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Earthquakes, Existing Buildings and
Seismic Design Codes in Turkey
A. Ilki & Z. Celep
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
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Arab J Sci Eng (2012) 37:365–380
DOI 10.1007/s13369-012-0183-8
R E S E A R C H A RT I C L E - C I V I L E N G I N E E R I N G
A. Ilki · Z. Celep
Earthquakes, Existing Buildings and Seismic Design Codes
in 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 tremendous
amount of experimental, analytical and numerical studies, significant contributions have been made for a better
understanding 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 more
realistic considerations of seismic demand, seismic response and seismic capacity. In this paper, after outlining the performance of existing buildings in Turkey during recent earthquakes (particularly Kocaeli 1999 and
Duzce 1999 Earthquakes), and by focusing on the observed common structural deficiencies, a brief summary
of the evolution of the Turkish Seismic Design Code in the last decades is presented. It is important to note
that the poor seismic performance of existing buildings in Turkey outlined in this study is not directly related
to the inefficiency of the relevant seismic design codes, but rather to extremely low quality construction and
the absence of a strict inspection system at the time of their construction. It should also be highlighted that the
lessons learnt from the catastrophic consequences of recent earthquakes, revisions in the seismic design code
and 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, Turkey
E-mail: [email protected]
Z. Celep
Civil 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 [1–3]. According
to 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, seismic
safety of existing building stock, is a major concern in Turkey, where various versions of seismic design code
have been published since 1940 always adopting more strict requirements. In spite of presence of a seismic
design code for quite a long time, catastrophic consequences were observed after all major earthquakes. These
consequences were mainly due to substandard construction practice in the absence of a strict inspection system, although the seismic design codes were reflecting the up-to-date seismic knowledge level. Parallel to the
developments in the field of earthquake engineering in the world, many vitally important concepts and details
have 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 and
Duzce Earthquakes, with a special focus on common structural deficiencies, the evolution of Turkish Seismic
Design Code is presented. For the sake of completeness, information on the demographical and the economical
characteristics 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 technical
engineering issues, but are also strongly dependant on the economical, social and cultural situation. The basic
demographic 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 that
Turkey spanning between Asia and Europe has quite a large area, compared to Japan and EU countries, and it
has a fairly high and young population. Population growth rate is around 1% Turkey’s gross domestic product
(GDP) is approximately one-seventh of that of Japan and GDP per capita in Turkey is around one-quarter of
that of Japan. Considering the consequences of past seismic events of similar magnitudes in Turkey, Japan and
USA (for example 1999 Kocaeli Earthquake in Turkey, 1995 Kobe Earthquake in Japan and 1989 Loma Prieta
Earthquake in USA) together with the economical data presented in Table 1, one can easily assess that the life
losses, the injuries and the extent of structural damages are closely related to the economic development of
the country.
