SEISMIC SAFETY SCREENING METHOD FOR ISTANBUL METROPOLITAN CITY

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1 SEISMIC SAFETY SCREENING METHOD FOR ISTANBUL METROPOLITAN CITY Pinar OZDEMIR 1 and Beyza TASKIN 1 ABSTRACT: Most of Turkey s existing building stock was constructed based on older design standards or non-engineered design. This doesn t mean that all of these buildings might be hazardous or suffering severe damage by earthquakes. It is necessary to identify those that are at risk and carry out reconstruction or seismic retrofit. This is a common problem of all earthquake-prone countries, is not unique problem for Turkey. But, reliability or inadequacy in construction procedure is unique to each country and for this reason, the method for seismic evaluation of existing buildings should be determined carefully. Also, selected procedure should be tested and calibrated based on the data compiled from damage surveys in relevant country. In this study proposed procedure is called Seismic Safety Screening Method, SSSM, which is adopted and modified from Japanese Seismic Index Method, JSIM. Both methods give seismic capacity index of buildings, which is checked against demand index, by referring the orientation, size and material properties. If seismic capacity index is less than seismic demand index, it indicates that the building might suffer severe damage or collapse. The method can be applied to medium-rise reinforced concrete buildings. The seismic capacity index gives effective information to quantitative understanding, not a subjective assessment data. This index is then modified by several coefficients that reflect the quality of materials and workmanship, and architectural features of Turkey s buildings. The calibration of several coefficients in this proposed method has been done by considering the studies carried out in the pilot area, Zeytinburnu, after Istanbul Earthquake Master Plan was completed. Nonlinear static or dynamic analyses results of several existing buildings are used to calibrate coefficients. Here, as an example, nonlinear dynamic analyses results of moderately damaged building by 1999 Marmara Earthquake and SSSM results are compared. It can be concluded that SSSM is suitable for Turkey s buildings. KEYWORDS: Istanbul Earthquake Master Plan, Seismic safety, Building evaluation, SSSM, JSIM, Reinforced-concrete building structures. 1. INTRODUCTION Advances in earthquake engineering, developments in measurement technology and enrichment of the strong motion database result in the improvements of seismic design codes almost every 20 years. Evaluation and determination of the earthquake safety of structures, especially in metropolitan cities is a seriously difficult issue. Having its settlement in two continents with a land area of 5196 km 2 and holding a population of 15 millions, Istanbul Metropolitan City suffered many disastrous historic earthquakes, still successfully preserving the heritage molten within a pot of engineered twenty-first century structures making the city one-of-its-kind. 1 Asst.Prof.Dr., Istanbul Technical University, Department of Civil Engineering, Maslak, 34469, Istanbul, Turkey. 1

2 Recently severe earthquakes with in the region have cost losses life and property. Determining the location of seismically vulnerable buildings structures in the current building stock is therefore a high priority task within the concept of urban renovation issues. In order to cope with the seismic safety evaluation of large number of existing buildings, it is necessary to use simplified techniques, which can predict the seismic safety of the existing buildings in relatively shorter time. The expected outcome from such an analysis is to determine which buildings are at relatively higher risk of collapse. Current approaches during seismic safety assessment lead different procedures; namely, sidewalk survey as in [1-2] unless the building stock of concern is engineered, registered and benchmark year is known; more complex procedures as in Index Methods [3] based on the structural dimensions and data; application of nonlinear static or dynamic procedures [4-6] for estimating demand and capacities. Considering the JICA report [7] exhibiting four probable scenario earthquakes it is anticipated that up to 70,000 building in Istanbul have the potential of heavy damage partial or total collapse. Taking into account that Istanbul Metropolitan city consist of almost 1,000,000 buildings it is well worth to investigate the risky areas. Istanbul Metropolitan City Municipality invited four local universities to prepare Istanbul Earthquake Master Plan [8], whose core is mainly based on inspection and evaluation of buildings as a firststep-pilot-study. Istanbul Technical University proposed one of the evaluation methods called Seismic Safety Screening Method, SSSM, which is adopted and modified from Japanese Seismic Index Method, JSIM. 2. SEISMIC SAFETY SCREENING METHOD Istanbul Technical University s researchers have proposed SSSM based on their experiences after the recent disastrous earthquakes of 1992 Erzincan, 1998 Adana-Ceyhan, 1999 Marmara and Duzce, 2003 Bingol and 2005 Karliova Earthquakes. Depending on inadequate control during construction, poor material quality and even non-engineered design make the sidewalk survey procedure non applicable to the existing building stock. Having the purpose of preventing misleading results SSSM is proposed hence it is trust worthy computational background which is simplified from dynamic theory. In the original Japanese form of the method [3], the seismic performance of a structure is evaluated using the seismic capacity index, I s. I s =E o S D T (1) where E o is a basic seismic capacity index, S D is a structural irregularity index and T is a time deterioration index. The obtained I s value should be compared with seismic demand index of structure I so, which is calculated by following equation. If seismic capacity index I s is less than seismic demand index I so, it indicates that collapse prevention performance level would not be met, the building might suffer severe damage or collapse. I so =E s Z G U (2) Here E s is index for basic seismic demand; Z is index for regional seismicity; G is index for local soil conditions; and U is index for usage. Three levels could be applied to medium-rise RC buildings according to the purpose of seismic capacity evaluation, and the structural characteristics of the building concerned. Equal energy concept is the main idea for the calculation of the basic seismic index, E o, which is obtained from multiplication of a strength index C and a ductility index F as in equation (3). 2

