Earthquakes, Existing Buildings and Seismic Design Codes in Turkey

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Arabian Journal for Science and

Engineering

ISSN 1319-8025

Volume 37

Number 2

Arab J Sci Eng (2012) 37:365-380

DOI 10.1007/s13369-012-0183-8

Earthquakes, Existing Buildings andSeismic Design Codes in Turkey

A. Ilki & Z. Celep


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Arab J Sci Eng (2012) 37:365380DOI 10.1007/s13369-012-0183-8

RESEARCH ARTICLE - CIVIL ENGINEERING

A. Ilki Z. Celep

Earthquakes, Existing Buildings and Seismic Design Codesin Turkey

Received: 1 November 2010 / Accepted: 9 March 2011 / Published online: 26 January 2012 King Fahd University of Petroleum and Minerals 2012

Abstract From worldwide observations made after the occurrence of earthquakes, as well as the tremendousamountof experimental, analytical andnumerical studies, significant contributionshave been made fora betterunderstanding of the characteristics of earthquakes, and effects of earthquakes on existing structural systems.Consequently, seismic design codes are revised in a parallel fashion by integrating new concepts towards morerealistic considerations of seismic demand, seismic response and seismic capacity. In this paper, after outlin-ing the performance of existing buildings in Turkey during recent earthquakes (particularly Kocaeli 1999 andDuzce 1999 Earthquakes), and by focusing on the observed common structural deficiencies, a brief summaryof the evolution of the Turkish Seismic Design Code in the last decades is presented. It is important to notethat the poor seismic performance of existing buildings in Turkey outlined in this study is not directly relatedto the inefficiency of the relevant seismic design codes, but rather to extremely low quality construction andthe absence of a strict inspection system at the time of their construction. It should also be highlighted that thelessons learnt from the catastrophic consequences of recent earthquakes, revisions in the seismic design codeand the developments in the material and workmanship characteristics have significantly improved the quality

of newer constructions in Turkey in the last decade.

Keywords Buildings Codes Damage Earthquake Performance Seismic Turkey

A. Ilki (B)Structural and Earthquake Engineering Laboratory, Istanbul Technical University, Istanbul, TurkeyE-mail: [email protected]

Z. CelepCivil Engineering Department, Istanbul Technical University, Istanbul, Turkey

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1 Introduction

In last two decades several catastrophic earthquakes hit Turkey causing thousands of casualties and injuries,as well as significant economic losses. Among these, Erzincan (1992), Kocaeli (1999) and Duzce (1999)earthquakes were the most severe ones, with magnitudes of 6.9, 7.4 and 7.2, respectively [13]. Accordingto a recent investigation, the probability of an earthquake with magnitude 7.0 or greater to affect Istanbul,

the cultural and economic center of Turkey, is around 41 4% in the next 30 years [4]. Therefore, seismicsafety of existing building stock, is a major concern in Turkey, where various versions of seismic design codehave been published since 1940 always adopting more strict requirements. In spite of presence of a seismicdesign code for quite a long time, catastrophic consequences were observed after all major earthquakes. Theseconsequences were mainly due to substandard construction practice in the absence of a strict inspection sys-tem, although the seismic design codes were reflecting the up-to-date seismic knowledge level. Parallel to thedevelopments in the field of earthquake engineering in the world, many vitally important concepts and detailshave been included in the seismic design codes at each revision.

In this paper, after briefly outlining the seismic performance of existing buildings against 1999 Kocaeli andDuzce Earthquakes, with a special focus on common structural deficiencies, the evolution of Turkish SeismicDesign Code is presented. For the sake of completeness, information on the demographical and the economicalcharacteristics of Turkey is also summarized briefly.

2 Demographic and Economic Data

As it is well known, the catastrophic consequences of severe earthquakes do not stem only from the technicalengineering issues, but are also strongly dependant on the economical, social and cultural situation. The basicdemographic and economical data of Turkey is presented in Table 1, together with the data obtained for Japan,European Union (EU) and United States of America (USA) for a better perception. Table 1 demonstrates thatTurkey spanning between Asia and Europe has quite a large area, compared to Japan and EU countries, and ithas a fairly high and young population. Population growth rate is around 1% Turkeys gross domestic product(GDP) is approximately one-seventh of that of Japan and GDP per capita in Turkey is around one-quarter ofthat of Japan. Considering the consequences of past seismic events of similar magnitudes in Turkey, Japan andUSA (for example 1999 Kocaeli Earthquake in Turkey, 1995 Kobe Earthquake in Japan and 1989 Loma PrietaEarthquake in USA) together with the economical data presented in Table 1, one can easily assess that the lifelosses, the injuries and the extent of structural damages are closely related to the economic development ofthe country.

3 Data on Existing Building Stock

Unfortunately, a reliable and scientific building inventory covering all areas of Turkey and reflecting the cur-rent situation is not available. However, information obtained in the census carried out by the State StatisticalInstitute [5] can be used for evaluating the type of buildings and their structural systems. According to thecensus results total number of buildings in Turkey by 2000 is 7,838,675. The distribution of buildings in termsof usage, number of stories and construction year are presented in Figs. 1, 2 and 3, respectively.

