposite sides of the clerestory or the clerestory roof is not by itself stable, then collapse can
occur. If the structure on both sides of the joint is stable, then differential movement of the
structure on opposite sides can result in severe damage. Long narrow buildings with one end
having an open store-front are also a common configuration that has a high degree of
torsional instability.
Expansion joints are a common feature of many large buildings of low- and mid-rise
construction, particularly in areas with significant seasonal temperature variation. They are
placed in buildings to relieve stresses induced by thermal expansion of the building frame as
well as to provide relief in exterior finishes (particularly roofing). The system of expansion
joints placed in a building will effectively divide it into separate structural units. Some
buildings with such joints have not been designed with a complete lateral force resisting
system for the structural segments on each side of the joints. This can result in collapse.
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
48 Master thesis
Another problem that can occur in buildings with expansion joints is pounding of the
adjacent structures (Figure 12-7). The severity of this problem is minimized somewhat if the
diaphragm levels on each side of the joint align so that the slabs of one structure do not act
as knife edges against the columns of the adjacent structure.
Structural Continuity and Inter-element deformations:
Structural continuity is an important factor for good seismic performance.
If all of the various components of a structure are not adequately tied together, the pieces
can move independently and in different directions.
This can result in dislodging elements from structures and the loss of bearing support for
vertical load carrying elements. Modern codes require that all elements of a structure be tied
together or that sufficient accommodation be made for the real displacements such that
failure does not occur. These considerations were often overlooked in older structures.
Common deficiencies include: inadequate anchorage of walls to diaphragms for out-of
plane and in-plane deformations use of sliding type beam bearing connections with
undersized bearing dimensions; inadequate attachment of architectural elements including
cladding, ceilings, and partitions to the structure; inadequate attachment of equipment and
utilities to the structure.
Excessive Lateral Flexibility:
Buildings with complete lateral force resisting systems but excessive flexibility in the
elements of their lateral force resisting systems have occasionally collapsed. Such buildings
can experience very large lateral displacements when subjected to ground shaking.
Structures with significant gravity loading can become unstable under large lateral
deformation, as a result of P-delta effects. Since flexible structures tend to have relatively
long fundamental periods of vibration, such structures tend to perform adequately when
located on sites with firm soils, as the energy content of ground shaking transmitted by such
sites to the structures is relatively limited. However, flexible structures located on sites with
deep soft soils can experience very large demands. Typically, structures with inter-story drift
ratios of 1% or less at real deformation levels behave acceptably.
Brittle elements:
Modern design practice for buildings expected to withstand strong ground shaking requires
the incorporation of ductile materials and detailing in the design of structures, such that
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
49 Master thesis
deformations substantially larger than those expected at normal service levels can be
tolerated without loss of structural capacity. Older construction rarely was provided with
this ductility. As a result, elements tend to be brittle, and can rapidly loose strength when
strained beyond their elastic or nominal capacities.
Unreinforced masonry walls can be composed of common clay brick, stone, hollow clay tile,
adobe, or concrete masonry materials.
Walls of these materials have limited strength, and very little ductility for in-plane demands.
Slender walls, with large ratios of unsupported length to thickness have often failed due to
out of- plane demands. Inadequate anchorage of these walls to diaphragms is a common
deficiency which contributes to poor out-of plane performance.
Non-ductile Concrete Frames. If adequately designed, moment resisting frames of reinforced
concrete can provide excellent behavior in strong earthquake shaking.
However, many earthquakes induced collapses of structures relying on non-ductile concrete
frames for their lateral resistance have occurred.
A number of problems can result in poor earthquake performance of concrete frames.
These include deficiencies in: shear capacity, joint shear capacity, placement of
reinforcement for load reversals, development of reinforcement, confinement of the concrete
and lateral support for reinforcing steel.
Shear failure of reinforced concrete columns and beams is a brittle failure mode and can
result in sudden loss of load carrying capacity and collapse. In frames with adequate
strength to remain elastic under real deformation levels, the beams and columns should
have greater shear capacity than required at these deformation levels. In frames which
experience flexural yielding at the joints under real deformation levels, the shear strength of
the elements must be greater than their flexural capacity or failure can result. The shear
strength capacity of members with relatively low axial compressive stress levels should be
limited to that provided by the reinforcing steel as the shear strength of the concrete in such
members quickly degrades under cyclic loading.
Shear failure of joints in moment resisting frames can also occur. The beam column joint of a
moment resisting frame can be subjected to very large shears, resulting from the transfer of
flexural stresses between the elements. Failure has occurred at such joints, particularly when
the lateral confinement reinforcement in the columns does not run continuously through the
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
50 Master thesis
joint zone. Frames with eccentric beam column joints or relatively slender beams tend to be
weaker than those without such features.
Moment resisting frames subjected to strong ground shaking will typically experience large
flexural load reversals at their joints. Some concrete frames designed primarily for gravity
load resistance have little if any positive beam reinforcing steel (located at the bottom face of
the beam) continuous through the beam column joint. As a result, the frames do not have
capacity to resist load reversals. For good performance, frames must have a minimum
percentage of the beam positive reinforcing developed continuously through the beam
column joints.
Inadequate development of reinforcing steel is another common problem. In frames with
inadequate strength to remain elastic at real deformation levels, the flexural reinforcing steel
will yield. Repeated cyclic loading of the bars into the yield range results in a breakdown of
the bond between the reinforcing steel and concrete, which can result in a loss of flexural
strength and frame instability.
Inadequate Concrete Confinement – Normal weight concrete elements with nominal lateral
reinforcement can withstand compressive strains on the order of 0.003 to 0.004.
Compressive strains in excess of this amount will result in crushing and spalling of the
concrete and degradation of the element's capacity to carry load. Strong ground shaking can
induce large compressive strains in concrete at flexural hinge regions of beam column joints.
Large compressive strains resulting from large overturning demands can also occur in
columns. Unless closely spaced lateral confinement reinforcing is provided, compressive
strains at real deformation levels in excess of about 0.004% in normal weight concrete and
0.002% in lightweight concrete can result in structural failure. This is not a concern for
members with low strain demands at real deformation levels.
Large tensile strains, particularly at flexural hinge regions of frames can also result in
member failure, unless closely spaced lateral reinforcement is provided. When a flexural
hinge forms, large tensile strains and elongation will occur in the longitudinal reinforcing
steel.
When structural response reverses, under cyclic motion, the elongated steel is forced into
compression, and if not provided with adequate lateral support, will buckle. In addition to
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
51 Master thesis
causing premature spalling of cover concrete, this can lead to low-cycle fatigue failure of the
reinforcing and loss of structural capacity.
Reinforced concrete and masonry walls can have many of the same problems described for
reinforced concrete frames, particularly if they are highly perforated by openings, or are tall
and slender. Generally, walls with relatively low levels of axial load, moderate quantities of
vertical reinforcing steel and shear capacities greater than their flexural capacities behave in
a ductile manner, while those without these features can be quite brittle. Wall failures can
occur as a result of excessive shear demands, as a result of crushing at the edges under
extreme flexural strains, or as a result of failure of the reinforcement, as previously described
for concrete frames. The most common wall failures occur in the spandrel beams present
over door and window openings. Very large stress concentrations occur in these elements,
often resulting in damage at relatively low levels of lateral load.
Once the spandrels have failed, overturning demands on individual piers can increase
substantially, and the stiffness and strength of the structure decrease.
Braced steel frame structures have been commonly damaged in earthquakes, but collapses have
been rare. The most common damage is to the bracing itself. Light rod braces often fracture,
as a result of a concentration of inelastic strain demands at the threaded portion of the rods.
In heavier structures, inelastic buckling of compression braces is also common.
Compression braces of intermediate slenderness, and non-compact section properties can
experience brittle fracture as a result of low-cycle fatigue induced by large secondary stresses
at buckled sections. Failure of bracing connections is also common, particularly when the
strength of the connection is less than the strength of the brace itself.
Highly eccentric brace connections tend to fail prematurely due to the large secondary
stresses induced by the eccentricities.
Although failure of braces is one mode of common failure, other failure modes can also
occur in these structures. One of the more common failure modes occurs in structures with
"chevron" type bracing, where the beam at the apex of the chevrons can be severely
deformed by large unbalanced force in the "tension" brace following buckling of the
"compression" brace. Some structural collapses have occurred as a result of braces which
were designed too strong, relative to other portions of the structure. Over-strength bracing
can place very large overturning demands on columns, resulting in buckling of this critical
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
52 Master thesis
gravity load carrying elements. Knee braced frames, in which the braces induce flexural
demands on columns can also result in premature column failure.
Inadequate diaphragms - Reliance on inadequate diaphragms can be another cause of
earthquake-induced collapse. Although the floors and roofs of most structures provide
diaphragm capacity, unless the structures were specifically designed to resist seismic loads,
these features are often grossly inadequate.
Common diaphragm deficiencies in buildings include inadequate shear capacity, inadequate
flexural capacity, extreme flexibility, poor connectivity to vertical elements of the lateral force resisting
system, and lack of continuity.
Diaphragms of differing materials have widely different shear strengths. Systems consisting
of cast-in-place concrete, composite systems of concrete filled metal deck, and horizontal
steel braced systems tend to have very large capacities and excellent ductility.
Diaphragms constructed of timber sheathing and certain metal decks have very limited
capacity but intermediate ductility. Diaphragms consisting of poorly bonded precast
concrete planks or of poured gypsum slabs tend to have very low shear capacity and
negligible ductility.
Flexural capacity of diaphragms should also be considered. Classic engineering evaluation
techniques of flexible diaphragms treat these elements as simply supported horizontal
beams, spanning between the various vertical elements of the lateral force resisting systems.
The diaphragm materials itself (timber sheathing, metal deck, diagonal braces, etc.) are
considered to act as the web of this beam while discrete continuous chord elements at the
edges of the member are provided to resist flexural demands. The presence of walls around
the perimeter of a diaphragm may alter the pattern of flexural demands. In such structures,
the walls themselves may directly resist the shear stresses at the boundaries of the
diaphragm such that the classic "simple beam" analogy is not valid. Regardless, a rational
stress path must exist such that the diaphragm remains in internal as well as external
equilibrium. A common deficiency in diaphragms is an absence of local flexural chords
around openings. This can greatly reduce the effectiveness of otherwise competent
diaphragms.
The basic functions of the diaphragm are to tie the elements of a structure together at a
given level and distribute inertial loads to the various vertical elements of the lateral force
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
53 Master thesis
resisting system. Diaphragms which are extremely flexible can result in very large interstory
drifts for supported elements such as walls subjected to out-of-plane loads. It is important
that the diaphragm have adequate stiffness to prevent excessive inter-story drifts from
developing. This problem tends to be most pronounced with diaphragms of timber
construction or those of unfilled metal deck construction.
Poor connectivity of the diaphragm to the vertical lateral force resisting elements is also
common, particularly in structures with relatively large diaphragms and isolated vertical
shear resisting elements. It is important that collectors be provided in such diaphragms to
transfer shears into the frames and walls.
Another common deficiency with regard to shear transfer is a physical separation between
the diaphragm web and the top of the vertical lateral force resisting elements.
Continuity is an important consideration for diaphragms constructed of materials with
limited tensile capacity including plywood, gypsum and concrete. Under the influence of
large concentrated inertial loads, such as generated by heavy masonry or concrete walls
supported at a diaphragm edge, diaphragms with limited tensile capacity can rip apart
unless directly provided with continuous elements to tie the structure together. In timber
diaphragms, continuity can best be provided through the framing members. In concrete
diaphragms, reinforcement must provide the required continuity.
Non-structural elements. Non-structural elements are those pieces of a structure which are not
intended by the designer to act as structural load carrying elements. Common non-structural
elements include non-load bearing walls, cladding, ceilings, ornamentation, and mechanical
and electrical services and utilities.
Non-load bearing walls including construction of hollow clay tile, concrete masonry, concrete,
and other materials are a common problem in structures. Often not directly considered by
the original structural designer of the building, these elements can have substantial influence
on the performance of a structure. They can alter its stiffness, deformation patterns, lateral
force resisting capacity and failure modes. Common problems include partial height walls
which can induce shear failures where they bear against the mid-height of columns, and
irregular placement of walls in a building which can create torsional problems and soft
stories. In addition to their effect on the behavior of the structure, partition walls can fail
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
54 Master thesis
either due to in-plane deformations or out-of-plane accelerations resulting in potential
personnel hazards as well as substantial architectural damage.
Buildings of recent construction often have curtain wall type cladding systems. A common
deficiency of such systems is an inability to withstand the large lateral deformations the
building experiences under strong ground motion. If the cladding has not been provided
with adequate deformation capacity, panels can crush or connections can fail, creating a
substantial falling hazard.
