## 1 Introduction  
The determination of the seismic performance of existing buildings has gained very much interest in recent years, and today there are a greater number of specifications and regulations containing provisions on this issue [1]. The seismic behavior of a building is directly related to the interaction between the three interconnected systems, which are superstructure, foundation and soil medium surrounding the foundation system [2]. Advances and experiences in earthquake engineering, reconnaissance surveys after strong earthquakes and academical studies about soil-structure interaction (SSI) have shown that old buildings designed by limited knowledge are far from meeting the current standards and performance objectives of new designs. Although there were studies in literature suggesting the use of force-based computational approaches for the modeling of SSI [3], the application of these methods has been very limited, and they have not found significant use in practice [1]. Particularly over the last two decades the widespread use of displacement-based methods, which include nonlinear calculations such as static pushover analysis, provide to investigate SSI beyond the elastic limits [3; 4]. Realistic estimations of both displacement capacities and seismic drift demands became possible by using nonlinear analysis methods. The damage observations and detailed structural analyses have shown that SSI could significantly alter both the capacity and demand-relatedstructural parameters (e.g., vibration period, drift capacity, etc.) and hence the seismic performance of buildings [5; 6]. All these observations and findings have shown that SSI effects should be considered necessary for the design and assessment of buildings. Earlier studies of SSI contained complex arithmetic formulas relating to wave propagation in several directions [1], and this approach made these studies difficult to comprehend. SSI is not covered in the undergraduate level and therefore, it is difficult for many engineers to apply these methods in design phase. While regulations and documents about SSI are available for engineers in countries such as USA [7], these sources have guided earthquake engineers only to a limited extent. In many countries, there are still no mandatory code regulations and directions that enforce the engineers to consider the SSI effects in design and assessment of buildings. SSI analyses are performed to investigate the effect of various soil conditions on the response of structures under seismic actions. There are two approaches for the calculation of SSI, namely the \"direct\" and \"substructure\" methods [8]. In the direct method, the structure and soil are modeled within a single finite element network in which the nonlinearities of the superstructure and the soil are represented as a whole, and structure and soil are analyzed together. On the other hand, direct method requires considerable calculation efforts and analysis duration, and therefore, it is not suitable when assessing a lot of buildings, as in the case of this study. The combined use of nonlinear time history analysis and direct method makes this situation more complicated. In the \"substructure method,\" on the other hand, the soil and the structure are considered as distinct systems. The behavior of the foundation and soil is represented by dynamic stiffness and damping coefficients and the effect of interaction between the soil and foundation is transmitted to the structure by means of dashpots and springs. This method considerably shortens the duration of analysis since the soil is not modeled directly. It is thus more favorable to use the substructure method in the studies when dealing with a large number of buildings. In this study, a lot of buildings (40 buildings) were considered and seismic response of these buildings under various soil conditions was investigated by using nonlinear time history analyses. Therefore, substructure method is preferred to investigate effect of SSI by considering the required efforts.