3 Data on Existing Building Stock
Unfortunately, a reliable and scientific building inventory covering all areas of Turkey and reflecting the current situation is not available. However, information obtained in the census carried out by the State Statistical
Institute [5] can be used for evaluating the type of buildings and their structural systems. According to the
census results total number of buildings in Turkey by 2000 is 7,838,675. The distribution of buildings in terms
of 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 Turkey. Some typical damages observed in existing reinforced concrete structures are shown in Fig. 4. Basic
weaknesses of reinforced concrete structures mostly observed can be classified as:
Table 1 Basic demographic and economic data
Country
Turkey
Japan
EU
Area (km2 )
780,000
378,000
4,300,000
Population
71,000,000a
127,000,000a
490,000,000a
Median age
28.6a
43.5a
NA
Population growth (%)
1.04a
−0.09a
0.16a
9
b
b
GDP (USD) 10
640
4.218
13.080b
GDP/capita (USD)
9.100b
33.100b
29.900b
b
b
GDP growth rate (%)
6.1
2.2
3.2b
a 2007 estimates
b 2006 estimates (source: https://www.cia.gov/library/publications/the-world-factbook/index.html)
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USA
9,800,000
301,000,000a
36.6a
0.89a
13.060b
43.800b
2.9b
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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 transverse
bars 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 confined
masonry due to their weak reinforced concrete structural systems. In such cases, infill walls resist the seismic
loads 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 of
the catastrophic consequences experienced after earthquakes. These errors, which cause premature failures of
structural members or structures, are basically due to improper application practices on-site. The most common
application 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, and
deterioration of structural system by time due to low-quality materials and lack of maintenance. Most common
problem 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 out
and 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 columns
and beams in reinforced concrete structures or walls in masonry structures), which are not conveniently located
or 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 of
a sufficient inspection mechanism, particularly a large number of buildings constructed before 1999 Kocaeli
Earthquake, do not satisfy requirements of the related national codes and standards. Consequently, they cannot
be 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 this
figure, 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 map
issued 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 the
population live in Zones I, II and III, respectively, while only approximately 15% of the population lives in
Zones IV and V. Furthermore, it is worth noting that further information on the history of the seismicity maps
of Turkey and additional seismic maps can be found elsewhere [10].
5 Evolution of Seismic Design Code in Turkey
While many catastrophic earthquakes have hit various areas of Turkey in history [11], the first major catastrophic
natural disaster experienced by Republic of Turkey was the Erzincan earthquake in 1939. The magnitude of
the 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 established
in 1940 by the Ministry of Public Works, followed by another version in 1942 annexed with a seismic zone
map. This seismic regulation was revised in 1944 within the articles of Law No. 4623 [12]. The law stated that
any 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 and
consequently no demolishment was done [12]. The seismic regulation was updated in 1949 and 1953 to reflect
the amendments of the seismic zone map without any major change in the code [13]. By the establishment
of Ministry of Reconstruction and Resettlement in 1958, the disaster prevention policy was upgraded and the
formulation of the base shear coefficient was revised in 1961 [2]. The next revisions in 1968 and 1975 brought
important enhancements to the seismic design and introduced the international developments to the engineering 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 important
detailing issues for seismic design. The most recent version of the code issued in 2007, particularly has been
a very important step towards the displacement-based design through the related requirements for the seismic
assessment of existing buildings and retrofitting. The evolution of the seismic design code is summarized in
more detail below.
5.1 1940 Seismic Regulation [14]
This was the first seismic regulation in Turkey. Besides several rules related to construction, materials and
workmanship, this code gave the fundamental base shear coefficient of 0.10 for calculation of the lateral seismic
load. In case of presence of the wind load (W ), the design lateral load (H ) is calculated by Eq. 1, where only
half of the live load (P) is considered in addition to the dead load (G). On the other hand, half of the wind
load (W ) is included as well. No specific distribution of the lateral load along the height of the building was
defined in this code.
P
W
H = 0.10 G +
+
(1)
2
2
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5.2 1944 Seismic Regulation [15]
This regulation included a seismic zone map having two seismic zones, Zones I and II. Areas outside Zones
I and II were considered to be safe in terms of seismicity. The fundamental base shear coefficient (the ratio
of the base shear force to the seismic weight of the building) was adopted to be 0.02–0.04 and 0.01–0.03 for
Zones 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]. Like
in previous version, in this regulation, the geotechnical conditions of the construction site and the structural
characteristics were not taken into account. Furthermore, the distribution of the lateral load along the height
of the building has still not been defined as well [1].