The authors of the present paper had chances to investigate the damages after several earthquakes in Tur-key. Some typical damages observed in existing reinforced concrete structures are shown in Fig. 4. Basicweaknesses of reinforced concrete structures mostly observed can be classified as:

Table 1 Basic demographic and economic data

Country Turkey Japan EU USA

Area (km2) 780,000 378,000 4,300,000 9,800,000Population 71,000,000a 127,000,000a 490,000,000a 301,000,000a

Median age 28.6a 43.5a NA 36.6a

Population growth (%) 1.04a 0.09a 0.16a 0.89aGDP (USD) 109 640b 4.218b 13.080b 13.060b

GDP/capita (USD) 9.100b 33.100b 29.900b 43.800b

GDP growth rate (%) 6.1b 2.2b 3.2b 2.9b

a 2007 estimatesb 2006 estimates (source: https://www.cia.gov/library/publications/the-world-factbook/index.html )

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Fig. 1 Building distribution according to usage (%)

Fig. 2 Building distribution according to the number of stories

Fig. 3 Building distribution according to the construction year (%)

Fig.4 Various failures due to a insufficient lap-splices of column reinforcement, b insufficient amount and detailing of transversebars and c insufficient shear strength of a short column

Insufficient lateral load capacity due to inadequate concrete strength and insufficient reinforcement, Insufficient lateral stiffness due to inadequate frame formation, Insufficient ductility due to inadequate lateral reinforcement, Inadequate detailing of longitudinal and transverse reinforcement, Insufficient stiffness of soft first stories, Insufficient strength of columns with respect to beams at joints.

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Many existing low-rise reinforced concrete frame buildings in Turkey can also be considered as confinedmasonry due to their weak reinforced concrete structural systems. In such cases, infill walls resist the seismicloads rather than weak reinforced concrete frames. Basic weaknesses of such buildings are:

Low ductility of infill walls, Lack of integrity of masonry units in walls, Insufficient integration of infill walls with frames, Poor quality of construction materials, Large window and door openings, Low out-of-plane strength and stiffness of infill walls.These basic weaknesses stemming from many different design and construction errors are main reasons ofthe catastrophic consequences experienced after earthquakes. These errors, which cause premature failures ofstructural members or structures, are basically due to improper application practices on-site. Themost commonapplication errors in reinforced concrete structures include low-strength concrete, insufficient transverse bars,inadequate lap-splices of column reinforcement, insufficient bond between plain round bars and concrete, anddeterioration of structural system by time due to low-quality materials and lack of maintenance. Most commonproblem related to design, especially due to architectural constraints, is irregularity of the structural system.

Among other common reasons of damage are inadequate consideration of ground conditions in the lay outand design of the structural system, addition of illegal stories to the existing structural systems without taking

necessary measures, and unconsciously removing or damaging certain structural members (such as columnsand beams in reinforced concrete structures or walls in masonry structures), which are notconveniently locatedor dimensioned for the occupants.

It is worth noting that most of the low-rise buildings in cities are legally engineered structures. Normally,they are expected to be designed and constructed with proper engineering service. However, due to lack ofa sufficient inspection mechanism, particularly a large number of buildings constructed before 1999 KocaeliEarthquake, do not satisfy requirements of the related national codes and standards. Consequently, they cannotbe classified as properly engineered buildings.

4 Seismicity in Turkey

The seismic zone map issued in 1972 and the current seismic risk map of Turkey are given in Fig. 5. In thisfigure, distribution of the historical hazardous earthquakes is also presented. As seen in Fig. 5, the seismic

zone maps are in agreement with the distribution of hazardous earthquakes. The previous seismic zone mapissued in 1972 had been valid until 1996 [8]. It should be noted that Zone I covers areas with highest risk,while Zone V covers the areas having minimum seismic risk. According to Ozmen [9], 45, 26 and 15% of thepopulation live in Zones I, II and III, respectively, while only approximately 15% of the population lives inZones IV and V. Furthermore, it is worth noting that further information on the history of the seismicity mapsof Turkey and additional seismic maps can be found elsewhere [10].

5 Evolution of Seismic Design Code in Turkey

While many catastrophic earthquakeshavehitvariousareasofTurkey in history [11], thefirstmajorcatastrophicnatural disaster experienced by Republic of Turkey was the Erzincan earthquake in 1939. The magnitude ofthe earthquake was 7.8 and caused a loss of more than 33,000 lives and destruction of 140,000 homes [ 1].This earthquake was a milestone for adoption of the concept of earthquake-resistant design and construction in

Turkey. Consequently, the first set of explicit legal provisions for earthquake-resistant design was establishedin 1940 by the Ministry of Public Works, followed by another version in 1942 annexed with a seismic zonemap. This seismic regulation was revised in 1944 within the articles of Law No. 4623 [12]. The law stated thatany building built without complying with the requirements of the regulation would be demolished. However,this stipulation (and its future versions) did not clearly state which authority is to do the demolishment andconsequently no demolishment was done [12]. The seismic regulation was updated in 1949 and 1953 to reflectthe amendments of the seismic zone map without any major change in the code [13]. By the establishmentof Ministry of Reconstruction and Resettlement in 1958, the disaster prevention policy was upgraded and theformulation of the base shear coefficient was revised in 1961 [2]. The next revisions in 1968 and 1975 broughtimportant enhancements to the seismic design and introduced the international developments to the engineer-ing society in Turkey. The concept of ductility was first time mentioned at member and structural levels in

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Fig. 5 a Seismic zone map in 1972 [1], b current seismic zone map (source: Ministry of Public Works and Settlements, http://www.deprem.gov.tr), and c historical hazardous earthquakes around Turkey (source: [6,7])

1975 code. The principles of the capacity design were introduced by the 1998 code together with importantdetailing issues for seismic design. The most recent version of the code issued in 2007, particularly has beena very important step towards the displacement-based design through the related requirements for the seismicassessment of existing buildings and retrofitting. The evolution of the seismic design code is summarized inmore detail below.