3 E o = C F (3) Strength index, C, is the smaller one of either the corresponding shear force obtained from capacity moment or shear strength itself. The ductility index, F, is a function of failure mode, which is chosen after the selection of the related shear value as an indicator of flexural or shear-dominant behavior of the structural member. The strength index, C is computed with the lateral strength of vertical members based on the assumption that beams are strong enough and the expected structural damage on beams can not be as important as vertical elements damages. The lateral strength of each vertical member is the minimum of the flexural strength and shear strength, equation (4). k C k Qmu ; k Q W su min (4) k C = k th vertical member s strength index; k Q mu = flexural strength of k th vertical element; k Q su = shear strength of k th vertical element; and W = total weight above the story concerned. The calculation should be done for both directions. Adopted method SSSM is the modification of the second level of JSIM. Since some columns are indirectly combined with the main frames of the structure, they do not fully participate in the lateral load transfer within the structural system for many of the Turkish type of structures. Therefore calculation of the shear force obtained from capacity moment is different than original method in the means of the inflection height coefficient. mu M u ho Q / (5) where is the coefficient for inflection height, and h 0 is the clear height of column or wall. If column is not connected to the frame is equal to 0. If it is connected then on the strong direction of column can be taken as 2 while on the weak direction it is 1.7. When walls are under consideration, coefficient becomes 1. The criteria to realize this selection is accepted as it is for Istanbul type of structures, however the coefficients for calculating a general score (index value) I S, are modified considering an average concrete strength of f ck =10 MPa which is a statistical concrete comprehensive strength, a minimum reinforcement ratio of =0.01 and very low levels of ductile behavior. The new coefficients and parameters are calibrated by applying the method to a 2401 number of buildings in Zeytinburnu pilot district of Istanbul Province. For the calibration of the SSSM, the results of nonlinear static analysis carried out for 12 of these buildings are utilized. During the nonlinear analyses, actual (as-built) structural characteristics of these buildings are used for input. During all steps of nonlinear analysis, the displacement profile is taken from the shape of the first mode in the corresponding direction. From the results of these analyses, base shear force capacities corresponding to collapse prevention performance level were determined for each building. Then the ratios of base shear force capacity to structural weight were calculated; in such as way that base shear coefficient can lead the calibration coefficients in the SSSM. Therefore, the final form of the SSSM is transformed into a very efficiently and rapidly applicable evaluation method for medium-rise reinforced concrete building structures. After the calibration, it is determined that the basic seismic demand index E s should be considered as 0.18, [9]. On the next page, Figure 1 shows the distribution of the evaluation results for the randomly picked 2400 buildings within surveyed area of the pilot district. 3

4 Building Stock's Percentage 14% Seismic Safety Screening Method Results Zeytinburnu Pilot Region 12% 10% 8% 6% 4% 2% 0% Seismic Capacity Index, I min Figure 1. Distribution of the buildings seismically risky in Zeytinburnu pilot district, İstanbul 3. EXAMPLE BUILDING A four story reinforced concrete building structure having equal story height of 2.80 m as investigated in details having the purpose of comparing non-linear dynamic analyses results with the SSSM index values. Figure 2 shows the ground story plans of residential condominium building with material quality of C14 concrete and S220 steel A S101 S102 S103 S104 S105 S106 S107 B S108 S109 S110 S111 C S112 D S113 E F S114 S115 S116 G H I S118 S119 S120 S121 S117 J S122 S123 K S124 S125 S126 L S127 S128 M S131 S132 S133 S130 S134 S GÜÇLENDIRILMEMIS DURUM: ZEMIN KAT KALIP PLANI Figure 2. Ground story plan 4