Ceilings are a frequent source of damage in earthquakes. Suspended plaster ceilings which
are not adequately braced to a nearby diaphragm are a particular problem. These heavy
ceiling systems can sway independently, much like a pendulum, and batter adjacent
structural elements including walls. This is a common mode of failure initiation in
unreinforced masonry buildings.
Exterior ornamentation on structures including parapets, statuary, balustrades, balconies and
similar items can also be problem areas. Often, these decorative elements have limited
capacity to resist earthquake induced lateral accelerations. Failure typically results in a
falling hazard.
Mechanical and Electrical Utilities must be maintained in a serviceable condition for structures
which are expected to remain functional following an earthquake. Even in less critical
facilities, shaking induced damage to these elements can result in substantial consequential
damage to architectural elements.
For example failed mechanical and electrical systems can result in fire initiation as well as in
flooding. Unfortunately, most mechanical and electrical systems in existing structures are
not adequately installed to prevent earthquake induced damage. Major equipment items are
not adequately anchored to the structure to prevent sliding or overturning. Piping and
conduit systems typically are not adequately braced and provisions have often not been
made for earthquake induced building deformation.
Poor construction quality has contributed to the earthquake induced failure of many properly
designed structures. Masonry structures tend to be particularly vulnerable. A number of
failures have occurred in reinforced masonry walls because grout had not been placed in
reinforced cells. Poor quality mortar is also common. In concrete structures, under strength
concrete has occasionally resulted in failures. Welded reinforcing steel splices are often quite
OVERVIEW ON CONSTRUCTION OF EXISTING BUILDINGS
55 Master thesis
brittle and can prematurely fail if proper procedures were not followed during construction.
Similar problems can occur at welded connections of steel structures. Timber buildings are
also susceptible to problems arising from poor construction quality, including such basic
errors as framing the structure differently than intended, or failing to provide the connectors
specified.
Deteriorated condition also contributes to earthquake induced failures. Common problems
include dry-rot and infestation damage to wood structures, rusting of steel and spalling of
concrete on marine structures, and weather deteriorated mortar in masonry structures.
Site characteristics are also too often overlooked by structural engineers with regard to
building performance. Unstable sites with propensities for liquefaction, lateral spreading,
land sliding or large earthquake induced differential settlements can lead to extensive
damage to structures which are otherwise adequately designed. It is critically important to
assess the nature and likely stability of the local geotechnical conditions as a first step in the
evaluation and retrofit of any existing structure.
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
56 Master thesis
5
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
Within the scope of specific project the investigation was basically devoted to development
of e practical and consistent methodology for structural state diagnosis. Basically, the
development procedure is regarded as an specific tool which will provide successful
identification of the basic parameters needed for elaboration of an optimal project for
revitalization of structural system, and it is predominantly based on application of
experimental non-destructive tests.
5.1 IN-SITU NON-DESTRUCTIVE TESTS AND DESTRUCTIVE LABORATORY TEST
In the framework of this topic particular attention should be paid to adopting of various in-
situ non-destructive test methods for identification of the basic parameters defining the
actual state of the existing structural materials, constituent elements, as well as the actual
state of the integral structural system. In this case the following methods have to be
particularly focused:
1. Ambient vibration test method
2. Forced vibration test method
3. Methods based on application of micro-seismic, micro-acoustic or other waves
These methods are briefly described as follows:
5.1.1 AMBIENT VIBRATION TEST METHOD
Widely applied and popular full-scale testing method for experimental definition of
structural dynamic characteristics is the ambient vibration testing method, which is based on
measuring the structural vibration caused by the ambient. The method is very fast and
relatively simple procedure can be performed on a structure in use, without disturbing its
normal functioning. Ambient vibration testing method is very convenient for testing of
structures whose lower mode shapes of vibration occur at frequencies below or near 1.0 Hz
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
57 Master thesis
in which range the output force of vibration generators has very low amplitudes and the
application of the forced vibration is limited.
The basic assumption used in this method is that the excitation forces are a stationary
random process, having an acceptably flat frequency spectrum. In such conditions, the
structures will vibrate and their response will contain their normal modes.
As ambient forces can be treated the wind, the traffic noise or some other micro-tremor and
impulsive forces like wave loading or periodical rotational forces of some automatic
machines. Even though, in a period of the time these excitations and consequently the
structure response to these ambient vibrations are nearly periodic, so for analysis of the
excited structural response to these ambient vibrations fast Fourier transform technique can
be used. The response of a linear system to e periodic force can be determined by combining
the responses to individual excitation terms in the Fourier series.
Usually, the wind as an excitation has relatively flat frequency spectrum and it is often used
for these measurements, because as mentioned, in such a case the structural response will
contain all normal modes.
5.1.2 FORCED VIBRATION TEST METHOD
The dynamic full scale testing of structures should be carried out interpreted on the basis of
a known theory and realization of the dynamic principles. The forced vibration testing
method is based on the principle of resonance when the structure is subjected to external
forces. In such a case, the governing differential equation of motion of a multi-degree-of-
freedom system is subjected to a vector of excitation forces is:
Where [M], [C] and [K] are mass, damping and stiffness matrices of the system, , and
are acceleration, velocity and displacement vectors and is the vector of the
external forces.
The theory of structural dynamics has shown that the free motion of a dynamic system as
well as the forced motion of such a system may be expressed in terms of the normal modes
of vibration and that the response may be obtained as the superposition of the solution of
independent modal equations. In other words, the normal modes may be used to transform
the system of coupled differential equations into a set of uncoupled differential equations in
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
58 Master thesis
which each equation contains only one dependent variable. In such way the modal
superposition method reduces the problem of finding the response of a multi-degree-of-
freedom system to the determination of the response of single degree-of-freedom systems.
The exciting force can be any time depending function, but the largest structural response
will be obtained if the excitation frequency is close or equal to the natural frequency of the r-
th /mode of the system or if the so called vibration resonant state of the structure in the r-th
mode is achieved.
5.1.3 SEISMO-ACOUSTICS BASED METHODS
In the case of micro seismo-acoustic based methods, impulse generated waves propagating
through the given medium are recorded at previously defined points by special recorders.
Having the recorded propagation times of longitudinal, transversal and surface waves along
known distance between exciter and recorders, propagation velocity can be determined.
Using this information, the dynamic values of the elasticity module (Edin), shear module
(τdin), Poisson coefficient (μdin), bulk density module (Kdin), etc. can be determined. Through
interpretation of obtained results, relative linear, volume or area material quality variation
was identified. By correlation of the dynamic and the static values, “static” parameters will
be also defined.
5.2 ADOPTING OF NONLINEAR FAILURE MODELS FOR STATE ANALYSIS
Considering the advantage of the newly developed theoretical models and the analysis
procedures for inelastic behavior and failure simulation of the construction material,
constituent structural components and the integral structure, in this sub-phase, included is
development of nonlinear failure models and computer software for state analysis of
structural systems under critical service load combinations and expected seismic and other
excessive load effects.
Finite element formulations that allow modeling at the material level using a point by point
basis, with different elements used for the concrete, reinforcing steel and possibly for their
interaction through bond would definitely be the ideal solution. Such modeling allows
representation of even minor details of the geometry of the members, and allows the history
of stresses and strains at every point of the structure to be followed. However, the
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
59 Master thesis
computational requirements of such an approach restrict its application to the analysis of the
response of individual members or sub-assemblages and therefore to date, the application of
such detailed modeling for entire structures under non-linear dynamic effects has not been
widely employed. However, given the hardware possibilities available nowadays 3D models
have assumed a leading role, wherein a member model is used as a founding unit and the
detailed stress-strain response at a large number of points over the cross-section of the
member is followed during the response analysis.
Having ruled out the practicability of using a general purpose finite element program, a
suitable platform had to be found that would allow the inelastic response of structures.
Many computer programs for the seismic response analysis of structures are available
amongst which DRAIN originally developed for 2-D frames but with later versions capable
of undertaking 3-D analysis, ANSR a non linear program specifically for 3-D structures,
IDARC 2D, SAP 2000 developed in California, RUAUMOKO developed in New Zealand,
OPENSEES, SEISMOSTRUCT and NORA 2000 developed in Macedonia. Although the
merits of each program is beyond the scope of this paper, in choosing a suitable analytical
engine various parameters had to be kept in mind, which included the availability of the
source code, the ability to analyze 3-D structures and finally the ability to model the spread
of inelasticity along the member length and across the section depth, thus allowing for
accurate estimation of damage distribution in reinforced concrete members. DRAIN-3DX
was finally chosen as a suitable platform given its widespread use by other researchers and
most importantly the possibility of adding the required information of the structure through
coding.
5.3 FRAGILITY MODELS AND EXPECTED DAMAGE
5.3.1 BASIC COMPONENTS OF RISK ASSESSMENT The disaster threat, or hazard, is any naturally occurring or human-induced phenomenon,
process or event with the potential to create loss to humans, to their welfare and their
environment. In other words, it is a general source of danger.
Many so-called "natural" hazards have both natural and human components. The
compound effect of natural and man-made hazards is better described by a term
environmental hazards defining a hazard environment as "extreme geophysical events,
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
60 Master thesis
biological processes and major technological accidents characterized by concentrated
releases of energy or materials that pose a largely unexpected threat to humans".
When the interaction between the human population and a hazard results in the loss, at
sufficiently large scale, of lives, of material possessions or of what is valued by humans, the
event is termed a disaster.
Risk is defined as "the probability of meeting danger or suffering harm or loss", or "the level
of loss or damage that can be predicted from a particular [environmental] hazard affecting a
particular place at particular time". In other words, hazard (or cause) may be defined as 'a
potential threat to humans and their welfare' and risk (or consequence) as 'the probability of
hazard occurrence’.
So a disaster may be seen as 'the realization of hazard', although there is no universally
agreed definition of the scale on 'which loss has to occur' in order to qualify the event as
disaster. In relation to disaster, risk has been more specifically described as the probability
that a disaster will occur, using the relative terms such as high risk, average risk and low risk
to indicate the degree of probability.
In engineering context risk is a way of describing the probability and consequences of a
disaster. Risk tries to identify the expected losses from the impact of a given threat to a given
vulnerable element over a specified time period.
Risk analysis includes four basic components:
Fig 5.1 Basic component of Risk Assessment
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
61 Master thesis
The exposure is defined as any element of human value (people and what they value)
sensitive to the realization of the hazard. Thus, people and/or what they value, e.g. the
elements at risk, are the essential point of reference for all risk assessments and for all
disasters. That is why when the large numbers of people exposed to hazard are killed,
injured or damaged in some way, the event is usually termed disaster. While hazard can
exist even in an uninhabited region, the risk can occur only in an area where people and
their possessions exist.
The location is defined as the position of the exposure relative to the hazard.
The vulnerability, i.e., the concept of vulnerability implies a measure of risk combined with
the level of social and economic ability to cope with the expected/resulting event. In its
simplest (physical) form, the vulnerability is defined as "the factors (of the community)
which allow a hazard to cause a disaster"; i.e. "the factors that increase the chances of a
community being unable to cope with an emergency". In an area exposed to multiple
hazards, vulnerability analysis should be carried out for each type of hazard.
The most essential and concerned physical vulnerability aspects are related to population,
buildings, infrastructure and agriculture.
The vulnerability of buildings is primarily controlled by factors such as characteristics of the
building site, their design, shape, materials used, construction techniques, maintenance and
proximity to other buildings. The weight attached to each of these factors will vary
according to the type of hazard encountered because different hazards produce different
forces affecting these structures.
The vulnerability of infrastructure is also specific, depending on the type of hazard.
Infrastructure may be considered in three broad groups; 1) transport systems (roads,
railways, bridges, airports, port facilities); 2) utilities (water, sewerage and electricity); and,
3) telecommunications. Hazard protection measures such as flood embankments are also
considered as a part of the infrastructure once they have been installed.
Risk management is the systematic application of management procedures and practices to
the tasks of identifying, analyzing, assessing, treating and monitoring risk. Risk
management involves a formal, quantitative evaluation of potential injury or loss over a
specified period of time, or the prospect of future mal-performance of a safety or security
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
62 Master thesis
systems. In the cases of emergencies or natural disasters, it is a measure of the occurrence
and severity of losses from a particular hazard.
5.3.2 PRE-DISASTER EVENT ACTIVITIES
The year-by-year development of earthquake engineering has enabled many countries
exposed to high seismic hazard to decrease the overall seismic risk to populated regions.