5.3 1961 Seismic Regulation [16]
In 1960s and 1970s, due to very rapid industrialization and urbanization, the amount of constructions increased
tremendously, particularly in cities such as Istanbul, Izmir and Bursa. Therefore, a great portion of the existing
buildings were demolished and reconstructed by increasing number of stories by considering 1961 Seismic
Regulation [8]. In this version of the regulation, parameters related to the seismic zone, type of the structural 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 in
this code to prevent excessive irregularities in plan and to minimize the potential negative effects of global
torsion. However, it was not mentioned how to deal with the torsion of the structure. The code required that
all parts of the building should resist seismic lateral load given in Eq. 2. In this equation, C and n were the
fundamental base shear coefficient and the live load reduction factor, respectively. The live load reduction
factor was given as 0.5 for ordinary buildings (residential buildings), whereas no load reduction was allowed
for densely populated buildings (theaters, hotels, factories and office buildings). The fundamental base shear
coefficient was calculated by Eq. 3, where Co was a coefficient depending on the height of the building, n1
was a coefficient related to soil conditions (Soil type I, II and III) and to type of structural system (reinforced
concrete or steel), and n2 was the seismic zone coefficient (Zones I and II). The numerical values of Co and
n1 are summarized in Tables 2 and 3, respectively. It should be noted that the interaction between soil and
structure was somehow taken into account through the coefficient n1 . The coefficient n2 was to be taken as 1.0
and 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 into
account properly in this code. Clearly, this negligence may result with unnecessarily high seismic design loads
for relatively high-rise structures, whereas the seismic design loads taken into account for low-rise structures
may be on the unsafe side.
H = C(G + n P) + W/2
(2)
C = Co n 1 n 2
(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% in
allowable 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 of Co depending on building height
Height of building (m)
Co coefficient
<16
16–22
22–28
28–34
34–40
> 40
0.06
0.07
0.08
0.09
0.10
+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
II
III
I rock, hard soil, II medium soil, III soft soil
0.8
0.9
1.0
0.6
0.8
1.0
5.4 1968 Seismic Regulation [17]
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 × story
height)]
• 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 reinforcement is to be doubled with respect to the mid-height of the column and mid-span of the beam).
• Confinement reinforcement requirement in the beam–column 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 this
equation, W is the total weight of the building to be considered for the seismic analysis (Eq. 5) and C is the
fundamental base shear coefficient to be calculated by Eq. 6. It should be noted that according to this code
no 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 residential
buildings. Co given in Eq. 6 is the seismic zone factor (0.06, 0.04 and 0.02, for Zones I, II and III, respectively),
α is the coefficient reflecting ground conditions (0.8, 1.0 and 1.2, for hard, medium and soft soil, respectively),
β 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, and
1.0 for ordinary buildings such as residential, office and industrial buildings, hotels, restaurants, etc.), and γ is
the 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 the
building (Eq. 8) to be used unless the period is not calculated by using a sophisticated method. In this equation
H and D are the height of the building (m) and the plan dimension of the building (m) in the direction of the
considered lateral load.
W =
Wi =
γ =
F = CW
(4)
G i + n i Pi (i: story number)
(5)
C = Co αβγ
(6)
γ = 1 (T ≤ 0.5 s)
(7a)
0.5
≥ 0.3 (T > 0.5 s)
T
(7b)
0.09H
T = √
D
(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. In
this equation, Fi , Wi and h i are the lateral forces applied to the ith story floor, the weight of the story and the
height of the story measured from the foundation level.
Wi h i
Fi = F Wi h i
(9)
The code permitted an increase of 50% in allowable stresses of concrete and steel in case of seismic design
assuming that the design is carried out by using allowable stress approach. Furthermore, an increase of 50 and
30% 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 amount
of longitudinal and transverse reinforcement should be placed, was not introduced in this code. In the code and
its latter revisions, the effect of earthquake was taken into account separately without considering the wind
load. Consequently, analysis against earthquake and wind loads are carried out separately and design is carried
out according to the most unfavorable case. In 1972, the seismic zone map was divided into five seismic risk
zones (Fig. 5a), including the zone with no seismic risk [2].