5.1 1940 Seismic Regulation [14]

This was the first seismic regulation in Turkey. Besides several rules related to construction, materials andworkmanship, this code gave thefundamental base shear coefficient of 0.10 forcalculation of the lateral seismicload. In case of presence of the wind load (W), the design lateral load (H) is calculated by Eq. 1, where onlyhalf of the live load ( P) is considered in addition to the dead load (G). On the other hand, half of the windload (W) is included as well. No specific distribution of the lateral load along the height of the building wasdefined in this code.

H= 0.10

G + P2

+ W

2(1)

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This regulation included a seismic zone map having two seismic zones, Zones I and II. Areas outside ZonesI and II were considered to be safe in terms of seismicity. The fundamental base shear coefficient (the ratioof the base shear force to the seismic weight of the building) was adopted to be 0.020.04 and 0.010.03 forZones I and II, respectively [2]. Selection of the appropriate value in these ranges was the responsibility of

design engineers. However, approval of the inspecting authority was required for the selected value [13]. Likein previous version, in this regulation, the geotechnical conditions of the construction site and the structuralcharacteristics were not taken into account. Furthermore, the distribution of the lateral load along the heightof the building has still not been defined as well [1].


In 1960s and 1970s, due to very rapid industrialization and urbanization, the amountof constructions increasedtremendously, particularly in cities such as Istanbul, Izmir and Bursa. Therefore, a great portion of the existingbuildings were demolished and reconstructed by increasing number of stories by considering 1961 SeismicRegulation [8]. In this version of the regulation, parameters related to the seismic zone, type of the struc-tural system and the ground conditions were taken into account for determining the base shear coefficient.Upper limit of the base shear coefficient is assumed to be 0.10 and the distribution of the seismic loads is

considered to be uniform along the height of the building. A qualitative recommendation was also present inthis code to prevent excessive irregularities in plan and to minimize the potential negative effects of globaltorsion. However, it was not mentioned how to deal with the torsion of the structure. The code required thatall parts of the building should resist seismic lateral load given in Eq. 2. In this equation, C and n were thefundamental base shear coefficient and the live load reduction factor, respectively. The live load reductionfactor was given as 0.5 for ordinary buildings (residential buildings), whereas no load reduction was allowedfor densely populated buildings (theaters, hotels, factories and office buildings). The fundamental base shearcoefficient was calculated by Eq. 3, where Co was a coefficient depending on the height of the building, n1was a coefficient related to soil conditions (Soil type I, II and III) and to type of structural system (reinforcedconcrete or steel), and n2 was the seismic zone coefficient (Zones I and II). The numerical values ofCo andn1 are summarized in Tables 2 and 3, respectively. It should be noted that the interaction between soil andstructure was somehow taken into account through the coefficient n1. The coefficient n2 was to be taken as 1.0and 0.6 for the seismic zones I and II, respectively. On the other hand, higher seismic demand on the lower

buildings due to their relatively higher stiffness, particularly in case of stiff soil conditions, was not taken intoaccount properly in this code. Clearly, this negligence may result with unnecessarily high seismic design loadsfor relatively high-rise structures, whereas the seismic design loads taken into account for low-rise structuresmay be on the unsafe side.

H = C(G + n P)+ W/2 (2)

C= Con1n2 (3)

It should be noted that when the wind load ( W) is higher than the design lateral load calculated by Eq. 2,the design lateral load is considered to be equal to the wind load. The code permitted an increase of 50% inallowable stresses in case of seismic design. The seismic zone map was revised in 1963 and the number of

seismic zones is increased from two to four including a zone where no seismic design is required [2].

Table 2 The coefficient ofCo depending on building height

Height of building (m) Co coefficient

40 +0.01/for each 3 m

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Table 3 The coefficient n1 according to building type and ground conditions

Ground type Reinforced concrete Steel

I 0.8 0.6II 0.9 0.8III 1.0 1.0

Irock, hard soil, IImedium soil, IIIsoft soil


This code brought significant enhancements to seismic-resistant design such as:

Definition of minimum dimensions for columns [depth of short side maximum (240 mm; 0.05 storyheight)]

Definition of minimum dimensions for beams (150 300 mm, depth 3 times of the slab thickness) Definition of minimum dimensions of shear walls [width maximum (200 mm and 0.04 story height)] Confinement reinforcement requirement for columns and beams in the vicinity of joints (transverse rein-

forcement is to be doubled with respect to the mid-height of the column and mid-span of the beam). Confinement reinforcement requirement in the beamcolumn joint

Consideration of dynamic characteristics of the building

Introduction of the building importance factor Inverse triangular distribution of the lateral forces Increase of the base shear force due to global torsion of the building, when the eccentricity between centers

of mass and rigidity exceeds 5% of the larger plan dimension of the building.

