Fragility-based assessment of public buildings in Turkey

Highlights

• This study evaluates seismic performances of public buildings in Turkey based on fragility curves.

• Lateral stiffness, strength and displacement capacities are determined by pushover analyses.

• Based on assessed capacities and demands, fragility curves are derived for each template designs.

• Material and detailing quality has limited effects on IO than LS and CP levels.

• Collapse probabilities of buildings were estimated according to Peak ground velocities.

Abstract

Public buildings which are mostly built of reinforced concrete with template designs constitute a great part of the vulnerable public building stock in Turkey as well as in several other countries prone to earthquakes. This study focuses on seismic fragility assessment of reinforced concrete public buildings with representative template designs, which have been designed according to the 1975 version of the Turkish seismic design code. Lateral stiffness, strength and displacement capacities of the selected template designs are determined by nonlinear static analyses in two principal directions. The inelastic dynamic characteristics of the template designs investigated are represented by equivalent single degree of freedom systems and their seismic deformation demands are calculated using a set of 100 strong ground motion records. Peak ground velocity is selected as the measure of seismic intensity because it has a good correlation with maximum inelastic displacements. Based on assessed capacities and demands, fragility curves are derived separately for each template designs. Buildings are grouped according to the number of storeys. The results revealed that the effect of concrete and detailing quality on IO performance level is more limited and less critical as the ground motion intensity increases. On the other hand, the probability of exceedence for LS and CP are closer to each other for each group of buildings. Finally, using constructed fragility curves, collapse probabilities of existing public buildings were estimated according to PGV values. The results are evaluated in terms of concrete quality and transverse reinforcement detailing.

Introduction

The damage caused by the recent devastating earthquakes in the last two decades drew attention to an ongoing problem in Turkey with regard to the seismic vulnerability of the public buildings. Many of them have been designed with templates according to the former version of Turkish seismic design code (TEC-1975) [1]. Field observations and many studies have clearly indicated that these types of buildings are highly vulnerable to seismic actions [2], [3], [4]. Especially, following after 1999 Kocaeli-Duzce (M_w = 7.4 and M_w = 7.1) and 1 May 2003 Bingöl (M_w = 6.2) earthquakes in Turkey, seismic safety of public buildings has been questioned. Because these buildings experienced considerable and irreversible damage compared to residential buildings. It is noteworthy that a significant number of the reinforced concrete (RC) buildings seriously damaged during 2003 Bingöl earthquake were public buildings such as schools, hospitals and other governmental facilities [5]. The collapse of many public buildings during Bingöl earthquake, where 83 children and one teacher died, alerted the country to the vulnerability of critical public buildings.

In Turkey, template designs developed by the Ministry of Public Works are used in all provinces for many of the public buildings intended for governmental services (administrative centers, health clinics, hospitals, schools, etc.) as a common practice to save up architectural fees and ensure quality control [4]. Although the used projects reveal minor differences from province to province, they are similar architecturally.

Public buildings which are mostly built of reinforced concrete with template designs constitute an important part of the vulnerable building stock in Turkey. The projects and the construction of existing public buildings that were built before 1998 were constructed in accordance with the regulations of 1984 version of Turkish building code (TBC-1984) [6] and 1975 version of Turkish earthquake code (TEC-1975) [1] which were in effect at that time. In general, public buildings designed without seismic considerations have significant deficiencies, such as discontinuity of positive moment reinforcement in beams, wide spacing of transverse shear reinforcement, poor material quality and workmanship. However, the earthquake and the construction regulations in Turkey underwent significant changes with the revisions made in 1998, 2000 and 2007 [7], [8], [9]. Meanwhile, it is unavoidable to experience destroying earthquakes in the near future for Turkey. Hence, the need for risk assessment and mitigation of seismic risk for existing public building stocks to predict the potential damage from future earthquakes is a task of extreme importance on the agenda of Turkish Government [10]. National attention focused on the problem of high seismic vulnerability of schools and hospitals. Seismic vulnerability of all strategic public buildings, including schools and hospitals as well as the infrastructure in medium and high seismic areas had to be evaluated urgently in order to start a seismic rehabilitation programme [4].

There have been a number of recent studies [11], [12], [13], [14], [15] that have focused on assessing the seismic risk of existing reinforced concrete residential buildings in Turkey in the context of emerging performance based design concepts. However, the problem of seismic vulnerability and risk of critical structures, especially of buildings where important public functions are carried out (such as schools and hospitals) has not been addressed in an intensive investigation by the use of fragility functions.

Fragility can be defined as the sensitivity of the assets to damage from intensity of earthquake excitation. Fragility analysis is an important component in performance-based design which is a fundamental concept of earthquake loss assessment. Fragility functions are useful tools for seismic loss estimation due to the earthquakes as a function of ground motion indices. They represent the probability of exceeding a damage limit state for a given structure type subjected to a seismic excitation [16]. The damage limits states can be defined based on global drift ratio, inter-story drift ratio or story shear force in fragility studies. Regular RC structures would have an almost uniform interstorey drift distribution along its height [17]. In literature, it can be found that there is a good correlation between the roof drift and maximum interstorey drift as shown in Fig. 1. Even though this case cannot be satisfied for irregular structures, drift ratio could be used as a damage indicator for regular structures. Besides, the use of global roof drift as the performance level thresholds can be found in several studies in the literature. [11], [13]. In this study, the global drift is chosen to identify the damage limit states, as the template designs do not comprise soft stories that may lead extreme local drift demands. The ground motion intensities in the fragility functions can be spectral quantities like peak ground motion values. In this sense, fragility curves grasp uncertainties related with structural capacity, damage limit state definition and record-to-record variability of ground motion intensities [11].

The main goal of this study is to develop the fragility curves for RC public buildings with representative template designs in Turkey which have been designed according to the former version of the Turkish seismic design code (TBC-1975) [1]. A realistic description of the fragility for such buildings is important since, considerable portion of the existing public buildings are in this category throughout Turkey and Turkey is undoubtedly under a significant risk [4]. Uncertainties in structural characteristics are taken into account by considering the variability in concrete strength, detailing, lateral strength and damage limit states. Using a set of 100 strong ground motions, the random natures of earthquakes have been taken into consideration. The fragility functions are determined separately for each template designs as the probabilities of exceeding the specified performance-based damage limit states under ground excitations, where the hazard parameter is expressed as PGV. In addition to this, considering whole template designs, effects of detailing and concrete quality on the performance of public buildings have been assessed.