## 1 Introduction
Academical studies considering the SSI have increased in the United States towards the end of the 2000s and some of them were summarized in the FEMA-440 report [3]. In this report, regulations and expressions are presented to explain how SSI can be considered in nonlinear static analyses. The findings of these studies were also included in US code specifications [4]. However, the expressions in FEMA-440 [3] and the ASCE-2007 [4] regulations are not recommended for nonlinear time history analyses and therefore this situation required new studies on this subject. The results of subsequent studies related with the SSI in performance-based earthquake engineering were summarized in 2012 and the method which can be used in non-linear time history analysis was proposed [1]. This approach was also used in this study during the analyses of selected building models and SSI was represented based on the expressions taken from these studies. There are many other studies addressing the non-building type of structures in the literature. [PERSON] [9] proposed algebraic formulas and tables that could be used to calculate the dynamic properties of foundations of different shapes. [PERSON] and [PERSON] [6] discussed whether SSI effects could be beneficial for structures. They have compared the seismic behavior of structures calculated by using traditional methods and by considering SSI effects. The authors showed that an increase in the natural vibration period may not always lead to a lower spectral acceleration response and noted that this may result in an unsafe structural assessment. [PERSON] et al. [10] studied the seismic analysis and design of bridge piers and proposed simple expressions for the calculation of kinematic effects. [PERSON] et al. [11] examined the seismic performance of empirical buildings with SSI and showed that the seismic performance of buildings varied significantly depending on the soil conditions. [PERSON] et al. [5] investigated the variations in SSI effects depending on the use of different demand calculation methods in multi-story buildings. Their research has shown that seismic performance evaluations are not within the reliable limits if the effects of SSI are ignored. Increasing number of academical studies and engineering reports imply that in the next generation codes SSI modeling will necessarily be required and consideration of SSI effects in design will be mandatory. However, buildings constructed before these findings will still be the weak point of the cities that are prone to seismic risk. Turkey is an earthquake-prone country and the majority of existing building stock, which consist of low and mid-rise reinforced concrete (RC) buildings, were designed without considering SSI. Therefore, the main purpose of the present study is to investigate the effects of SSI on the seismic response of existing buildings. For this purpose, 40 RC buildings selected from Turkey were investigated. TEC-2007 [12] and TBEC-2018 [13] include the regulations which define the seismic performance assessment of existing buildings. However, both Turkish earthquake codes do not comprise the definitions or formulations which explain how to model SSI in design and assessment. In this study, residential RC buildings that constitute three-, four-, five- and six-story buildings were selected, and they were mainly classified into two groups as \"old\" and \"new\" buildings according to their construction dates [14]. TEC-1998 [15] is accepted as the reference code to distinguish buildings since this code applied the capacity design principle at the first time and considerably increased the design forces and limited the displacement demands in terms of drift ratios. This situation significantly differs the strength and stiffness capacity of existing buildings constructed before and after TEC-1998 in Turkey. Moreover, mandatory building control law, earthquake insurance and improvements in the workmanship and material qualities after the 1999 Marmara earthquakes increased the safety of newer buildings with respect to old ones.