In this code, base shear force (F) to consider the effects of earthquakes is to be calculated by Eq. 4. In thisequation, W is the total weight of the building to be considered for the seismic analysis (Eq. 5) and C is thefundamental base shear coefficient to be calculated by Eq. 6. It should be noted that according to this codeno live load reduction factor is allowed for buildings such as theaters, schools, stadiums, storage facilities.However, a live load reduction factor of 0.5 is given for health facilities, hotels, administrative or residentialbuildings. Co given in Eq. 6 is the seismic zone factor (0.06, 0.04 and 0.02, for Zones I, II and III, respectively), is the coefficient reflecting ground conditions (0.8, 1.0 and 1.2, for hard, medium and soft soil, respectively), is the building importance factor (1.5 for important or densely populated buildings such as communication

buildings, hospitals, fire stations, museums, schools, stadiums, theaters, train stations, religious buildings, and1.0 for ordinary buildings such as residential, office and industrial buildings, hotels, restaurants, etc.), and isthe dynamic coefficient to be calculated by Eq. 7a or 7b depending on the fundamental period of the building(T) in seconds. A simple equation is also given in the code for calculation of the fundamental period of thebuilding (Eq. 8) to be used unless the period is not calculated by using a sophisticated method. In this equation

Hand D are the height of the building (m) and the plan dimension of the building (m) in the direction of theconsidered lateral load.

F = CW (4)

W =

Wi =

Gi + ni Pi (i : story number) (5)

C= Co (6)

= 1 (T 0.5 s) (7a)

= 0.5T

0.3 (T > 0.5 s) (7b)

T = 0.09HD

(s) (8)

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According to the code, the lateral forces are to be distributed to floor levels along the height by using Eq. 9. Inthis equation, Fi , Wi and hi are the lateral forces applied to the i th story floor, the weight of the story and theheight of the story measured from the foundation level.

Fi = FWi hi

Wi hi(9)

The code permitted an increase of 50% in allowable stresses of concrete and steel in case of seismic designassuming that the design is carried out by using allowable stress approach. Furthermore, an increase of 50 and30% in the ground allowable stresses was permitted for the ground types I (hard soil) and II (medium soil),respectively. However, it should be noted that the concept of shear wall end zones, where an increased amountof longitudinal and transverse reinforcement should be placed, was not introduced in this code. In the code andits latter revisions, the effect of earthquake was taken into account separately without considering the windload. Consequently, analysis against earthquake and wind loads are carried out separately and design is carriedout according to the most unfavorable case. In 1972, the seismic zone map was divided into five seismic riskzones (Fig. 5a), including the zone with no seismic risk [2].


1975 Seismic Regulation has been valid for more than 20 years. Therefore, as seen in Fig. 3, a great portion ofexisting buildings were designed and constructed, while this code was in effect. The code was the first code,in which the term ductility was used explicitly. Furthermore, the base shear force was given as a function ofstructural ductility for the first time implicitly according to the lateral load resisting system of the structure byintroducing a structure type coefficient. Other important improvements in the code were:

Inclusion of more detailed principles related to seismic-resistant detailing Inclusion of details about minimum cross-sectional dimensions and minimum reinforcement ratios for

structural members Inclusion of more detailed requirements related to confinement Inclusion of a quantitative shear design for beamcolumn joints Inclusion of the ground dominant period into the equation given for determination of the spectrum coeffi-

cient Inclusion of an explicit definition of irregular buildings (although the definitions of irregularities were notsufficiently detailed)

Inclusion of the requirement of the modal analysis for irregular or high-rise structures (H> 75 m) Introduction of the concept of increased longitudinal reinforcement at end zones of shear walls Consideration of an additional eccentricity of 5% of the largest plan dimension of the building.

However, it should be noted that while an increase of longitudinal reinforcement at the end zones of the shearwalls was introduced in the code, the confinement of longitudinal bars in these end zones was not required.

In the code, the base shear force was to be calculated by Eq. 10. In this equation C, Co, K, Sand Iare thefundamental base shear coefficient, the seismic zone coefficient (0.10, 0.08, 0.06 and 0.04, for Zones I, II, IIIand IV, respectively), the structure type coefficient, the dynamic coefficient and the building importance factor,respectively. The values of the structure type coefficient K, which was actually introduced for considerationof ductility capacity of various structural systems, are given in Table 4 depending on the type of the structure.As seen in this table, relatively lower ductility capacity of the shear wall structures is taken into account byincreasing designbase shear force in thisversion of theseismicdesigncode. Thedynamiccoefficient (spectrumcoefficient) is to be evaluated by Eq. 11, where To is the effective period of the ground in seconds. It should benoted that the dynamic coefficient should be assumed as 1.0 for one and two-story structures and all masonrybuildings. For the fundamental period of buildings, in addition to Eq. 8, which has already been given in the1968 regulation, an alternative formula was given as well (Eq. 12). In Eq. 12, Nis the number of stories. Thebuilding importance factor,Iwas almost same as in the 1968 code (either 1.0 for ordinary buildings, or 1.5 forimportant or densely populated buildings.