Traditionally, the engineering response to the problem has almost been solely structural, i.e.,
(1) to demolish the highly vulnerable building and replace it by a modern one, designed and
built according to adopted seismic regulations, standards and codes, or (2) to repair and
strengthen it in order to improve its seismic resistance and overall safety. Although very
effective in the case of isolated buildings, the amount of capital resources that should be
placed in when the building class is considered becomes so high that only few countries can
afford it. Earthquake damage can also be reduced by proper land-use policy which should be
gradually realized through the processes of general, physical and urban planning. To
decrease earthquake damage, the urbanization pattern (land occupancy, adopted structural
typology, distribution of material property and its density concentrations, etc.) should
comply with the level and the spatial distribution of the expected seismic hazard.
5.3.2.1 Criteria for Defining Acceptable Level of Seismic Risk
An appropriate earthquake-resistant design may be defined as one that provides adequate
safety against injury and loss of life, minimum damage to property and ensures continuity of
vital services, achieving this at an acceptable cost. However, it is a well-recognized fact, that
to provide complete protection against earthquakes is not economically feasible. It is
generally accepted that earthquake resistant design criteria should satisfy the following
conditions:
To resist minor earthquakes, without damage
To resist moderate earthquakes without structural damage but with
some non-structural damage
To resist major earthquakes, of intensity and severity of the strongest
experienced in the area without collapse, and with limited structural
and vital nonstructural damage.
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
63 Master thesis
In most structures, it is expected that structural damage, even in a major earthquake, could
be limited to repairable level. The achievement of acceptable earthquake-resistant design
criteria requires an understanding of how structures deform when exposed to earthquake
shaking, and how the materials the structure is made from behave when subjected to these
deformations. Several levels of understanding are required. The first is the elastic response
of structures during which the earthquake vibrations do not produce any structural damage
or any plastic deformations. The second is the large-amplitude non-linear vibrations of
structures when plastic deformations, cracking and other types of damage may be sustained,
but not to the degree that the structure is near to failure. The third is the very large-
amplitude vibration with increased damage to the point of collapse. Consideration of elastic
structural vibrations is required in order to understand how structures will behave when
subjected to minor and moderate ground shaking that should cause no structural damage.
This is the most likely ground shaking that structures will experience. However, very strong
shaking may occur during the life of the structure, even through the probability is low, and
in this case the ground shaking may be so severe that the structure is damaged. Economic
considerations require that ordinary structures be designed for controlled damage when
exposed to very strong ground shaking. Life safety considerations require that ordinary
structures be designed so as not to collapse in the event of a maximum credible shaking, but,
on the other hand economic considerations show that it is not feasible to design ordinary
structures to resist such intensive ground motions without damage. The percentage of
structures that will experience such intense ground shaking during their lifetime is quite
small and most will never experience such a strong shaking. Therefore, it is economically
desirable to reduce the design for structures in general, even though same structures will
require damage repairs. On the other hand, special structures, because of the cost, the
potential hazard they pose, the need to maintain operations, etc., require special
consideration. For example, attention should be given to how the structures or facilities will
perform during future earthquakes; what is acceptable infrequent damage; how much
should be invested in providing earthquake resistance. Such questions are certain to arise
when designing high-rise buildings, large dams, nuclear power plants, long-span bridges, oil
refineries, LNG storage facilities, offshore platforms, chemical process facilities, port and
harbor facilities, and other similarly complex and costly installations. It is very important
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
64 Master thesis
that correct earthquake engineering decisions be made for such projects, from the standpoint
of safety as well as of cost. All social goals incorporate values that must be weighed against
the costs of achieving various objectives. Several factors must therefore be considered in
defining "acceptable risks" to life and property in relation to the costs and outlays required.
There is no uniform level of acceptable risk.
Acceptable safety levels vary with time, place, frequency of natural disaster occurrence; they
must be related to costs; and they are influenced by socio-economic and technology
development factors, and applied practice.
5.3.2.2 Planning for Earthquake Disaster Mitigation
Action within this segment usually takes the form of specific programs intended to reduce
the effects of disaster on nation or community. There are various definitions of disaster
mitigation. The widely used one defines it as" Measures aimed at reducing the impact of an
environmental (natural or manmade) disaster on a nation or community". With this
definition, the basis premises is that whilst it may be possible to prevent some disaster
effects, other effects will obviously persist, but they can be modified or reduced provided
appropriate action is taken.
Mitigation activities can be broadly classified into structural and non-structural mitigation
measures.
The typical structural mitigation measures are:
Construction of structures to resist the forces generated by environmental
hazards (earthquake, high winds, flood, etc.)
Strengthen existing structures to render them more resistant against the
environmental hazard forces.
The structural measures should be developed based on:
Adequate site planning
Assessment of forces created by the potential environmental hazards
The planning and analysis of structural measures to resist such forces
The design and proper detailing of structural components
Construction with suitable materials
Good workmanship under adequate supervision
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
65 Master thesis
Earthquake disaster prevention arrangements could be best achieved with continuous
implementation of the following activities: permanent recording of the earthquakes by
seismological and strong motion observation systems; strong motion instrumentation of
buildings and structures; continuous earthquake data analysis and studies of earthquake
phenomena for the needs of general, physical and urban planning, seismic design and
construction of new structures and prevention of existing structures and utilities; continuous
improvement of seismic zoning maps of the region and micro zoning maps of the urban
areas as well as seismic design and construction codes and requirements; continuous
education and improvement of knowledge of scientists, engineers and planners for the need
of application of established scientific basis in the process of physical and urban planning,
design and construction in the region.
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
66 Master thesis
Structural Input
Geographical Identification
Soil Input Seismic Input
Soil Conditions PGAStructural
Properties Code Reqirements
Pushover Analyses
DAMAGE ASSESSMENT OF BUILDINGS
Response Spectra (1)
Demand Spectra
Strength Reduction Factor Rμ
Capacity Curve
BilinearCapacity Model
CapacitySpectrum
Damage State
Fragility Curve
p
Regression Analysis
Performance Points-PP
Damage Assessment
Fig. 5.2 Schematic presentation of Damage Assessment
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
67 Master thesis
The static pushover procedure has been presented and developed over the past twenty year
(Saiidi and Sozen, 1981; Fajfar and Gaspersic, 1996; Bracci et al., 1997; Krawinkler and
Seneviratna, 1998; Kiattivisanchai, 2001; Imarb, 2002). The method is also described and
recommended as a tool for design and assessment purposes by the National Earthquake
Hazard Reduction Program 'NEHRP' guidelines for seismic rehabilitation of existing
buildings and by the Applied Technology Council (1996) guidelines for seismic evaluation
and retrofit of concrete buildings. Moreover, the technique is accepted by the Structural
Engineers Association of California 'SEAOC Vision 2000' (1995) among other analysis
procedures with various levels of complexity.
The recent advent of performance based design has brought the nonlinear static pushover
analysis procedure to the forefront. Pushover analysis is a static, nonlinear procedure in
which the magnitude of the structural loading is incrementally increased in accordance with
a certain predefined pattern. With the increase in the magnitude of the loading, weak links
and failure modes of the structure are found. The loading is monotonic with the effects of
the cyclic behavior and load reversals being estimated by using a modified monotonic force-
deformation criteria and with damping approximations.
Static pushover analysis is an attempt by the structural engineering profession to evaluate
the real strength of the structure and it promises to be a useful and effective tool for
performance based design.
The nonlinear static pushover analysis is a sample option for estimating the strength
capacity of building in the post-elastic range. The technique can also be used to highlight
potential weak areas in the structure. This procedure involves applying a predefined lateral
load pattern that is distributed along the building height. The lateral forces are then
monotonically increased in constant proportion with a displacement control at the top of the
building until a certain level of deformation is reached. The target top displacement may be
the deformation expected in the design earthquake in case of designing a new structure or
the drift corresponding to structural collapse for assessment purposes. The method allows
tracing the sequence of yielding and failure on the member and the structure level as well as
the progress of the overall capacity curve of the structure.
The capacity spectrum method, by means of a graphical procedure, compares the capacity
of the structure with the demands of the earthquake ground motion on the structure.
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
68 Master thesis
Therefore, both must be defined with the same set of coordinates, which is defined as
Acceleration-Displacement Response Spectrum format (ADRS), means of Sa vs. Sd in
which the period can be represented by lines radiating from the origin. The intersection
between capacity and demand spectrum having the same ductility factor, μ, is the
performance point that represents the maximum structural force and displacement expected
for the demand earthquake ground motion.
Fig.5.3 (a) Capacity Spectrum (b) Demand Spectrum
(c) Demand Spectrum superimposed Capacity Spectrum in ADRS format
(a) The Capacity of the structure is represented by a nonlinear force-displacement
curve, which is referred as a pushover curve and explained in details in point 3
above for our structure using bilinear model as well as Spectrum Capacity.
(b) The demands of the earthquake are represented by Response Spectra. Earthquake
loading on structures is often characterized by its effect on simple structural
systems, namely Single Degree Of Freedom systems through response spectra, so
in other words, response spectra are the maximum response quantities
(Displacement, Velocity and Acceleration) of a SDOF plotted as a function of its
period (T) for a given earthquake record. Therefore, each earthquake is
characterized by it own set of spectra.
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
69 Master thesis
Depending on the material behavior involved in the structural response, spectra are
generally divided in two main categories: elastic and inelastic.
Linear Elastic Response Spectra is assumed by code with 5% damping which corresponds to
ductility μ = 1, while Inelastic Response Spectra represent the modified linear elastic
response spectra in order to represent the effects of inelastic response by substituting higher
damping response spectra to account for hysteretic nonlinear response of the structure.
The general trend in developing inelastic response spectra is from their elastic counterparts,
by means of a reduction factor R, called the force reduction factor (or q in Eurocode 8). This
factor is designed to drag the system into inelastic response, thus causing it to dissipate
energy through inelastic deformations. The dissipated energy is usually expressed is terms of
the ductility � of the system, and is a function of its period of vibration T.
There are different approaches to define R – � – T relationship. From Displacement –
based approach damping ratio is defined in function of ductility, according to equation
)05.095.01(105.0 μππ
ξ −−+=
From FEMA 356 based in the graph R vs. T
For long period system μ=R
For short period system 1=R
For intermediate period system 12 −= μR
From Vidic et al. Model (1994)
For T ≥ TC μ=R (1)
For T ≤ TC 1)1( +⋅−=CT
TR μ (2)
Having in mind that the response spectra are a maximum response quantities of
Acceleration as well as Displacement, is in our interest to calculate Elastic Displacement
Response Spectra from their acceleration counterparts using the definition of the pseudo
acceleration bellow: )()( 2 tuwtA = ⇒ dMAXMAX ST
uwASa 2
22 4π
=== and then
224
)()( T
TSTS ae
de π=
STRUCTURAL DIAGNOSIS AND DAMAGE ASSESSMENT
70 Master thesis
So, each ordinate of spectral acceleration that is associated with the period T is converted
into corresponding spectral displacement ordinate by multiplying with the factor 22
4πT .
Now Demand Spectrum in ADRS format is to plot spectral acceleration versus spectral
displacement, for elastic and inelastic spectrum.
The most sophisticated level of analysis available to the designer for the purpose of
predicting design forces and displacements under seismic attack is Dynamic inelastic time-
history analysis. This involves stepwise solution in the time domain of the multi-degree-of-
freedom equations of motion representing a multistory building response. It requires one or
more design accelerograms representing the design earthquake. These are normally
generated as artificial earthquakes analytically or by “massaging” recorded accelerograms to
provide the requisite elastic spectral response. Since structural response will depend on the
strengths and stiffness of the various structural elements of the building, which will not
generally be known at the preliminary stages of a design, it is unsuitable for defining design
force levels.
Following EUROCODE 8 as alternative to a linear method, nonlinear methods may also be
used such as:
Nonlinear time history (dynamic) analysis
Nonlinear static (pushover) analysis
Fragility curves represent one of the possible forms of the earthquake intensity – damage to
structures relationship. A random point on the fragility curve shows the conditional
probability that the damage under an earthquake of a given intensity will exceed a certain
damage state. These functions are defined on the basis of defined discrete damage states and
the fitted probability distributions of damage under earthquakes of certain intensity.
The performance point represents the maximum structural force and displacement expected
for the demand earthquake ground motion, based on the intersection between capacity and
demand spectrum having the same ductility factor, μ.
Superposing performance point to the fragility curves, a Damage assessment is compute for
existing structure.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
71 Master thesis
6
MEASURES AIMED TO REDUCE THE IMPACT
OF ENVIRONMENTAL HAZARDS
Due to recent catastrophic earthquakes in the Mediterranean region, a large number of
residential buildings, schools, hospitals and other public, administrative and industrial
buildings, as well as other facilities of local and regional infrastructure have been severely
damaged. The largest number of damaged buildings is in the state that their use is not
permissible before adequate repair and repair or strengthening of the basic structural (load
carrying) system, nonstructural elements and installations is performed. In order to assure
appropriate safety and normal functioning of damaged buildings, it will be important to
recognize that these buildings will be exposed in future to a large number of small and,
moderate to strong earthquakes and with significant probability of exposure to catastrophic
earthquakes with large magnitudes similar to those in the past. To meet the requirements for
economic development and aseismic design, systematic scientific and applied research
should be carried out for the purpose of seismic risk evaluation, definition of economically
justified and technically consistent design criteria, and improvement of structural systems,
capable to withstand the expected earthquake effects.