5.5 1975 Seismic Regulation [18]
1975 Seismic Regulation has been valid for more than 20 years. Therefore, as seen in Fig. 3, a great portion of
existing 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 of
structural ductility for the first time implicitly according to the lateral load resisting system of the structure by
introducing 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 beam–column joints
• Inclusion of the ground dominant period into the equation given for determination of the spectrum coefficient
• Inclusion of an explicit definition of irregular buildings (although the definitions of irregularities were not
sufficiently 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 shear
walls 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, S and I are the
fundamental base shear coefficient, the seismic zone coefficient (0.10, 0.08, 0.06 and 0.04, for Zones I, II, III
and 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 consideration
of 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 by
increasing design base shear force in this version of the seismic design code. The dynamic coefficient (spectrum
coefficient) is to be evaluated by Eq. 11, where To is the effective period of the ground in seconds. It should be
noted that the dynamic coefficient should be assumed as 1.0 for one and two-story structures and all masonry
buildings. For the fundamental period of buildings, in addition to Eq. 8, which has already been given in the
1968 regulation, an alternative formula was given as well (Eq. 12). In Eq. 12, N is the number of stories. The
building importance factor, I was almost same as in the 1968 code (either 1.0 for ordinary buildings, or 1.5 for
important or densely populated buildings.
C = Co K S I ≥
123
Co
2
(10)
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Table 4 The structure type coefficient, K
Ka
Structure type
Ductile framesb
(a) 0.60, (b) 0.80, (c) 1.00
Non-ductile framesb
(a) 1.20, (b) 1.50, (c) 1.50
Steel frames with bracingsb
(a) 1.20, (b) 1.50, (c) 1.60
Shear wall–ductile framesb,c
(a) 0.80, (b) 1.00, (c) 1.20
Shear wall structures without frames
1.33
Masonry buildings
1.50
Others
1.00
a The minimum value of K is 1.0 for one or two-story structures
b Having (a) reinforced concrete or reinforced masonry infill walls, (b) unreinforced masonry infill walls, (c) light weight or few
infill walls, or prefabricated concrete infill walls
c The ductile frames should resist at least 25% of the lateral loads
S=
1
≤ 1.0
|0.8 + T − To |
T = (0.07 − 0.10)N
(11)
(12)
The distribution of the base shear force along the height of the building was again adopted according to the
first mode shape of the building (inverse triangular distribution), with an additional singular force to be applied
to the top story (Ft ), to take implicitly into account the effects of higher modes approximately as given in Eqs.
13 and 14. According to this version of the code, Ft was assumed to be zero for low-rise buildings (H/D ≤3).
Wi h i
Fi = (F − Ft ) Wi h i
Ft = 0.004F
H
D
(13)
2
≤ 0.15F
(14)
It is important to note that the permission to increase the allowable stresses of concrete and steel was reduced
to 33% from 50% in the code. Additionally, the permitted increase of the ground allowable stress was reduced
to 33% for the ground types I, II and III. No increase for the ground allowable stresses was permitted for
the ground type IV, as well as for the concrete and steel allowable stresses for structures on the ground type
IV. It should be noted that while the building weight to be considered for calculation of the base shear force
was similar to the 1968 code (Eq. 5), the values of live load reduction factor (n) were slightly revised. This
value was 0.8 for storage type structures, 0.6 for schools, theaters, concert halls, shops, dormitories, and 0.3
for residential buildings, offices, hospitals, hotels. It is important to emphasize that the reinforced concrete
design and construction code in Turkey was revised in 1984 and 2000 [19,20]. After 1984, while still the use of
allowable stress design was permitted, the ultimate strength design was encouraged explicitly. Consequently,
after mid-1980s, design engineers began to use the ultimate strength design instead of the allowable stress
approach. In the most recent version of the reinforced concrete design and construction code published in
2000, the use of the allowable stress design is not permitted any more. In 1996, the seismic zone map has been
revised once more (Fig. 5b). In the revised seismic zone map, which still has five seismic zones, the area for
Zone I is significantly increased.