C= Co K S I Co

2(10)

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Table 5 The building importance factor, I

Purpose of occupancy or type of building Importancefactor (I)

1. Buildings to be utilized after the earthquake and buildings containing hazardous materials(a) Buildings required to be utilized immediately after the earthquake (hospitals, fire fighting buildings,

telecommunication facilities, transportation stations and terminals, power generation and distribution

facilities, official administration buildings, etc.)(b) Buildings containing or storing toxic, explosive and flammable materials, etc. 1.5

2. Intensively and long-term occupied buildings and buildings preserving valuable goods(a) Schools, dormitories, military barracks, prisons, etc.(b) Museums 1.4

3. Intensively but short-term occupied buildingsSport facilities, cinema, theatre and concert halls, etc. 1.2

4. Other buildingsBuildings other than defined above (residential and office buildings, hotels, building-like industrialstructures, etc.) 1.0

Table 6 Characteristic spectrum periods

Local site class TA (s) TB (s)

Z1 0.10 0.30

Z2 0.15 0.40Z3 0.15 0.60Z4 0.20 0.90

The most important advances introduced through the 1998 code are:

Inclusion of the detailed capacity design principles Explicit definition of the design earthquake in terms of occurrence probability Explicit definition of the acceptable structural performance under the design earthquake Definition of the elastic design spectrum Definition of the seismic load reduction factor depending on the structural characteristics, including

dynamic properties and ductility of the structural system and the over-strength factor

Inclusion of detailed requirements on confinement and explicit rules for reinforcement detailing

Quantitative definition of irregularities.

The capacity design principles in the code provide that plastic hinges form at beams by assuring that columnsare stronger than beams framing into the same joint. Furthermore, the shear capacity of beams and columnsas well as shear walls is kept higher than their bending capacity, so that ductile failure is ensured in case ofseismic loads higher than that considered in seismic design.

The design earthquake considered in the code corresponds to an earthquake with the return period of475 years for ordinary buildings (for building importance factor 1.0) and 2,475 years for the most importantbuildings (for building importance factor 1.5). The probabilities of exceedence for these two cases are 10 and2% in 50 years, respectively. In the code, thespectral acceleration coefficientA(T) is givenby Eq. 15, where Ao,

Iand S(T) are the effective seismic acceleration coefficient (seismic zone coefficient), the building importancefactor and the elastic spectrum coefficient evaluated for 5% damping ratio. The effective seismic accelerationcoefficient (Ao) is to be taken as 0.40, 0.30, 0.20 and 0.10, for the seismic zones I, II, III and IV, respectively,

(Fig. 5b). The building importance factor (I) is given with more details in the code compared to its previousversions, (Table 5). Spectrum coefficient (S(T)) is determined through Eqs. 16a, 16b, 16c as a function of thefundamental period the building (T) and the characteristic spectrum periods (TA and TB), which are to bedetermined depending on the ground type. The characteristic spectrum periods for various ground conditionsare given in Table 6. In this table, it is apparent that Z1 represents strongest ground conditions, while Z4corresponds to the weakest. The variation of spectrum coefficient with respect to the fundamental period ofthe building is shown in Fig. 6. It should be noted that the fundamental period of the building can be calculatedby Eq. 17 or Eq. 18. Equation 17 is the well-known Rayleigh equation, where mi is the mass of the i th story,Ff i is the fictitious lateral load acting on the i th story and d f i is the corresponding displacement of the i thstory in the direction of F f i . On the other hand, Eq. 18 is an empirical relation, where Ct is a coefficient anddepends on structural system of the building (0.08 for steel frames, 0.07 for reinforced concrete frames, 0.05

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(b)

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 0.5 1 1.5 2

T(s)

Ao

IS(T)/R

a(T)

Zone 1

Zone 2

Zone 3

Zone 4

TBTA

S(T) = 2.5 (TB/ T )0.8

S(T)

2.5

1.0

T

Fig. 6 a Elastic spectrum coefficient S(T) and b spectral acceleration coefficient depending on the fundamental building periodTfor four seismic zones

for shear wall buildings) and HN is the total height of the building.

A(T) = AoI S(T) (15)

S(T) = 1 + 1.5 TTA

(0 T TA) (16a)

S(T) = 2.5 (TA T TB) (16b)

S(T) = 2.5

TB

T

0.8(T TB) (16c)

T1 = 2

Ni=1

mi d2f i/

Ni=1

Ff i df i (17)

T1 = T1A = CtH3/4N (18)

For using inelastic capacity of the structures (at least partially), certain level of inelastic deformations (con-trolled damages) beyond elastic limits are allowed under the design earthquake explicitly, provided that thebuilding does not collapse, life safety is ensured and damages are kept within the controlled limits. For uti-lizing inelastic deformations, the structural system should have a certain level of ductility. According to thecode, the buildings can be designed considering two levels of ductility; normal or high. There are severalrules, particularly in terms of the application of the capacity design, construction details and irregularities forclassifying the structural systems as normal or high ductility. Since inelastic deformations are allowed, thelateral load demand evaluated by using the elastic design spectrum is reduced depending on the characteristicsof the structural system by the seismic load reduction factor Ra(T) given by Eqs. 19a, 19b. Obviously, if thestructural system possess thecharacteristics such that the system canbe classified as a high ductility system, thereduction in lateral loads is higher than that of a normal ductility structural system. The variation of the spectral