6.1 REPAIR AND STRENGTHENING BASIC CONCEPTS
Buildings decay due to weather, load effects and foundation settlement etc. The building, if
it has to resist an earthquake shock it should be safe under normal load and resist the lateral
load without collapse. The types of intervention necessary to enhance the performance of
the building can be broadly grouped under the following three categories - Repair,
Restoration and Strengthening.
Strengthening: reconstruction or renewal of any part of an existing building to provide better
structural capacity than that of the original building.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
72 Master thesis
Repair: reconstruction or renewal of any part of a damaged or deteriorated building to
provide the same level of strength and/or ductility which the building had prior to the
damage.
Remodeling: reconstruction or renewal of any part of an existing building due to change of
usage or occupancy.
Fig 6.1 Restoration scheme
Retrofit: concepts including strengthening, remodeling and repair.
Rehabilitation: reconstruction or renewal of a damaged building to provide the same level of
function which the building had prior to the damage.
Restoration: concepts including both retrofit and rehabilitation.
The purpose of repairs is to rectify the observed defects and bring the building to
reasonable architectural shape so that all services start functioning. This enables the use of
building for its intended purpose.
Repairs do not improve structural strength or stability. In fact a repaired building may be
deceptive. It may hide the structural defects. Outwardly it may appear good. It may suffer
from structural weakness.
Such weakness may cause collapse during future earthquakes.
Repairs include following interventions
i. Patching cracks and plastering.
ii. Fixing doors, windows, broken glass panes.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
73 Master thesis
iii. Setting right electrical installation, wiring etc.
iv. Fixing services such as gas lime, plumbing services including water
pipes, sewerage line etc.
v. Rebuilding non-structural walls, partition walls, plastering etc.
vi. Re-fixing roof tiles
vii. Repair to flooring and correcting slope for drainage etc
viii. Providing decorative finishes, white washing.
ix. Painting wood work.
x. Attending to root leakage during rain etc.
In fact, repairs will hide the existing structural defects and hence do not guarantee for good
performance when the structure is shaken by an earthquake.
The main purpose is to structurally treat the building with an aim to restore its original
strength. This intervention is undertaken for a damaged building if one is sure that the
original strength provides an adequate level of safety for future earthquake disasters.
The action will involve cutting portions of wall and rebuilding them, inserting support,
underpinning foundation, strengthening a weak component etc.
Some of the common restoration techniques are:
a) Removal of a partition or defective wall and rebuilding it with richer
mortar
b) Crack sealing using epoxy to regain the strength of a structural
component.
c) Adding wire mesh on either side of a cracked component, crack
stitching etc. with a view to strengthen it.
Seismic forces are the most serious dynamic forces. Although various approaches to make
the building safe by minimizing natural period, increasing damping etc, are practiced most
common intervention for non-engineered building is strengthening it by various measures.
The seismic resistance of old existing non-engineered buildings is lowered with passage of
time due to material property degradation and structural strength loss. This deterioration
may occur due to climatic, biological or chemical causes. Strengthening is undertaken to
enhance the original strength to the current requirement so that the desired protection of
lives can be guaranteed as per the current codes of practice against possible future
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
74 Master thesis
earthquakes. The level of strengthening should also consider the remaining life of the
structure being strengthened.
Strengthening of a building will involve either component strength enhancement or
structural system modification or both. It is expected to improve the overall strength in the
following ways:
Increasing the lateral load resistance by reinforcing or by introducing new walls and
columns.
Introducing the continuity between the
components of the structure to achieve
ductile performance. This will include
connection of wall with roof, including
bands and ties between walls and
introducing connections between roof
and walls and wall to wall.
Eliminating existing weakness in an existing building by introducing symmetry in
plan, changing location of mass, reducing large openings etc.
Avoiding brittle modes of failure. This will include improving anchorage and
providing bracings in walls.
The extent of modification has to be determined based on the principle of introducing
sufficient anchorage of all elements, providing bracing to vertical load carrying members in
order to avoid premature mode of failure and to ensure continuity of all structural
components in a building.
About repair materials, cement and steel are common materials that are used for repair
work. Various types of cement with properties such as shrinkage compensating, low heat
and sulphate resistance are preferred for specific repair applications. Steel in the form of
bolts, threaded rods, angles, channels and high strength pre-stressed steel can also be used.
Fiber reinforced polymers also have been widely used as one of the innovative method for
repair and strengthening of existing buildings.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
75 Master thesis
6.2 INTERVENTION TECHNIQUES
Among all the techniques there is an essential division between intervention methods:
Local Techniques
FEMA 273 and NZDC explain that the component is allowed to resist large deformation levels without failure by improving the deformation capacity or ductility of the component, without necessarily increasing the strength. For example, placement of a jacket around a reinforced concrete column to improve its confinement increases its ability to deform without spalling or degrading reinforcement splices. As per FEMA 273, the cross section of selected structural components can be reduced to increase their flexibility and response displacement capacity. According to Eurocode 8, local or overall modification of damaged or undamaged elements (repair or strengthening) can be done, considering their stiffness, strength and/or ductility. It also suggests full replacement of inadequate or heavily damaged elements. Structural rehabilitation, as defined in UNIDO manual, may also consist of a modification of the existing structural members so that their individual strength and/or ductility are improved. As a result, the respective characteristics of the structure are influenced (e.g., jacketing of the columns), even though the overall structural scheme is unmodified.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
76 Master thesis
Global Techniques
Large lateral deformations induced in the structure due to ground shaking, impose high ductility demand on the components of the structure. Also flexible structures with components having inadequate ductility behave poorly. It is essential that such structures be stiffened at a global level. FEMA 273 and NZDC propose the addition of new braced frames or shear walls within an existing structure for increasing the stiffness. While some existing structures have inadequate strength, which result into inelastic behavior at very, low levels of earthquake forces and cause large inelastic deformation demands throughout the structure. By strengthening the structure, the threshold of lateral force at which the damage initiates, increased. Moment resisting frames can be provided as they are more flexible and add strength to the structure without significantly increasing its stiffness, as per these two documents. Eurocode 8 suggests addition of new structural elements like bracings or infill walls; steel, timber or reinforced concrete belts in masonry construction; etc. or addition of a new structural system to take the seismic action. As per UNIDO manual, strengthening of whole structure can be undertaken to improve its lateral force resistance, stiffness and ductility. This can be achieved through the addition of new structural members to increase the respective characteristics of the structure, like bracing in a frame or skeleton structure or new shear walls in a shear wall structure in order to reduce the eccentricity of the masses. A new lateral force resistant structure can be introduced to act integrally with the existing system to resist seismic forces (e.g., stiff shear walls introduced in a flexible frame or skeleton structure). Such an intervention produces significant changes of the stress distribution in the structure as well as in the structural layout. FEMA 273 and Eurocode 8 also suggest mass reduction of the structural system, wherever possible.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
77 Master Thesis
6.2.1 BONDING MATERIALS
1. Epoxy resins
These are used for the following:
a. to bond plastic concrete to hardened concrete
b. to bond rigid materials to one another
c. for patch work
d. For painting over concrete to give color, resistance to chemicals, water and to give
abrasion resistance.
They are excellent binding agents. The low viscosity resins can be injected into small cracks.
The higher viscosity material is used as binding agent and for filling larger holes and cracks.
2. Epoxy mortar
The Epoxy mortar is made using epoxy resins and suitable sized aggregate (sand). They
have high compressive strength, high tensile strength and low modulus of elasticity. In
cement mortar or concrete, the inclusion of epoxy can be considered as an incorporation of
a second binder into the mix. The polymer mortars are two phase systems which forms co-
matrix with cement. In cementations water phase, fine polymer particles of size 0.1 to 0.2
microns are dispersed. In cement polymer system, the polymer particles join and chain link
reinforcing and there by increasing tensile and flexural strength. They achieve greater
plasticity and tend to reduce the shrinkage stress. Hence they vastly improve the property of
plain cement mortar.
3. Gypsum cement mortar
Based on hydraulic binder these readymade formulations are tailor made to give repair
mortar material which is flowable and shrinkage free. Hence they can be applied in
complicated locations and only addition of water is required at site. Cementicious mortars
such as gypsum cement mortar have limited use for structural applications and are intended
for hand/trowel applications
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
78 Master Thesis
4. Quick-setting cement mortar
These are patented mortars generally having two components. They come in pre-packed
condition.
They can be classified as:
Unmodified cementicious
Polyester or Epoxy resin based
Polymer modified and cementicious
Cement/ pozzolanic -modified
Various surface treatments used are shown in Table below, indicating their properties and
the defects they are intended to treat.
Table 6.1 Properties of Quick cement mortar
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
79 Master Thesis
5. Micro-concrete
Based on hydraulic binders these readymade formulations are tailor made to give concrete
which is flow able and free of shrinkage. They are applied in complicated location and in
thin sections such as those met with in jacketing. They can be made either as type A
(normal strength) or type B (high strength) depending on requirement.
Table 6.2 Properties of Micro-concrete
6. Fiber-reinforced concrete
Fiber reinforced concrete has better tensile strength and toughness compared to
conventional concrete.
They have also improved energy absorption capacity. These compositions offer high tensile
strength, durability, ductility and enhanced energy absorption capacity. They are being
increasingly used for structural strengthening.
7. Mechanical anchors
Mechanical type of anchors employs wedging action to provide anchorage. These can
provide resistance against shear and tension. Some of these anchorages are specialized
patented products which can be designed for a required tension or shear force.
8. Metal plates, steel and aluminum etc.
Application of bonded steel plate can increase the flexural and load carrying capacity,
improve stiffness thus reducing deflection and cracking and enhance shear capacity. The
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
80 Master Thesis
process of strengthening is usually bonding additional reinforcement to the external faces of
the structure. The steel plate is attached to the structure either by anchor bolt or by chemical
or epoxy bonding. Bolts are often used in conjunction with adhesive to provide mechanical
anchor for the plate at the ends to prevent premature de-bonding due to peeling. The bolting
also help support the plates whilst the adhesive cures. Sometimes instead of steel plates for
strengthening brickwork aluminum plates have also been used.
9. Ferro cement
Ferro cement is a type of reinforced cement mortar commonly made of hydraulic cement
mortar reinforced with closely spaced layers of small diameter wire mesh”. The mesh may
be made of metallic or other suitable materials. Fineness of mortar matrix and its
composition should be compatible with opening and tightness of the reinforcing system, it is
meant to encapsulate. The matrix may contain discontinuous fibers.
Well designed Ferro cement wraps can be an economical alternative to CFRP wraps
especially for non engineered construction.
Some of the common forms of repair and strengthening techniques for local and global
intervention are described below:
6.2.2 STRENGTHENING TECHNIQUES
1. Shotcrete
Shotcrete is a process in which compressed air forces mortar through a nozzle to be sprayed
on a surface of a building at a high velocity. The materials used in shotcrete are generally
same as those used for conventional mortar. The reinforcement provided is generally
welded wire fabric and deformed bars tacked onto the surface.
Shotcrete is applied using either wet or dry process. The wet mix consists of cement and
aggregate premixed with water and the pump pushes the mixture through the hose and
nozzle. Compressed air is introduced at the nozzle to increase the velocity of application.In
the dry mix process, compressed air propels premixed mortar and damp aggregate and at
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
81 Master Thesis
the nozzle end water is added through a separate hose. The dry mix and water through the
second hose are projected on to a prepared surface.
Generally Shotcrete gun nozzle is held at 0.6 to 1.8 meter from the surface. In most cases
Shotcrete can be applied in a single application for the required thickness. It is versatile as it
can also be applied on curved or irregular surface. Its strength after application and its good
physical characteristics make it ideal for strengthening weak members.
This method is used for local and global intervention as well, it local where we speak about
a certain element and global when mostly of elements are involved in this intervention. Here
in are present some examples where this technique is chosen as the most appropriate:
Fig. 6.2 Shear-Drift (Storey 1) Infilled and Bare Frame
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
82 Master Thesis
Fig.6.3 Application of Shotcrete
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
83 Master Thesis
Column Shear-out (Overturning negative effects)
Fig 6.4 Column Shear-out
Fig 6.5 Shotcrete negative effect
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
84 Master Thesis
2. Injection of small cracks
Even fine cracks in load bearing members, which are un-reinforced, like masonry or plain
concrete, will reduce their resistance against loads. Hence they should be marked carefully
and critical ones repaired.