5 Base Shear Force (kn). Non-linear dynamic analyses have been performed using DRAIN-2DX [10] under the effect of seven strong motions recorded during 1999 Marmara and Duzce earthquakes for the sample building in Yalova, which was suffered moderate damage during the 1999 Marmara Earthquake. Absolute acceleration response spectra of the above mentioned seven strong motion records are plotted against design spectra for to types of soil are represented rock Z1 and very soft soil Z4 respectively as defined in the Turkish Earthquake Code [11] S a (t)/a max Sakarya EW Yarımca EW Yarımca NS Düzce EW Düzce NS Bolu EW Düzce EW Z1 Z Periyot,T (sn) Figure 3. Comparison of the absolute acceleration response spectra Time variation of base shear for each of the seven earthquake records are computed for each principle axis. Figure 5 illustrates base shear force time histories of y direction of the building for Marmara 99 earthquake Yarimca EW record YARIMCA EW Time (s) Figure 4. Time history of base shear From the results of these analyses, the maximum values of base shear force for each of the seven earthquake records for the y direction of the building are summarized on Table 1. The ratios of maximum base shear force to structural weight were calculated which for determining the most suitable basic seismic demand index, E s for Turkey. The basic seismic demand index, E s is not only verified by a base shear coefficient, but also by the data compiled from calculated basic seismic capacity index, E o values of damaged or non-damaged buildings after the earthquakes. In this 5

6 example, if the mean value of base shear coefficients is calculated of the five earthquake records for Marmara 99 earthquake, the score is The seismic capacity index for the y direction of the building is calculated according to SSSM, as the value of Comparison between the seismic capacity index value and the calibration score in Zeytinburnu indicates that building performance level is higher than collapse prevention level. Since the seismic capacity index value is just around the limit value, it can be expected that building might be damaged as in reality. Table 1. Base shear maxima of the building Marmara Earthquake Records Düzce Earthquake Records Sakarya Yarımca Yarımca Düzce Düzce Bolu EW Düzce EW EW EW NS EW NS Base Shear (kn) Base Shear Coefficient/ Basic Seismic Demand Index, E s Basic Seismic Capacity Index, E o CONCLUSIONS Based on the experiential study reported in the paper the Seismic Safety Screening Method, SSSM, which is adopted and modified from Japanese Seismic Index Method, JSIM, can be used for estimating the existing seismic safety levels for medium/low rise RC buildings since the results obtained by the SSSM and the structural analysis are in good agreement. The seismic capacity index gives effective information to quantitative understanding, not a subjective assessment data. Calibrated coefficients are provisional and still much effort is needed to apply this method to a large selection of RC buildings in different seismic regions, and verify the results by the structural analyses. 5. REFERENCES 1. FEMA 154: Rapid Visual Screening of Buildings for Potential Seismic Hazards - A Handbook, Federal Emergency Management Agency, Washington D.C., FEMA 310: Handbook for the Seismic Evaluation of Buildings -A Prestandard, Federal Emergency Management Agency, Washington D.C., Standard for Seismic Evaluation of Existing Reinforced Concrete Buildings, Japan Building Disaster Prevention Association, Computers&Structures Inc., SAP 2000 Nonlinear Version 7, CSI, Berkeley, California, Prakash, V., Powell, G.H. and Champbell, S., DRAIN-2DX Base Program Description and User Guide, SEMM Report, UCB/SEMM-93/17, University of California, Berkeley, California, Powell, G.H., DRAIN-2DX Element Description and User Guide for Element type01, type04, type06, type09 and type15, SEMM Report, UCB/SEMM-93/18, University of California, Berkeley, California, The Study on a Disaster Prevention / Mitigation Basic Plan in Istanbul, Japan International Cooperation, Istanbul Metropolitan Municipality, Istanbul, Earthquake Master Plan for Istanbul, Istanbul Metropolitan Municipality Construction Directorate Geotechnical and Earthquake Investigations Department, Istanbul, Ozdemir, P., Boduroglu, M.H. and Ilki, A., Seismic Safety Screening Method, Proceeding for SPEAR International Workshop, Joint Research Centre, Ispra, April 4-5, Yilmaz, E., Assesment of the Structural Performance of a RC Building Strengthened after Kocaeli 99 Earthquake Based on Non-linear Dynamic Analysis, Master Thesis, Institute of Science and Technology, Istanbul Technical University, Istanbul, Turkey, Earthquake Disaster Prevention, Part III, Specification for Structures to be Built in Disaster Areas, Ministry of Public Works and Settlement Government of Republic of Turkey,