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Table 7 The structural system coefficients, R

Building structural system Systems Systemsof normal of highductility level ductility level

(1) Cast-in-situ reinforced concrete buildings(1.1) Buildings in which seismic loads are fully resisted by frames 4 8

(1.2) Buildings in which seismic loads are fully resisted by coupled structural walls 4 7(1.3) Buildings in which seismic loads are fully resisted by solid structural walls 4 6(1.4) Buildings in which seismic loads are jointly resisted by frames and solid and/ 4 7

or coupled structural walls(2) Precast reinforced concrete buildings

(2.1) Buildings in which seismic loads are fully resisted by frames 3 6with connections capable of cyclic moment transfer

(2.2) Buildings in which seismic loads are fully resisted by single-story hinged 5frames with fixed-in bases

(2.3) Buildings in which seismic loads are fully resisted by prefabricated solid 4structural walls

(2.4) Buildings in which seismic loads are jointly resisted by frames with connectionscapable of cyclic moment transfer and cast-in-situ solid and/or coupled structural walls 3 5

(3) Steel buildings(3.1) Buildings in which seismic loads are fully resisted by frames 5 8(3.2) Buildings in which seismic loads are fully resisted by single-story hinged frames

with fixed-in bases 4 6(3.3) Buildings in which seismic loads are fully resisted by braced frames or cast-in-situreinforced concrete structural walls(a) Concentrically braced frames 3 (b) Eccentrically braced frames 7(c) Reinforced concrete structural walls 4 6

(3.4) Buildings in which seismic loads are jointly resisted by frames and braced frames orcast-in-situ reinforced concrete structural walls(a) Concentrically braced frames 4 (b) Eccentrically braced frames 8(c) Reinforced concrete structural walls 4 7

acceleration coefficient A(T) = AoIS(T)/Ra(T) for the four different seismicity levels is also presented inFig. 6. The spectral acceleration coefficients are obtained for an ordinary (building importance factor I= 1.0)

reinforced concrete frame building with a seismic load reduction factor of 4 (typical for a reinforced concreteframe of normal ductility) constructed on the soil class Z1(TA = 0.1 s,TB = 0.3 s).

Ra(T) = 1.5 + (R 1.5)T

TA(T TA) (19a)

Ra(T) = R (T > TA) (19b)Asseen inEqs. 19a, 19b the seismic load reduction factor Racan be calculated as a function of the structural

system coefficient,R, which can be determined through Table 7. It should be noted that the value of the seismicload reduction factor does not represent the structural system ductility only, but it includes the over-strengthfactor as well. Finally, the reduced base shear force (Vt) can be calculated by Eq. 20, where W is the totalweight of the building to be calculated in a similar method as in the 1975 Code by considering dead load and

reduced live load. Furthermore, in this revision the lateral drift limits were also revised.

Vt = WA(T)

Ra(T) 0.10AoI W (20)

Requirements on the combinations of the seismic loads with the other loads are given in the related codes fortypical buildings, such as the reinforced concrete design and construction code [20]. In this code, the wind andseismic loads are not considered in one single combination together. Some typical design combinations givenin this code are 1.4G + 1.6Q, G + Q + E, 0.9G + E, G + 1.3Q + 1.3Wand 0.9G + 1.3 W, where G, Q, Eand Wrepresent dead, live, seismic and wind loads, respectively.

It is worth noting that while the code was quite comprehensive in terms of reinforced concrete structures,recommendations on steel structures were not equally detailed. This was due to the fact that number of steel

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structures was very low compared to reinforced concrete structures before the 1999 earthquakes. In contrast tothe fact that all types of structural systems have almost same level of seismicsafety, if designed and constructedproperly, the damage experienced by inadequate reinforced concrete structures (so-called reinforced concretestructures, built without receiving proper engineering service) caused a misconception such that reinforcedconcrete structures are not seismically safe, but steel structures are. Consequently, while still being in mar-ginal numbers with respect to reinforced concrete structures, the amount of steel construction has increased

significantly after the 1999 earthquakes, necessitating more comprehensive seismic provisions in the code.


Based on the demand of people and the official institutions for earthquake safe environment after the earth-quakes experienced in 1999, many structures were investigated in terms of seismic safety and some of thesewere retrofitted. However, due to lack of official guidelines and standards about seismic safety assessment andretrofitting, in many cases non-standard and sometimes inappropriate approaches were being used by designengineers while analyzing or retrofitting the existing buildings. Therefore, the most recent version of the seis-mic design code published in 2007 includes the issues on seismic safety assessment of existing buildings andretrofitting comprehensively. The code has only minor revisions in the provisions related to reinforced concretebuildings to be newly designed. However, the seismic safety requirements for steel structures, which were notaddressed in sufficient comprehensiveness in the previous codes, are covered comprehensively in the code.With this version, the title of the code, which was Regulation for structures in disaster areas since 1961, hasbeen changed as Regulation for buildings in seismic areas. Consequently, issues related with other disasters(such as flood and fire) are removed from the code.