5.6 1998 Seismic Regulation [21]
After more than 20 years of the publication of the 1975 code, a revised version was published in 1998 just 1 year
before the catastrophic earthquakes experienced in 1999. Therefore, the 1998 code and the earthquakes experienced created a milestone in terms of earthquake-resistant design and construction, as well as the demand
of public for safe housing. Presently, most engineers in Turkey believe that the buildings constructed after
1998–1999 are much safer against earthquakes than the older buildings.
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Table 5 The building importance factor, I
Purpose of occupancy or type of building
Importance
factor (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.
2. Intensively and long-term occupied buildings and buildings preserving valuable goods
(a) Schools, dormitories, military barracks, prisons, etc.
(b) Museums
3. Intensively but short-term occupied buildings
Sport facilities, cinema, theatre and concert halls, etc.
4. Other buildings
Buildings other than defined above (residential and office buildings, hotels, building-like industrial
structures, etc.)
1.5
1.4
1.2
1.0
Table 6 Characteristic spectrum periods
Local site class
T A (s)
T B (s)
Z1
Z2
Z3
Z4
0.10
0.15
0.15
0.20
0.30
0.40
0.60
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 columns
are stronger than beams framing into the same joint. Furthermore, the shear capacity of beams and columns
as well as shear walls is kept higher than their bending capacity, so that ductile failure is ensured in case of
seismic loads higher than that considered in seismic design.
The design earthquake considered in the code corresponds to an earthquake with the return period of
475 years for ordinary buildings (for building importance factor 1.0) and 2,475 years for the most important
buildings (for building importance factor 1.5). The probabilities of exceedence for these two cases are 10 and
2% in 50 years, respectively. In the code, the spectral acceleration coefficient A(T) is given by Eq. 15, where Ao ,
I and S(T) are the effective seismic acceleration coefficient (seismic zone coefficient), the building importance
factor and the elastic spectrum coefficient evaluated for 5% damping ratio. The effective seismic acceleration
coefficient (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 previous
versions, (Table 5). Spectrum coefficient (S(T)) is determined through Eqs. 16a, 16b, 16c as a function of the
fundamental period the building (T) and the characteristic spectrum periods (T A and T B ), which are to be
determined depending on the ground type. The characteristic spectrum periods for various ground conditions
are given in Table 6. In this table, it is apparent that Z1 represents strongest ground conditions, while Z4
corresponds to the weakest. The variation of spectrum coefficient with respect to the fundamental period of
the building is shown in Fig. 6. It should be noted that the fundamental period of the building can be calculated
by Eq. 17 or Eq. 18. Equation 17 is the well-known Rayleigh equation, where mi is the mass of the ith story,
F f i is the fictitious lateral load acting on the ith story and d f i is the corresponding displacement of the ith
story in the direction of F f i . On the other hand, Eq. 18 is an empirical relation, where Ct is a coefficient and
depends on structural system of the building (0.08 for steel frames, 0.07 for reinforced concrete frames, ≤ 0.05
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375
(a)
S(T)
2.5
S(T) = 2.5 (TB / T )0.8
1.0
TA
T
TB
A o I S(T)/R a(T)
(b) 0.30
Zone 1
0.25
Zone 2
0.20
Zone 3
Zone 4
0.15
0.10
0.05
0.00
0
0.5
1
1.5
2
T(s)
Fig. 6 a Elastic spectrum coefficient S(T) and b spectral acceleration coefficient depending on the fundamental building period
T for four seismic zones
for shear wall buildings) and H N is the total height of the building.