Cracks in width smaller than 0.75mm can be effectively repaired by pressure injection of
epoxy.
The surfaces are thoroughly cleaned of loose materials and plastic injection ports are placed
along the length of the cracks on both sides and secured in place with epoxy seal (Fig-……).
The ports can be installed at intervals approximately equal to thickness of the element being
repaired.
Fig 6.6 Grout of epoxy injection Fig 6.7 Crack injection system
After hardening of the seal low viscosity epoxy resin is injected into one port at a time
sequentially beginning at the port of the
lowest level and moving upwards.
The resin is pushed through the packer till
it is seen flowing from the other end or a
port at a level higher than where it is
injected Fig 6.8. The injection port is
closed at this juncture and the packer is
moved to the next higher port.
Fig. 6.8 Grout or Epoxy injection in an existing weak wall
This operation is repeated sequentially until we reach the top most port and the whole crack
is filled with grout.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
85 Master Thesis
Longer cracks will permit longer packer spacing depending on thickness of the member.
This technique can be used for all types member (beams, columns, walls or slabs). They can
be utilized to repair small cracks in individual masonry blocks or fill large continuous
cracks. Vacuum injection has a typical fill level of 95% and can fill fine cracks even as small
as 0.025mm.
3. Large cracks and crushed material
For cracks wider than 6mm and where brickwork or concrete is crushed, the following
procedure is suitable:
a) Loose material in the crack is removed and
any of the repair mortar mentioned is
filled.
b) If necessary the crack is dressed to have a
“V groove” at both faces.
c) A place where cracks are wide fillers like
flat stone chips can be used Fig. 6.9.
d) Additional shear and/or flexural
reinforcement are provided in the location
of the repairs based on structural necessity. Fig. 6.9.
e) The added steel has to be protected properly by sufficient polymer mortar to prevent
it from corrosion Fig. 6.10.
f) In case of walls or roof slabs additional
mesh reinforcement is included either on
one or both sides. This mesh
reinforcement is generally nailed, tacked
and tied by binding wire (Fig. 6.10).
g) To prevent widening of the cracks they
can be stitched. The stitching consists of
drilling small holes of size 6 to 10mm on
both side of the crack cleaning the holes
Fig. 6.10 and anchoring legs of stitching dogs with short legs.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
86 Master Thesis
Fig. 6.11 Repair by stitching the cracks
The stitching dogs are variable length and orientation as shown. The spacing of
reinforcement should be reduced at the ends of the crack. Stitching will not close the
crack but prevent further propagation and widening.
4. Reinforced Concrete Jacketing
Jacketing should be applied in case of heavy damaged columns or in the case of column
strength. Because of the increased capacity of the columns, this is really a strengthening
procedure although it can also provide repair. Jacketing can be preformed by means of
adding reinforced concrete, a steel profile skeleton or steel encasement.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
87 Master Thesis
Reinforced concrete jacketing according to the available space conditions around the
column can be preformed by adding jacketing to one, two,
three or four sides of concrete column sections. It is strongly
recommended that columns be jacketed on all four sides for
best performance in future. In case one, two or three sides
jacketing is all that is possible, the concrete cover in the
jacketed parts of existing must be chipped away so new ties
can be welded to existing ties. In case of a four-sides jacketing,
only roughening of the surface of the existing column may be
required unless greater load transfer is desired.
Jacketing only in the story space without reinforcement
penetrating through the floor structure can improve the local
axial and shear strength of the column. However, the flexural
column strength is not improved and the column-to-beam joint is
not strengthened. Thus, the total frame structure may show poor
behavior in future earthquakes. Jacketing only within the story is
e local strengthening which does not improve seismic response
unless significant shear walls are also added. Adequate column
flexural strength can be achieved by passing the new longitudinal
reinforcement through holes drilled in the slab and
placing new concrete in the beam-column joint region.
Special attention should be paid to the good confinement
of the longitudinal reinforcement in the region of the
floor beams.
In case of a one side jacket, adequate connection
between existing and new concrete must be achieved by
good detailing and closely spaced, well anchored,
additional transversal reinforcement. The following solution can be applied:
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
88 Master Thesis
Anchorage by ties to the existing longitudinal reinforcement. Welding is not
necessary, but chipping free space for passing the hooks of the additional ties is
necessary.
Welding of additional ties to the existing column. The concrete cover in tie
region must be removed and every new tie must be welded to the existing one.
Connection by bent bars welded to the longitudinal reinforcement. The concrete
must be chipped only in the welding region up to the longitudinal reinforcement.
By this way, concrete keys capable of transmitting shear forces are formed. The
bent bars allow direct transfer of forces between longitudinal reinforcement. In
lieu of welded bent bars, a vertical steel plate can be welded between existing and
new longitudinal bars.
In the usual case of four-sides jacketing, several solution are possible as presented in Figure,
as follows:
Jacketing can be achieved with welded wire fabric and new concrete cover. This
solution improves the local column
ductility, but the flexural column strength
is not significantly increased because of
the impossibility of eh wire fabric passing
through the floor structure. The fabric
should be completely encase the column
with a long overlap or consist of two
sections with an adequate overlap on two
opposite sides.
Jacketing with connecting bent bars. The
added longitudinal reinforcement is connected to the existing reinforcement by
welding of bent bars. This jacket type is applied for large column cross sections
where the middle reinforcement cannot be confined by new ties.
Jacketing with ties. Concentration of newly-added longitudinal reinforcement at
the corners of the cross section allows an adequate confinement of all
longitudinal bars. The jacket should be of sufficient thickness with closely spaced
ties to provide confinement. With the new longitudinal reinforcement passing
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
89 Master Thesis
through holes drilled in the slab, this procedure a continuous connection of the
jackets to the upper story and lower story columns.
Reinforced concrete jacketing should also conform the following provisions:
The strength of the new materials must be equal or grater than those of the existing
column. Concrete should be at least 5 MPa greater than the strength of the existing
concrete.
The thickness of the jacket should be at least 4
cm for shotcrete application or 10 cm for cast in
situ concrete.
The area of longitudinal reinforcement should
not be less than 0.01 and no more than 0.06
times the gross area of the jacket section. The
reinforcement should not be less than four bars
for four-sided jacketing and bar diameter should
be at least 14 mm.
Ties should be arranged so that every corner and
alternate longitudinal should have lateral support provided by corners of the ties with
an included angle of no more than 135 degrees. No intermediate bar should be
farther than 10 cm from corners of the ties. In some cases, it will be necessary to drill
into the core of existing column and epoxy hooked ties the hole or drill completely
through the existing column core to install new ties.
The diameter of ties (except welded wire fabric) should be minimum 8 mm, but not
less than 1/3 of the longitudinal bar
diameter.
Vertical spacing of ties shall not exceed 20
cm, while the spacing close to the joints
within a length of 1/4 of the clear height
should not exceed 10 cm. In addition, it is
advisable that the spacing of ties should not
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
90 Master Thesis
exceed the thickness of the jacket.
Jackets can be installed either as conventional or special cast-in-situ concrete or by shotcrete
(gunite). For both methods, the existing concrete cleaned of all loose material, dust and
rease. The surface should be thoroughly moistened before placing the concrete or shotcrete.
5. Steel Profile Jacketing
Steel profile skeleton jacketing consists of four longitudinal angle profiles placed one at each
corner of the existing reinforced concrete column and connected together in e skeleton with
transverse steel straps as shown in fig. They are welded to the angle profiles and can be
either round bars (minimum diameter of 12
mm) or steel straps (minimum size of 25/4
mm). The angle profile size should be no less
than L 50/50/5. Gaps and voids between
the angle profiles and the surface of the
existing column must be filled with non-
shrinking cement grout or resin grout. A
covering with concrete or shotcrete
reinforcement with welded fabrics is efficient
for corrosion or fire protection. Tight bearing between the angle profiles and the floor
structures, important for transmitting the forces, is achieved by an angle profile collar
formed around the column perimeter directly in contact with the surfaces of the floor
structures. In general, an improvement of the ductile behavior and an increase of the axial
load capacity of the strengthened column is achieved. However, the stiffness remains
relatively unchanged.
This technique is implemented by the following steps as shown in Fig.6.11:
1. Removing the concrete cover.
2. Cleaning the reinforcement steel bars using a wire brush or a sand compressor.
3. Coating the steel bars with an epoxy material that would prevent corrosion.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
91 Master Thesis
4. Installing the steel
jacket with the required
size and thickness,
according to the design,
and making openings to
pour through them the
epoxy material that
would guarantee the
needed bond between
the concrete column
and the steel jacket.
5. Filling the space
between the concrete
column and the steel
jacket with an
appropriate epoxy
material.
Fig. 6.11. Increasing the cross-sectional area of column by steel jacketing
In some cases, where the column is needed to carry bending moment and transfer it
successfully through the floors, one should install a steel collar at the neck of the column by
means of bolts or a suitable bonding material.
Fig.6.12. Shows a column which was strengthened with steel angles.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
92 Master Thesis
6. Fiber or reinforced polymer (FRP and CFRP materials)
Carbon fiber reinforced plastic (CFRP) material consists of strong and stiff carbon fibers
(approx. 7 micrometer diameter) embedded in an epoxy resin matrix. It has high strength to
weight ratios and corrosion resistance thus helps reduce maintenance cost. They are
manufactured in long lengths by pultrusion process, with unidirectional fibers ranging from
50-150mm wide and 1.2-1.4 mm thick having fiber volume content greater than 65%. They
are black in color and are about 10 times as strong as mild steel but one fifth of its weight.
They have low density, high fatigue strength, and high wear resistance, vibration absorption
and dimensional stability, high thermal and chemical stability. They are suitable to be used
as a protective and strengthening “jacket”. CFRP wraps can be used to strengthen masonry
piers and walls especially for seismic loading.
The term strengthening means the structural enhancement of existing weak members in
such a way as to restore or increase their ultimate strength in bending, shear or direct
tension and compression. Strengthening options may vary for specific case, depending on
the problems to be addressed, and individual attention is needed to assess the suitability of
the proposed intervention.
7. Base Isolation
An overall advantage of base isolation is
reduction in demands on the elements of
the structure. This technique is most
effective for relatively stiff low rise
buildings with large mass compared to
light, flexible structures. However, base
isolation is technically complex and
costly to implement and can be
considered for historic structures or
where a performance level greater than life safety is required. FEMA 273, NZDC and
Eurocode 8 propose base isolation as an option for seismic rehabilitation. However, they
generally refer to specialist literature for details of analysis and design.
MEASURES AIMED TO REDUCE THE IMPACT OF ENVIRONMENTAL HAZARDS
93 Master Thesis
8. Supplemental Energy Dissipation
Energy dissipation helps in the
overall reduction in displacements
of the structure. This technique is
most effective in structures that are
relatively flexible and have some
inelastic deformation capacity and
can be less costly compared to base
isolation. As per FEMA 273 and
NZDC devices such as hydraulic
cylinders, yielding plates, or
friction pads, can be used for
energy dissipation. According to
Eurocode 8, addition of local
friction, global damping devices or
active control at appropriate
locations of the building can be
done.
Design of strengthening scheme
using energy dissipation devices are
very sensitive to characteristics of
devices used and therefore, require
a higher degree of sophistication in
the analysis and design.
Fig.6.13. Energy dissipation devices. Different for each type of structure
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
92 Master Thesis
7
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
The Capacity Spectrum Method (CSM) compares the capacity of a structure to resist
lateral forces to the demands of earthquake response spectra in a graphical
presentation that allows a visual evaluation of how the structure will perform when
subjected to earthquake ground motion. The method is easily understandable and
generally consistent with other methods that take into account the nonlinear behavior
of structures subjected to strong motion earthquake ground movements.
The capacity is represented by a lateral load force-displacement diagram that takes
into account the sequential yielding of structural elements as the structure is laterally
displaced beyond its linear-elastic limits. This procedure is sometimes referred to as a
pushover curve. The lateral load force diagram is proportioned to represent the
fundamental mode of the building. The forced displacement diagram is calculated in
terms of lateral roof displacement (û ) and total R lateral force at the base of the
building (V). In order to be directly comparable to demand response spectra, û and V
are converted to a spectral set of coordinates (Figure 2) by using the R dynamic
characteristics of the fundamental mode to represent the structure as a single-degree-
of freedom structure. The procedure also allows the inclusion of higher mode effects.
The demands of the earthquake are represented by response spectra. Linear elastic
response spectra, assumed at 5% damping, are modified to represent the effects of
inelastic response by substituting higher damped response spectra to account for
hysteretic nonlinear response of the structure.