The most important advances introduced through the 2007 version of the code are:

Inclusion of a new extensive chapter on seismic safety assessment and retrofitting of existing buildings Inclusion of a linear elastic method for seismic safety assessment considering the inelastic behavior in

terms of approximate allowable demand/capacity ratios given depending on the damage level Inclusionof theperformance-based assessmentprinciplesforexistingstructures in seismicsafetyevaluation

and retrofitting Inclusion of different levels of design earthquakes (such as service, design and maximum earthquakes) and

performance levels (such as immediate occupancy, life safety and collapse prevention) to be consideredfor various types of buildings

Inclusion of single-mode and multi-mode push-over analysis for seismic safety assessment and retrofitting Inclusion of nonlinear time history analysis Inclusionof principles anddetails relatedwithconventional retrofitting techniques (such as concrete jacket-

ing, strengthening with steel members, and shear wall additions) and retrofitting using innovative materials(such as fiber reinforced polymers).

As known, in performance-based assessment, seismic performance of the building is determined based on theextent anddistribution of structural memberdamages. In the code, the damagelevels are determined dependingon the concrete compressive strain at the extreme compression fiber (either on the cover or core depending onthe damage level) and tensile reinforcement strain, which are calculated through the rotations of plastic hingeswhen push-over analysis is carried out. When distributed plasticity assumption is used, the critical strains canbe evaluated directly.

5.8 Change of Code Recommended Seismic Load in Time

Based on the above explanations, a summary of the variation of the base shear coefficient over time for afour-story reinforced concrete building having ductile frames and located on Z2 type ground in the south partof Istanbul is presented in Fig. 7. It should be noted that slight changes of the given values may be possiblebased on assumptions related with the dynamic characteristics of the building and the ground. Furthermore,it should be taken into consideration that Istanbul was designated as Seismic Zone II until 1996. After 1996,south part of Istanbul has been designated as Seismic Zone I. While there are buildings of various heights indifferent parts of Turkey, in cities majorityof existing buildings consist of four to five-story reinforced concreteframe buildings similar to the building considered for the calculation of the base shear coefficients in Fig. 7.

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0.00

0.03

0.06

0.09

0.12

0.15

1944 1961 1968 1975 1998 2007

Year

Fund

amentalbaseshear

coefficient

Fig. 7 The variation of the base shear coefficient required by the code by time (excluding the 1940 regulation)

5.9 Other Issues

It is important to note that while the Turkish Seismic Design Code has been upgraded in certain time intervals,it is not possible to claim that buildings have been constructed following the codes valid in the time of theirconstruction due to lack of sufficient enforcement of the code. Unfortunately, only a small portion of theexisting buildings has been constructed in accordance with the Seismic Design Code until the 1999 Kocaeliearthquake, which has been a milestone in terms of public awareness. This earthquake has deeply affectedpublic and constructors in terms of potential threat to human lives and economy. Interestingly, the experienceddisaster has been far more effective on the awareness of the public and the attitude of constructors than therevisions in the Seismic Design Code.

An interesting example of non-compliance with the code regulations is the requirement on the seismicjoints between the adjacent buildings. Although several requirements on the seismic joints are present in thecode since 1940 Seismic Regulation, one can hardly see any proper seismic joint between existing adjacentbuildings, even nowadays.

It should be noted that further information on Turkish Seismic Design Code and its evolution by time canbe found elsewhere [1,2,8,13,23].

6 Conclusions

Milestones of evolution of seismic design in Turkey can be summarized as below:By 1940 Seismic Regulation:

The first set of rules for seismic design was introduced and a fundamental base shear coefficient of 0.10was considered.

By 1944 Seismic Regulation:

Seismic zones were included (after the revision in 1942). Fundamental base shear coefficient was revised(reduced) as 0.040.01.


Soilstructure interaction was taken into account implicitly based on type of the structure and the groundtype.


Ductility concept wassomehow introduced implicitly through columnandbeam confinement in thevicinityof joints. Confinement of the joint cores by transverse bars was required.

Minimum dimensions for columns, beams and shear walls for seismic design were defined. Dynamic characteristics of buildings were considered in the evaluation of base shear force. Building importance factor was introduced. Inverse triangular distribution of lateral forces was adopted. Torsional irregularity was taken into account.By 1975 Seismic Regulation:

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Ductility wasmentionedexplicitly both in member andstructural levels and taken intoconsideration duringboth analysis and design.

Seismic zone map issued in 1972 was taken into consideration. More detailed principles related with seismic-resistant detailing were introduced. Simple definition for classification of irregular and regular structures based on the structural configuration

was introduced.

Dynamic analysis requirement for irregular and high-rise structures was included. Quantitative shear design for joints was required. Concept of increased longitudinal reinforcement at the end zones of the walls was introduced.By 1998 Regulation:

Capacity design principles were introduced. Explicit definition of design earthquake in terms of occurrence probability was included. Explicit definition of acceptable structural performance against design earthquake was given. Elastic design spectrum was defined. Seismic load reduction factor as a function of ductility was introduced. More detailed definition of building importance factor was included. More detailed requirements on confinement and explicit rules for reinforcement detailing were included.

Definition and classification of irregularities were given quantitatively.

By 2007 Regulation:

New and extensive chapter is added on seismic safety assessment and retrofitting. This chapter includeselastic and inelastic performance based analysis approaches.