A(T ) = Ao I S(T )
S(T ) = 1 + 1.5
T
(0 ≤ T ≤ T A )
TA
S(T ) = 2.5 (T A ≤ T ≤ TB )
TB
T
(15)
(16a)
(16b)
0.8
(T ≥ TB )
(16c)
N
N
m i d 2f i /
Ff id f i
T1 = 2π (17)
S(T ) = 2.5
i=1
i=1
3/4
T1 ∼
= T1A = Ct HN
(18)
For using inelastic capacity of the structures (at least partially), certain level of inelastic deformations (controlled damages) beyond elastic limits are allowed under the design earthquake explicitly, provided that the
building does not collapse, life safety is ensured and damages are kept within the controlled limits. For utilizing inelastic deformations, the structural system should have a certain level of ductility. According to the
code, the buildings can be designed considering two levels of ductility; normal or high. There are several
rules, particularly in terms of the application of the capacity design, construction details and irregularities for
classifying the structural systems as normal or high ductility. Since inelastic deformations are allowed, the
lateral load demand evaluated by using the elastic design spectrum is reduced depending on the characteristics
of the structural system by the seismic load reduction factor Ra (T) given by Eqs. 19a, 19b. Obviously, if the
structural system possess the characteristics such that the system can be classified as a high ductility system, the
reduction 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
Systems
of normal
of high
ductility level ductility level
(1) Cast-in-situ reinforced concrete buildings
(1.1) Buildings in which seismic loads are fully resisted by frames
(1.2) Buildings in which seismic loads are fully resisted by coupled structural walls
(1.3) Buildings in which seismic loads are fully resisted by solid structural walls
(1.4) Buildings in which seismic loads are jointly resisted by frames and solid and/
or coupled structural walls
(2) Precast reinforced concrete buildings
(2.1) Buildings in which seismic loads are fully resisted by frames
with connections capable of cyclic moment transfer
(2.2) Buildings in which seismic loads are fully resisted by single-story hinged
frames with fixed-in bases
(2.3) Buildings in which seismic loads are fully resisted by prefabricated solid
structural walls
(2.4) Buildings in which seismic loads are jointly resisted by frames with connections
capable of cyclic moment transfer and cast-in-situ solid and/or coupled structural walls
(3) Steel buildings
(3.1) Buildings in which seismic loads are fully resisted by frames
(3.2) Buildings in which seismic loads are fully resisted by single-story hinged frames
with fixed-in bases
(3.3) Buildings in which seismic loads are fully resisted by braced frames or cast-in-situ
reinforced concrete structural walls
(a) Concentrically braced frames
(b) Eccentrically braced frames
(c) Reinforced concrete structural walls
(3.4) Buildings in which seismic loads are jointly resisted by frames and braced frames or
cast-in-situ reinforced concrete structural walls
(a) Concentrically braced frames
(b) Eccentrically braced frames
(c) Reinforced concrete structural walls
4
4
4
4
8
7
6
7
3
6
–
5
–
4
3
5
5
8
4
6
3
–
4
–
7
6
4
–
4
–
8
7
acceleration coefficient A(T ) = Ao IS(T )/Ra (T ) for the four different seismicity levels is also presented in
Fig. 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 concrete
frame of normal ductility) constructed on the soil class Z1(T A = 0.1 s, TB = 0.3 s).
Ra (T ) = 1.5 + (R − 1.5)
T
(T ≤ TA )
TA
Ra (T ) = R (T > T A )
(19a)
(19b)
As seen in Eqs. 19a, 19b the seismic load reduction factor Ra can 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 seismic
load reduction factor does not represent the structural system ductility only, but it includes the over-strength
factor as well. Finally, the reduced base shear force (Vt ) can be calculated by Eq. 20, where W is the total
weight 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 = W
A(T )
≥ 0.10 Ao I W
Ra (T )
(20)
Requirements on the combinations of the seismic loads with the other loads are given in the related codes for
typical buildings, such as the reinforced concrete design and construction code [20]. In this code, the wind and
seismic loads are not considered in one single combination together. Some typical design combinations given
in this code are 1.4G + 1.6Q, G + Q + E, 0.9G + E, G + 1.3Q + 1.3W and 0.9G + 1.3 W, where G, Q, E and W
represent 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 to
the fact that all types of structural systems have almost same level of seismic safety, if designed and constructed
properly, the damage experienced by inadequate reinforced concrete structures (so-called reinforced concrete
structures, built without receiving proper engineering service) caused a misconception such that reinforced
concrete structures are not seismically safe, but steel structures are. Consequently, while still being in marginal 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.