Response spectra have traditionally been plotted with S (acceleration in units of
gravity) vs T a (period in seconds) coordinates or tripartite log coordinates. In order to
more visually illustrate the relationship between accelerations and displacements, the
S vs T coordinate system for the a response spectra is converted to a set of coordinates
defined by S and S . When the spectral a d values are plotted in this acceleration-
displacement response spectrum format (ADRS), the period can be represented by
lines radiating from the origin (Mahaney et al. 1993).
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
93 Master Thesis
Structural Input
Geographical Identification
Soil Input Seismic Input
Soil Conditions PGA
Response Spectra (1)
Demand Spectra
Strength Reduction Factor Rμ
StructuralProperties
Code Reqirements
Capacity Curve
BilinearCapacity Model
CapacitySpectrum
Damage State
Fragility Curve
Pushover Analyses Sap2000/Idarc2D
Regression Analysis
Performance Points-PP
Damage Assessment
DAMAGE ASSESSMENT OF BUILDINGS
An existing 5 story building is chosen as example in this thesis, which is a building
settle in downtown Tirana, (constructed around 1996-1997).
This analysis passes through different steps presented in fig.7.1;
The real capacity of the existing building is evaluated using pushover analysis then is
converted in ADRS format which compared with demand spectra for Albanian Code
gives the performance point. A set of fragility curves developed inhere present a
different probability of exceedance for a certain damage state and compared with
defined performance point presents the appropriate state of the building, in which is
predicted the probability of exceedance for different levels of damage.
The same procedure is repeated for the repaired model. After all, the comparison of
the calculations and conclusions for this both models is giving the best scenario for the
repaired model.
Fig. 7.1 Steps for Damage Assessment
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
94 Master Thesis
7.1 TECHNICAL CHARACTERISTICS/ DESCRIPTION OF THE BUILDING
Means: Description of the structure for vertical loads and other structural patterns (facade
walls, partition walls, material used, etc.), material characteristics, loading conditions.
Typical characteristics of building are presented in table below:
Structural Description
Nr Description of Elements
1. Frame Structure No. of typical frame 4
2. Isolated Footing No. of footing 18
3. Floor structure Prefabricated ribbed slab X-X direction
4. Defined Concrete Using sclerometer Mostly B-20
5.
Reinforcement Lab testing Fy= 240 Mpa
6. Stories No. of story 5
7. Height of building 5 story x 3.06m/each 15.3m
8. Area of floor First to second floor Area 315 m2
Third to fifth floor Area 140 m2
9. Partition walls Ceramic hollow bricks T= 12cm,25 cm
10. Reinforced Concrete Columns
Dimension 30x30cm Concrete B-20
11.
Reinforced Concrete Beams
Dimension 50x30cm Concrete B-20
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
95 Master Thesis
11 x 17 / 28 12345678910
4 x 17 / 28 123
Dho
me
gjum
i 2
Dho
me
gjum
i 1
Dho
me
gjum
i 3
Dh.
dite
+ a
neks
Wc
Wc
Kor
ridor
Fig 7.2 Plan of floor and photo of façade for
Existing building
The diagrams of concrete and steel used for analyses as input in IDARC2D
Fig. 7.3. Concrete Stress-Strain curve Fig. 7.4. Reinforcement Stress-Strain curve
Concrete Fc (Mpa) Ec (Mpa) EPso (%) Ft (Mpa) EPsu (%) Model
1. 20 21500 0.2 2.4 0.35 Exist
2. 25 24800 0.2 3 0.35 Repair
Reinf. Steel Fs (Mpa) Fsu (Mpa) Es (Mpa) ESH (Mpa) EPSH (%) Model
1. 240 350 199949 3.33 3 Exist
2. 400 560 199949 3.33 3 Repair
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
96 Master Thesis
Fig. 7.5. 3D Model of existing building
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
97 Master Thesis
7.1.1 ANALYSIS OF VERTICAL LOADS
• Dead Load for 11.=mγ (safety coefficient for the material)
Slab:
o Tile 1cm → 23 022.01.1201.0 mton
mtonm =××
o Mortar 2cm → 23 0396.01.18.102.0 mton
mtonm =××
o Sand 10cm → 23 176.01.16.11.0 mton
mtonm =××
o Plaster 2cm → 23 0396.01.18.102.0 mton
mtonm =××
o Slab 14 cm → 23 385.01.15.214.0 mton
mtonm =××
o Partition walls → 225.0 mton=
Total sum of loads for m2 22 0.990.0 mkN
mton ==
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
98 Master Thesis
• Live Load for 21.=fγ (safety coefficient)
Assuming that use of the building will be for residential propose we decide.
For interior areas_________ 222 4.224.02.12.0 mkN
mton
mtonP ==×=
For balconies________ 222 33.02.125.0 mkN
mton
mtonP ==×=
These loads are applied as punctual force in the nodes of the structure. The value of
these forces is determined from the combination of the dead loads and live loads,
according to code requirement, which are used as nodal weights in IDARC2D. The
values of nodal weight are presented in the following table for both orthogonal
directions, being the same for all stories.
From the geometry three typical frames are chosen to be analyzed, those are the most
loaded elements of structure.
Tributary Area (m2)
Frame 2-2 Area
around axis A-A
Area around axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8
Story level 2 16.4 16.4 13.6 16 16 8 Story level 3 16.4 16.4 11 0 0 0 Story level 4 16.4 16.4 11 0 0 0 Story level 5 16.4 16.4 11 0 0 0
Nodal weight (kN) Story level 1 231.035 231.035 191.215 225.4 225.4 112.7
Story level 2 231.035 231.035 191.215 225.4 225.4 112.7 Story level 3 231.035 231.035 157.025 0 0 0 Story level 4 231.035 231.035 157.025 0 0 0
Story level 5 231.035 231.035 157.025 0 0 0
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
99 Master Thesis
Tributary Area (m2)
Frame 3-3 Area
around axis A-A
Area around axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8 Story level 2 16.4 16.4 13.6 16 16 8 Story level 3 16.4 16.4 11 0 0 0 Story level 4 16.4 16.4 11 0 0 0 Story level 5 16.4 16.4 11 0 0 0
Nodal weight (kN) Story level 1 231.035 231.035 191.215 225.4 225.4 112.7 Story level 2 231.035 231.035 191.215 225.4 225.4 112.7 Story level 3 231.035 231.035 157.025 0 0 0
Story level 4 231.035 231.035 157.025 0 0 0
Story level 5 231.035 231.035 157.025 0 0 0
Tributary Area (m2)
Frame 4-4 Area
around axis A-A
Area around axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8 Story level 2 16.4 16.4 13.6 16 16 8 Story level 3 14.4 14.4 3 0 0 0 Story level 4 14.4 14.4 3 0 0 0 Story level 5 14.4 14.4 3 0 0 0
Nodal weight (kN) Story level 1 231.035 231.035 191.215 225.4 225.4 112.7 Story level 2 231.035 231.035 191.215 225.4 225.4 112.7 Story level 3 204.735 204.735 51.825 0 0 0 Story level 4 204.735 204.735 51.825 0 0 0
Story level 5 204.735 204.735 51.825 0 0 0
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
100 Master Thesis
7.1.2 BUILDING`S GEOMETRY
Graphical presentation of characteristics building’s floor plans and cross-sections.
Floor plans are not symmetrically in direction X-X as well as in direction Y-Y; they also
differ from one level to one other. Below are presented all the plans of the building;
Fig.7.6 Plan of foundation footing N-3.20
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
101 Master Thesis
Fig.7.7 Plan of columns N+0.00
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
102 Master Thesis
Fig.7.8 Plan of structure N+3.06,+6.12
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
103 Master Thesis
Fig.7.9 Plan of structure N+9.18, +12.24, +15.3
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
104 Master Thesis
The frames are not regular in elevation for both orthogonal direction (X-X &Y-Y). Below
are presented the frames in X-X direction (most loaded frames), with the corresponding
columns and beams types. There is also given the area of bottom and top longitudinal
reinforcement for all the beams.
Column and Beam Type Column and Beam Number
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
105 Master Thesis
The reinforcement of the typical columns and beams are presented in whole frame, as given
below:
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
106 Master Thesis
7.2 BUILDING CAPACITY CURVE AND ADRS MODELS
7.2.1 PUSHOVER AND CAPACITY MODEL IN BOTH ORTHOGONAL BUILDING DIRECTIONS.
The static pushover procedure has been presented and developed over the past twenty year
(Saiidi and Sozen, 1981; Fajfar and Gaspersic, 1996; Bracci et al., 1997; Krawinkler and
Seneviratna, 1998; Kiattivisanchai, 2001; Imarb, 2002). The method is also described and
recommended as a tool for design and assessment purposes by the National Earthquake
Hazard Reduction Program 'NEHRP' guidelines for seismic rehabilitation of existing
buildings and by the Applied Technology Council (1996) guidelines for seismic evaluation
and retrofit of concrete buildings. Moreover, the technique is accepted by the Structural
Engineers Association of California 'SEAOC Vision 2000' (1995) among other analysis
procedures with various levels of complexity.
The recent advent of performance based design has brought the nonlinear static pushover
analysis procedure to the forefront. Pushover analysis is a static, nonlinear procedure in
which the magnitude of the structural loading is incrementally increased in accordance with
a certain predefined pattern. With the increase in the magnitude of the loading, weak links
and failure modes of the structure are found. The loading is monotonic with the effects of
the cyclic behavior and load reversals being estimated by using a modified monotonic force-
deformation criteria and with damping approximations.
Static pushover analysis is an attempt by the structural engineering profession to evaluate
the real strength of the structure and it promises to be a useful and effective tool for
performance based design. The nonlinear static pushover analysis is a sample option for
estimating the strength capacity of building in the post-elastic range. The technique can also
be used to highlight potential weak areas in the structure. This procedure involves applying
a predefined lateral load pattern that is distributed along the building height. The lateral
forces are then monotonically increased in constant proportion with a displacement control
at the top of the building until a certain level of deformation is reached. The target top
displacement may be the deformation expected in the design earthquake in case of designing
a new structure or the drift corresponding to structural collapse for assessment purposes.
The method allows tracing the sequence of yielding and failure on the member and the
structure level as well as the progress of the overall capacity curve of the structure.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
107 Master Thesis
From the configuration of the building that the 6h floor was difficult to model in IDARC2D,
so we skip it and its weight and mass were distributed appropriate in the other floors. Our
model now has 5 floors. Previous to perform Push-Over analysis we ask to the program to
push the building until the structure in the top story to have the upper limit for displacement
= 1% of building height. Once the program is run, from the out put is possible to get the
capacity curves for each story and overall building in both orthogonal directions.
The Capacity Curve can be approximated using bilinear approach, in order to define the
basic states of the structure, yielding point as well as ultimate point.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
108 Master Thesis
• Graphical presentation of mode shapes and drifts stories.
Analyzing fundamental period of structure before Pushover 1.39s, gives a sense of a flexible
structure, because is bigger than 0.5-0.6s, which will an expected period.
BEFORE PUSHOVER exist
Fundamental Period = 1.39 s
STORY FIRST EIGEN
VECTOR (normalized) ϕ
5 1.000 4 0.877 3 0.671 2 0.388 1 0.197 0 0.000
AFTER PUSHOVER exist Fundamental Period = 8.88 s
STORY FIRST EIGEN
VECTOR (normalized) ϕ
5 1.000 4 0.997 3 0.953 2 0.491 1 0.268 0 0.000
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
109 Master Thesis
& 3
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
110 Master Thesis
7.2.2 BUILDING BILINEAR CAPACITY MODEL IN ADRS FORMAT.
The application of the capacity spectrum technique requires that both the demand response
spectra and structural capacity (or pushover) curve be plotted in the spectral acceleration, Sa
and spectral displacement, Sd domain, or the so-called Acceleration-Displacement
Response Spectra (ADRS).
To construct the capacity spectrum, capacity curve of the multi-storey building is converted
into the capacity curve of the equivalent single degree of freedom (SDOF) systems based on
the capacity curve, which in terms of base shear and lateral roof displacement, is obtained
from pushover analysis and also based on its dynamic characteristic. Any point (V and Δ roof) on the capacity curve can be converted to the corresponding point Sa and Sd by using
the equations the following equation.
MgVSa
α= &
R
R
PFSd
ϕΔ
=
Where V: Shear Base of the structure
M: Total Mass of the structure
∆R: Roof Displacement
α: Effective Mass Ratio
PFøR: Roof participation Factor
The dynamic characteristics as Period (T) and Mode shapes (�) together with the lumped
floor mass (mx) are needed for determination of the effective mass ration and the roof
participation factor, according the equation below.