Different design earthquakes and performance levels are defined for various types of buildings. Principlesanddetailsof retrofittingtechniqueseitherusingconventional oradvancedmaterials are included.The authors believe that performance-based design, which is included in the most recent version of the TurkishSeismic Design Code for seismic safety assessment and retrofitting of existing buildings, will progress rapidlyto be utilized in seismic-resistant design of new structures as well.

Acknowledgments The authors acknowledge Turkish Earthquake Foundation and Prof. Nahit Kumbasar for providing previous

versions of the Seismic Design Code.

References

1. AIJ/JSCE/BU: Damage report on 1992 Erzincan earthquake, Turkey. Technical Report by Joint Reconnaissance Team ofArchitectural Institute of Japan, Japan Society of Civil Engineers, Bogazici University, Istanbul (1993)

2. AIJ/JSCE/JGS: Report on the damage investigation of the 1999 Kocaeli earthquake in Turkey. Technical Report by JointReconnaissance Team of Architectural Institute of Japan, Japan Society of Civil Engineers, The Japanese GeotechnicalSociety (2001)

3. Aydan, O.; Ulusay, R.; Kumsar, H.; Tuncay, E.: Site investigation and engineering evaluation of the Duzce-Bolu earthquakeof November 12, 1999. Technical Report, Turkish Earthquake Foundation, TDV/DR 0951 (2000)

4. Parsons, T.: Recalculated probability of M 7 earthquakes beneath the Sea of Marmara, Turkey. J. Geophys. Res. SolidEarth 109, Art. No. B05304 (2004)

5. State Statistical Institute, Results of Census in 2000. Ankara (2000)

6. The study on a disaster prevention/mitigation basic plan in Istanbul including seismic microzonation in the Republic ofTurkey. Technical Report prepared for JICA and IMM, Pacific Consultants, Oyo Corporation (2002)7. Utsu, T.: Table of world historical earthquakes (1990)8. Aydinoglu, M.N.: From seismic coefficient to performance based design: 40 years of earthquake engineering from an

engineers viewpoint. In: Proceedings of 6th Nat. Conf. on Earthquake Engineering, Maya Basin Yayin, 2007, pp. 1541(in Turkish with English abstract)

9. Ozmen, B.: Damage due to August 17, 1999 Izmit Bay Earthquake. Technical Report, Turkish Earthquake Foundation,TDV/DR 01053 (2000)

10. Pampal, S.; Ozmen, B.: Development of earthquake zoning maps of Turkey. In: Proceedings of Sixth National Conferenceon Earthquake Engineering, Maya Basin Yayin, 2007 (in Turkish with English abstract)

11. Ambraseys, N.N.: Comparison of frequency of occurrence of earthquakes with slip rates from long-term seismicity data:the cases of Gulf of Corinth, Sea of Marmara and Dead Sea fault zone. Geophys. J. Int. 165, 516526 (2006)

12. MWH Engineering and Consultancy.: Country strategy paper for natural disasters in Turkey. Draft Final Report submittedto Japan International Cooperation Agency (2004)

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13. Gulkan, P.: Building code enforcement at municipal level in Turkey: failure of public policy for effective building hazardsmitigation. Disaster Management Implementation and Research Center, Middle East Technical University (2000)

14. Ministry of PublicWorks.: ZelzeleMntkalar MuvakkatYap Talimatnamesi. (Temporary building regulation in earthquakezones.) Devlet Matbaas, Ankara (1940) (in Turkish)

15. Ministry of Public Works.: ZelzeleMntkalarMuvakkatYap Talimatnamesi. (Temporary building regulationin earthquakezones.) Yeni Cezaevi Matbaas, Ankara (1944) (in Turkish)

16. Ministry of Public Works and Housing.: Afet Blgelerinde Yaplacak Yaplar Hakknda Ynetmelik. (Regulation for struc-tures in disaster areas.) Devrim Basm ve Cilt Evi, Ankara (1961) (in Turkish)

17. Ministry of Public Works and Housing.: Afet Blgelerinde Yaplacak Yaplar Hakknda Ynetmelik. (Regulation for struc-tures in disaster areas.) Basbakanlk Devlet Matbaas, Ankara (1968) (in Turkish)

18. Ministry of Public Works and Housing.: Afet Blgelerinde Yaplacak Yaplar Hakknda Ynetmelik. (Regulation for struc-tures in disaster areas.) Official Gazette (1975) (in Turkish)

19. TSE-TS-500.: Requirements for design and construction of reinforced concrete structures. Turkish Standards Institution,Ankara (1984) (in Turkish)

20. TSE-TS-500.: Requirements for design and construction of reinforced concrete structures. Turkish Standards Institution,Ankara (2000) (in Turkish)

21. Ministry of Public Works and Housing.: Afet Blgelerinde Yaplacak Yaplar Hakknda Ynetmelik. (Regulation for struc-tures in disaster areas.) Official Gazette (1998) (in Turkish)

22. Ministry of Public Works and Housing.: Deprem Blgelerinde Yaplacak Binalar Hakknda Ynetmelik. (Regulation forbuildings in seismic areas.) Official Gazette (2007) (in Turkish)

23. Bayulke, N.N.: Reedition history of seismic design code of Turkey: historical development of the earthquake resistantbuilding design code of Turkey. Bull. IISEE 26, 413429 (1992)

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Documents

Earthquakes, Existing Buildings and Seismic Design Codes in Turkey