5.7 2007 Seismic Regulation [22]
Based on the demand of people and the official institutions for earthquake safe environment after the earthquakes experienced in 1999, many structures were investigated in terms of seismic safety and some of these
were retrofitted. However, due to lack of official guidelines and standards about seismic safety assessment and
retrofitting, in many cases non-standard and sometimes inappropriate approaches were being used by design
engineers while analyzing or retrofitting the existing buildings. Therefore, the most recent version of the seismic design code published in 2007 includes the issues on seismic safety assessment of existing buildings and
retrofitting comprehensively. The code has only minor revisions in the provisions related to reinforced concrete
buildings to be newly designed. However, the seismic safety requirements for steel structures, which were not
addressed 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, has
been 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
• Inclusion of the performance-based assessment principles for existing structures in seismic safety evaluation
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 considered
for 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
• Inclusion of principles and details related with conventional retrofitting techniques (such as concrete jacketing, 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 the
extent and distribution of structural member damages. In the code, the damage levels are determined depending
on the concrete compressive strain at the extreme compression fiber (either on the cover or core depending on
the damage level) and tensile reinforcement strain, which are calculated through the rotations of plastic hinges
when push-over analysis is carried out. When distributed plasticity assumption is used, the critical strains can
be 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 a
four-story reinforced concrete building having ductile frames and located on Z2 type ground in the south part
of Istanbul is presented in Fig. 7. It should be noted that slight changes of the given values may be possible
based 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 in
different parts of Turkey, in cities majority of existing buildings consist of four to five-story reinforced concrete
frame buildings similar to the building considered for the calculation of the base shear coefficients in Fig. 7.
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Fundamental base shear
coefficient
378
0.15
0.12
0.09
0.06
0.03
0.00
1944
1961
1968
1975
1998
2007
Year
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 their
construction due to lack of sufficient enforcement of the code. Unfortunately, only a small portion of the
existing buildings has been constructed in accordance with the Seismic Design Code until the 1999 Kocaeli
earthquake, which has been a milestone in terms of public awareness. This earthquake has deeply affected
public and constructors in terms of potential threat to human lives and economy. Interestingly, the experienced
disaster has been far more effective on the awareness of the public and the attitude of constructors than the
revisions in the Seismic Design Code.
An interesting example of non-compliance with the code regulations is the requirement on the seismic
joints between the adjacent buildings. Although several requirements on the seismic joints are present in the
code since 1940 Seismic Regulation, one can hardly see any proper seismic joint between existing adjacent
buildings, even nowadays.
It should be noted that further information on Turkish Seismic Design Code and its evolution by time can
be 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 0.10
was considered.
By 1944 Seismic Regulation:
• Seismic zones were included (after the revision in 1942). Fundamental base shear coefficient was revised
(reduced) as 0.04–0.01.
By 1961 Seismic Regulation:
• Soil–structure interaction was taken into account implicitly based on type of the structure and the ground
type.
By 1968 Seismic Regulation:
• Ductility concept was somehow introduced implicitly through column and beam confinement in the vicinity
of 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 was mentioned explicitly both in member and structural levels and taken into consideration during
both 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 includes
elastic and inelastic performance based analysis approaches.
• Different design earthquakes and performance levels are defined for various types of buildings.
• Principles and details of retrofitting techniques either using conventional or advanced materials are included.
The authors believe that performance-based design, which is included in the most recent version of the Turkish
Seismic Design Code for seismic safety assessment and retrofitting of existing buildings, will progress rapidly
to 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.
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