∑∑
∑⋅⋅
∗=
)()(
2
2
ϕϕ
αxx
xx
mmm
& RRm
mPF ϕ
ϕ
ϕϕ ⎟
⎟
⎠
⎞
⎜⎜
⎝
⎛=
∑∑
2
The transformation will be done for the bilinear capacity curve, which helps us to identify
the design capacity (DC), yielding capacity (YC) and ultimate capacity (UC) of the
structure, for both orthogonal directions. In order to perform the ADRS format of the
bilinear capacity curve for our structure, its lumped floor masses and its dynamic
characteristics have been taken from IDARC2D, which are presented in the table below.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
111 Master Thesis
IDARC(x-x): OUTPUT X-X.OUT
STORY FIRST EIGEN
VECTOR (normalized) ϕ
PERIOD (s)
5 1.000 0.19566 4 0.877 0.23536 3 0.671 0.31867 2 0.388 0.58007 1 0.197 1.39327
STORY BUILDING HEIGHT (m)
FLOOR MASSES (kN*s2/m)
5
15.30
176 4 176 3 176 2 352 1 352
DYNAMIC CHARACTERISTICS
Σmi (kN*s2/m) (Σmi ∗ϕ1)^2 Σmi ∗ϕ1^2
EFFECTIVE MASS
RATIO (α)
ROOF PARTICIPATION FACTOR (PFϕR)
1232 429123.823 457.9794037 0.76029 1.43036
BILINEAR CAPACITY CURVE
BASE SHEAR COEFFICIENT
(%W)
OVERALL DEFORMATION
(%H)
0.00 0.00 0.07 0.30 0.07 1.10
μ max 3.67
BILINEAR in ADRS
Sa (g) Sd (cm)
0.00 0.00 0.09 3.21 0.10 11.78
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
112 Master Thesis
7.3 SEISMIC RESPONSE OF EXISTING BUILDING
7.3.1 CAPACITY SPECTRUM METHOD (CSM)
The capacity spectrum method, by means of a graphical procedure, compares the
capacity of the structure with the demands of the earthquake ground motion on the
structure. Therefore, both must be defined with the same set of coordinates, which is
defined as Acceleration-Displacement Response Spectrum format (ADRS), means of Sa
vs. Sd in which the period can be represented by lines radiating from the origin. The
intersection between capacity and demand spectrum having the same ductility factor, μ,
is the performance point that represents the maximum structural force and displacement
expected for the demand earthquake ground motion.
Fig.7.10 (a) Capacity Spectrum (b) Demand Spectrum
(c) Demand Spectrum superimposed Capacity Spectrum in ADRS format
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
113 Master Thesis
(a) The Capacity of the structure is represented by a nonlinear force-displacement
curve, which is referred as a pushover curve and explained in details in point 3
above for our structure using bilinear model as well as Spectrum Capacity.
(b) The demands of the earthquake are represented by Response Spectra. Earthquake
loading on structures is often characterized by its effect on simple structural
systems, namely Single Degree Of Freedom systems through response spectra, so
in other words, response spectra are the maximum response quantities
(Displacement, Velocity and Acceleration) of a SDOF plotted as a function of its
period (T) for a given earthquake record. Therefore, each earthquake is
characterized by it own set of spectra.
Depending on the material behavior involved in the structural response, spectra are
generally divided in two main categories: elastic and inelastic.
Linear Elastic Response Spectra is assumed by code with 5% damping which
corresponds to ductility μ = 1, while Inelastic Response Spectra represent the modified
linear elastic response spectra in order to represent the effects of inelastic response by
substituting higher damping response spectra to account for hysteretic nonlinear
response of the structure.
The general trend in developing inelastic response spectra is from their elastic
counterparts, by means of a reduction factor R, called the force reduction factor (or q in
Eurocode 8). This factor is designed to drag the system into inelastic response, thus
causing it to dissipate energy through inelastic deformations. The dissipated energy is
usually expressed is terms of the ductility μ of the system, and is a function of its period
of vibration T.
There are different approaches to define R – μ – T relationship.
From Displacement – based approach damping ratio is defined in function of ductility,
according to equation )05.095.01(105.0 μμπ
ξ −−+=
From FEMA 356 based in the graph R vs. T
For long period system μ=R
For short period system 1=R
For intermediate period system 12 −= μR
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
114 Master Thesis
From Vidic et al. Model (1994)
For T ≥ TC μ=R (1)
For T ≤ TC 1)1( +⋅−=CT
TR μ (2)
Response spectra is the maximum response quantities of Acceleration as well as
Displacement, is in our interest to calculate Elastic Displacement Response Spectra from their
acceleration counterparts using the definition of the pseudo-acceleration bellow:
)()( 2 tuwtA = ⇒ dMAXMAX ST
uwASa 2
22 4π
=== and then 224
)()( T
TSTS ae
de π=
So, each ordinate of spectral acceleration that is associated with the period T is converted
into corresponding spectral displacement ordinate by multiplying with the factor 22
4πT .
Now Demand Spectrum in ADRS format is to plot spectral acceleration versus spectral
displacement, for elastic and inelastic spectrum.
7.3.2 DEMAND SPECTRA
From the Albanian seismicity map given, we assume earthquake’s magnitude over than 5.5
for Tirana city. (See chapter 2) According to microzonification for the city of Tirana and
with the value of PGA=0.32g, we find return period which is equal to yearsTR 475= , this
value correspond to probability of exceedance 10% for 50 years life exposure of the building.
This value corresponds to Non-collapse requirement according to EC-8.
Calculate the annual probability of exceedance.
( )R
LR P
TT−
−=1ln
TR → Return Period
yearsP
TTR
LR 475
10150
1=
−−=
−−=
).ln()ln( PR → Probability of exceedance=10%
%.. 2100021047511
====RT
P TL → Life exposure
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
115 Master Thesis
So there is 0.21% chance that in any given year TR will occur.
From Site investigation soil is Subsoil class B, according to EC-8 Table 3.1
Ground
Type
Description of stratigraphic profile
Parameters
Vs,30(m/s)
NSPT
(blows/30cm)
Cu (kPa)
B
Deposits of very dense sand, gravel, or very stiff clay, at least several tens of m in thickness, characterized by a gradual increase of mechanical properties with depth
360-800
>50
>250
Tc=0.4s
RTSaTSa e )()( =
In order to represent the family of ductility demand spectra, an elastic spectra is scaled to
inelastic spectra corresponding to ductility μ = 2, 4 and 6, using equation (1) and (2) for
Acceleration spectra as well for Displacement spectra. And then its corresponding
percentage of critical damping can be determined using the formula mention above.
→ μ= 1 05.0105.0195.01105.0 =⎟
⎠
⎞⎜⎝
⎛−−+=
πξ or 5% of critical damping
→ μ= 2 132.0205.0295.01105.0 =⎟
⎠
⎞⎜⎝
⎛−−+=
πξ or 13.2% of critical damping
→ μ= 4 185.0405.0495.01105.0 =⎟
⎠
⎞⎜⎝
⎛−−+=
πξ or 18.5% of critical damping
→ μ = 6 2.0605.0695.01105.0 =⎟⎟
⎠
⎞⎜⎜⎝
⎛−−+=
πξ or 20% of critical damping
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
116 Master Thesis
)()( TSdR
TSd eμ
=
7.3.3 PERFORMANCE POINT ESTIMATION-INTERSECTION OF BILINEAR CAPACITY MODEL
AND NONLINEAR CONSTANT DUCTILITY DEMAND SPECTRA.
The performance point is an intersection between capacity and demand spectrum having the
same ductility factor μ, plotted in ADRS format. This intersection represents the inelastic
response of the structure.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
117 Master Thesis
From graphical presentation we conclude that bilinear capacity spectrum doesn`t extend
through the envelope of the demand curve, that means building will not survive. The
performance point is achieved in point 2, where capacity curve with ductility 1.5 is the same
value to ductility of demand spectra.
The table gives the components of interest according to performance point.
Right from these analyses there is a need of intervention in structure integrity, where the
technique of repairing and strengthening are indispensible.
Below are presented same picture done during the intervention where the poor quality of
construction take attention.
Fig.7.6 Photo of Beam-Column joint
Point 2
PGA %g
Sd (cm)
Ductility μ
Damping %
Base Shear V (kN)
Roof Displacement ∆s (cm)
X-X 0.09 4.88 1.52 10.1 846.3 6.97
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
118 Master Thesis
Fig.7.7 Photo of excavated footing Fig. 7.8 Photo of Distorted Column
7.4 BUILDING SEISMIC RESPONSE IN REPAIRED MODEL
Too many traditional techniques exist (mentioned in chapter 6) for repair and strengthening
of this building, but because of collaboration with factories that are mostly concerned in
concrete technology the owner preferred Reinforced Concrete Jacketing.
So, our design is focus on this method.
During the analyses of existing building five main problems were among the others:
1. The foundation footings were not appropriate designed for actual spectra of loading.
2. Building doesn`t possess enough capacity for surviving next earthquake.
3. Yielding Cracks happen in column and beams.
4. Not good distribution of floor masses.
5. Poor quality of construction.
The analyses for repaired model is passing through the same steps as presented for existing
model.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
119 Master Thesis
7.4.1 BUILDING`S GEOMETRY
To improve all these deficiencies a new model is proposed, as steps presented below:
1. Strengthening and upgrading of footing with addition of grade beam.
2. Repairing and strengthening of columns
3. Integration of new shear walls, for seismic resistance
4. Extending the floor in 3,4, and 5th story, in order to balance story masses
5. Improving the structure by implementing good material.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
120 Master Thesis
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
121 Master Thesis
7.4.2 ANALYSIS OF VERTICAL LOADS
Tributary Area (m2)
Frame 2-2 Area around
axis A-A Area around
axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8
Story level 2 16.4 16.4 13.6 16 16 8
Story level 3 16.4 16.4 13.6 16 0 0
Story level 4 16.4 16.4 13.6 16 0 0
Story level 5 16.4 16.4 13.6 16 0 0
Nodal weight (kN)
Story level 1 231.035 231.035 191.215 225.4 225.4 112.7
Story level 2 231.035 231.035 191.215 225.4 225.4 112.7
Story level 3 231.035 231.035 191.215 225.4 0 0
Story level 4 231.035 231.035 191.215 225.4 0 0
Story level 5 231.035 231.035 191.215 225.4 0 0
Tributary Area (m2)
Frame 3-3 Area around
axis A-A Area around
axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8
Story level 2 16.4 16.4 13.6 16 16 8
Story level 3 16.4 16.4 11 0 0 0
Story level 4 16.4 16.4 11 0 0 0
Story level 5 16.4 16.4 11 0 0 0
Nodal weight (kN)
Story level 1 231.035 231.035 191.215 225.4 225.4 112.7
Story level 2 231.035 231.035 191.215 225.4 225.4 112.7
Story level 3 231.035 231.035 157.025 0 0 0
Story level 4 231.035 231.035 157.025 0 0 0
Story level 5 231.035 231.035 157.025 0 0 0
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
122 Master Thesis
Tributary Area (m2)
Frame 4-4 Area around
axis A-A Area around
axis B-B
Area around
axis C-C
Area around
axis D-D
Area around
axis E-E
Area around axis F-F
Story level 1 16.4 16.4 13.6 16 16 8
Story level 2 16.4 16.4 13.6 16 16 8
Story level 3 14.4 14.4 3 0 0 0
Story level 4 14.4 14.4 3 0 0 0
Story level 5 14.4 14.4 3 0 0 0
Nodal weight (kN)
Story level 1 231.035 231.035 191.215 225.4 225.4 112.7
Story level 2 231.035 231.035 191.215 225.4 225.4 112.7
Story level 3 204.735 204.735 51.825 0 0 0
Story level 4 204.735 204.735 51.825 0 0 0
Story level 5 204.735 204.735 51.825 0 0 0
Dead, self and life load already defined in 7.1.2
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
123 Master Thesis
7.4.3 BUILDING CAPACITY CURVE AND ADRS MODELS
The following graphics are presented in comparison mode between existing model and
repaired one.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
124 Master Thesis
• Graphical presentation of mode shapes and drifts stories.
BEFORE PUSHOVER repaired
Fundamental Period = 0.69 s
STORY FIRST EIGEN
VECTOR (normalized) ϕ
5 1.000 4 0.787 3 0.478 2 0.234 1 0.07 0 0.000
AFTER PUSHOVER repaired
Fundamental Period = 5.7 s
STORY FIRST EIGEN
VECTOR (normalized) ϕ
5 1.000 4 0.799 3 0.590 2 0.382 1 0.179 0 0.000
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
125 Master Thesis
& 3
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
126 Master Thesis
7.4.4 BUILDING BILINEAR CAPACITY MODEL IN ADRS FORMAT
IDARC(x-x): OUTPUT X-X.OUT
STORY FIRST EIGEN
VECTOR (normalized) ϕ
PERIOD (s)
5 1.000 0.05402 4 0.787 0.08798 3 0.478 0.13079 2 0.234 0.25045 1 0.070 0.73094
STORY BUILDING HEIGHT (m)
FLOOR MASSES (kN*s2/m)
5
15.30
248 4 248 3 248 2 371 1 371
DYNAMIC CHARACTERISTICS
Σmi (kN*s2/m) (Σmi ∗ϕ1)^2 Σmi ∗ϕ1^2 EFFECTIVE
MASS RATIO (α)
ROOF PARTICIPATION FACTOR (PFϕR)
1485 454037.761 479.8543297 0.63716 1.40422
BILINEAR CAPACITY CURVE
BASE SHEAR COEFFICIENT
(%W)
OVERALL DEFORMATION
(%H)
0.00 0.00 0.12 0.17 0.15 1.00
μ max 5.90
BILINEAR in ADRS
Sa (g) Sd(cm)
0.00 0.00 0.1883 1.85
0.24 10.92
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
127 Master Thesis
7.4.5 PERFORMANCE POINT FOR REPAIRED MODEL
From graphical presentation bilinear capacity spectrum extend through the envelope of the
demand curve that means building will survive. The performance point is achieved in point
2, where capacity curve with ductility 1.7 is the same value to ductility of demand spectra.
The table gives the components of interest according to performance point.
This table comparing to existing one change in each properties, as:
1. PGA and Base Shear for repaired model is bigger showing that building is
capable to sustain more external forces.
2. Ductility and damping is increased.
3. Roof displacement is decreased according the values of existing model.
So, the basic conception for RC structures like enough strength, ductility and stiffness is
fulfilled.
Point 2
PGA %g
Sd (cm)
Ductility μ
Damping %
Base Shear V (kN)
Roof Displacement ∆s (cm)
X-X 0.2 3.15 1.7 11.6 1835.6 4.42
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
128 Master Thesis
7.4.6 BUILDING FRAGILITY MODELS AND EXPECTED DAMAGE
Fragility curves represent one of the possible forms of the earthquake intensity – damage to
structures relationship. A random point on the fragility curve shows the conditional
probability that the damage under an earthquake of a given intensity will exceed a certain
damage state.
These functions are defined on the basis of defined discrete damage states and the fitted
probability distributions of damage under earthquakes of certain intensity.
Fragility curves using Binominal Distribution Approach
Pushover / CBA Method
The CBA method is based on the following assumptions:
• The models of capacity and fragility curves are assumed to be HAZUS-type [6];
• The building model types and characteristics are taken from RISK-UE Buildings
Typology Matrix (BTM) [2];
• Definition of the seismic coefficient in accordance with: the period of construction (level
of seismic protection), seismicity, local soil conditions and dynamic characteristics of the
structures [7-10];
• To estimate the drift ratio at the threshold of structural damage state a modified
Park&Ang damage index is adopted [3, 4]; and,
• Definition of the overstrength and ductility ratios is to be done from the codes,
experimental data or expert judgment.
The building fragility functions define the conditional probability of being in or exceeding a
certain damage state noted as: Slight, Moderate, Extensive and Complete. They are
modeled as cumulative lognormal distribution and each function is defined by its median
value and lognormal standard deviation (β) values as follows
dS Seismic hazard parameter
dsdS , The median value of spectral displacement at which the building reaches the
threshold of the damage state ds,
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
129 Master Thesis
Sd
Sa
Sd1 Sd2 Sd3 Sd4
Dy Du
dsβ Is the standard deviation of the natural logarithm of spectral displacement of
damage state, ds.
Φ Is the standard normal cumulative distribution function.
The procedure for calculating fragility curves pass through the steps listed below:
1. Definition of damage threshold levels:
→ Slight DySd 7.01 =
→ Moderate DySd =2
→ Extensive ( )DyDuDySd −+= 25.03
→ Collapse DuSd =4
2. Definition of lognormal Standard Deviation (�ds) for each damage state as a function
of ultimate ductility, Y
Uu Δ
Δ=μ :
→ Slight )(07.025.01 uds Ln μβ ⋅+=
→ Moderate )(18.02.02 uds Ln μβ ⋅+=
→ Extensive )(40.01.03 uds Ln μβ ⋅+=
→ Collapse )(5.015.04 uds Ln μβ ⋅+=
3. Calculate the probability of exceed by of Cumulative normal Distribution for each
damage state: [ ]⎥⎥⎦
⎤
⎢⎢⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛Φ=
dsd
d
dsd S
SLnSdsP
,
1/β
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
130 Master Thesis
For repaired model:
Dy = 1.85 cm Du = 10.9 cm 9.5=uμ
Damage State
Median
dsdS , (cm)
Standard Deviation
dsiβ
Cumulative probability for exceeding certain damage state
[ ]dSdsP /
Slight 1.29 0.3742 0.500 0.246 0.077 0.020 Moderate 1.85 0.5194 0.83 0.500 0.162 0.044 Extensive 4.11 0.8097 0.999 0.938 0.500 0.174 Collapse 10.92 1.0371 1.000 1.000 0.886 0.500
Model Sd
(cm)
Probability of exceed for each Damage State
Slight Moderate Extensive Collapse
Exist 4.88 100% 96.9% 58.3% 21.9%
Repaired 3.15 99% 84.7% 37% 11.5%
Table. Correlation of Damage index and Damage states
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
131 Master Thesis
7.4.7 CONCLUSION
I. Existing Building
1. Existing building suffer yielding cracks in columns and beams.
2. Total displacement on top of building 6.9cm which is bigger than code
recommendation.
3. The structure is very flexible and fundamental period with value = 1.39s is more than
expected period for this type of structure, which would be in rage of 0.5-0.6s
4. Capacity curve expressed in ADRS format doesn`t extend through plot of demand
spectra meaning that building will not survive future earthquakes.
5. From site investigation in whole building is evaluated even problems in technology
during time construction, like distorted column presented in picture 7.8.
6. According plot of fragility curves and description of damage state performance point
present:
There is 100% of exceeding slight damage or light cracking throughout.
There is 96.9% of exceeding moderate damage or severe cracking, localized
spalling.
There is 58.3% of exceeding extensive damage or crushing of concrete,
reinforcement exposed.
There is 21.9% of exceeding collapse damage.
7. Maximum PGA carried by structure is 0.09 % of g
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
132 Master Thesis
II. Repaired Building
1. Regarding failure mode of repaired model cracks happen before in beams and
columns doesn`t not suffer yielding cracks.
2. Capacity curve expressed in ADRS format extend the plot of demand spectra
meaning the building will survive future earthquakes.
3. The structure is more rigid and stiff, its fundamental period = 0.69s and total
displacement = 4.4cm are less than previous model and within code provisions.
4. Maximum PGA and Base shear value in repaired model are at least double of
existing one, showing the model is capable to withstand bigger outside forces.
5. According plot of fragility curves and description of damage state performance point
present:
There is 99% of exceeding slight damage or light cracking throughout.
There is 84.7% of exceeding moderate damage or severe cracking, localized
spalling.
There is 37% of exceeding extensive damage or crushing of concrete,
reinforcement exposed.
There is 11.5% of exceeding collapse damage.
6. The model posses more ductility and damping, properties which are very important
in seismic buildings.
7. Reinforced concrete jacketing is a suitable method and easy technology to be
implemented in site. During site works all the recommendation mention in this thesis
were adopted, a special attention was done during reinforcing the beam column
joint. In continue are presented some photo of site works.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
133 Master Thesis
Fig. 7.9 Photo of Reinforced Footing
Fig. 7.10 Photo of Reinforced Column
Fig. 7.11 Photo of Reinforced Column-Beam Joint
Fig. 7.12 Photo of Covering with Epoxy-Resin Fig. 7.13 Photo of Epoxy-Resin for Injection
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
134 Master Thesis
Fig. 7.14 Photo of Reinforcement passing through concrete slab
Fig. 7.15 Photo of Column and Shear Wall Jacketing
Fig. 7.16 Photo of RC Beam-Colum Joint
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
135 Master Thesis
APPENDIX
A. Calculation of Bent bars for Columns
Max. Shear Force Q=130kN
For one side of Column: P=2.5* ø2*√ (Bp * Bs)
Bs (reinforcement yielding strength) = 3520 kg/cm2 Bp (28 days concrete strength) = 200 kg/cm2
Choose bar ø=12mm P=2.5*1.22*√ (200 * 3520) =3021kg or 30.2kN So, for one meter 130kN/30.2kN = 4.3 bent bars.
We define bar ø=12mm/15cm for each side of column
B. Cost estimation
Object: Repairing and Strengthening of Existing Structure Address: Tirana, Albania
No Description Unit Amount Price/unit Value (euro)
I. Excavation Works 1 Excavation and transportation of soil m3 300 26 7,800 2 Leveling the gravel m2 236 1.5 354 Sum I Euro 8,154 II. Reinforced concrete Works 1 Concrete B30 for foundation m3 240 82 19,680 2 Concrete B30 for columns m3 37 121 4,477 3 Reinforced steel kg 15000 0.85 12,750
Sum II Euro 36,907 III. Special Works 1 Demolishing the plaster m2 151 2.7 408 2 Boring holes and filling with epoxy resin HIT-150 holes 2500 1.6 4,000 3 Covering old concrete with epoxy Sika-dur m2 200 2.5 500
Sum III Euro 4,908 Total Sum I-III Euro 49,969
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
136 Master Thesis
REFERENCES 1-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-1, Family House Type-1 (B+GF+1S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-01-P1, Skopje, February 2000.
2-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure: Prototype-2, Family House Type-2 (GF+1S+R)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-07-P2, Skopje, February 2000.
3-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-3, Family House Type-3 (GF+3S+R)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-08-P3, Skopje, February 2000.
4-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure: Prototype-4, Residential Building-1 (B+GF+6S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-02-P4, Skopje, February 2000.
5-0. Ristic, D., "Inventive GVCS-System for Seismic Protection of Building Structure:
Prototype-5, Residential Building-2 (2B+GF+14S+R)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-03-P5, Skopje, February 2000.
6-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-6, Patent Center (B+GF+2S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-09-P6, Skopje, February 2000.
7-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-7, Children’s Education Center (B+GF+1S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-010-P7, Skopje, February 2000.
8-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-8, Hotel Type-1 (B+GF+5S+R)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-04-P8, Skopje, February 2000.
9-0. Ristic, D., "Inventive GVCS-System for Seismic Protection of Building Structure:
Prototype-9, Hotel Type-2 (2B+GF+17S+5S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-011-P9, Skopje, February 2000.
EFFICIENT METHOD FOR REPAIR AND STRENGTHENING
137 Master Thesis
10-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure: Prototype-10, School Building (B+GF+1S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-05-P10, Skopje, February 2000.
11-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-11, Post Building (B+GF+1S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-012-P11, Skopje, February 2000.
12-0. Ristic, D., "Inventive GOSEB-System for Seismic Protection of Building Structure:
Prototype-12, Hospital Building (B+GF+3S)”; Inventive Cycle: “Seismo-Safe Cities of the Future”, Prototype Structures, Ministry of Development, IPPO, Report DR-2000-06-P12, Skopje, February 2000.
13-0. Milutinovic, Z., “Planning for Seismic Risk Reduction”.Institute of Earthquake
Engineering and Engineering Seismology (IZIIS), Skopje, Macedonia 2006. 14-0. “Earthquake Loss Estimation Methodology. HAZUS 99”. Federal Emergency Management
Agency (FEMA), Washington 1999. 15-0. Milutinovic, Z., Trendafiloski, G., “Chapter 3. Vulnerability Assessment Methods” RISK-UE
Methodolody. 2003. 16-0. “Eurocode 8: Design of Structures for earthquake resistance” European Committee for
Standardization January, 2003. 17-0. T.Paulay, M.J.N. Priestly, “Seismic Design of Reinforced Concrete and Masonry Buildings”
March, 1991. 18-0. Necenska-Cvetanovska, G,. Aseismic Design of Reinforcement Concrete, Masonry and Steel
Structures, March 2006, Skopje, Macedonia. 19-0. Chopra, Anil K.,“Dynamic of Structure”, New Jersey. 1995.
20-0. Taskov, Lj., “Dynamic of Structure”, .Institute of Earthquake Engineering and Engineering
Seismology (IZIIS), Skopje 2005, Macedonia. 21-0. Gravilovic, P., “Repair and strengthening of existing structures”, Institute of Earthquake
Engineering and Engineering Seismology (IZIIS), Skopje 2004, Macedonia. 22-0. Boonyapinyo, V., “Seismic capacity evaluation by pushover analysis” April, 2006.
23-0. IDARC2D V.6 computer program, Buffalo University, USA 2004.
24-0. SAP2000 V.10.09 Structural Analysis Program, Computer and Structures, Inc, Berkeley
University 1995