## 5 Comparison of SSI Effects on the Seismic Performance of Existing Buildings  
Obtained results clearly show that interaction between soil and the structure affects both calculation of displacement capacities and demands in existing buildings. This situation implies that seismic performance of these buildings should also be examined since the performance is determined by comparing both capacity and demand.  
Calculation of capacity curves revealed that interaction between soil and the structure shifts the total displacement capacity of buildings instead of increasing them. Assessments performed after pushover analyses show that plastic deformation capacity of buildings remains almost constant (Figure 9) while yield displacements increase. Decreasing displacement ductilities corresponding to soft soils are the indicator of this situation (Figure 10). Changes in the elastic slope of the building capacity curves also imply the elongations in vibration periods and hence higher seismic displacement demands in buildings (Figure 5). Figures 12-15 clearly represent this increase in demands from stiff to soft soil sites. All these capacity- and demand-related changes in the results are the indicator of possible seismic performance changes in selected buildings.  
Seismic performance of investigated buildings was determined by using Turkish Earthquake Code regulations and the effect of SSI on the collapse risk of old and new buildings was compared. At this point, it should be reminded again that the aim of this study is not to investigate the seismic performance of selected buildings according to one specific code regulations. The aim of this study is to understand whether consideration of SSI can change the capacity and demand calculations (and hence the performance estimations) of existing buildings. For this reason, the number of collapsed buildings under various soil conditions was determined and compared.  
In Turkish Earthquake Code, seismic performance of buildings is determined by using strain-based damage assessment method. Furthermore, accumulation of damages, whether in beams or columns, contributes to damaged columns and the shear capacity of stories are additional factors which are considered by the Turkish Earthquake Code for the seismic performance assessment. In Figure 24, changes in the number of collapsed buildings located on stiff to soft soil sites are presented. Increasing damage trends show that interaction between soil and the structure is much more significant, especially in soft soils, and a number of collapsed buildings on soft soils sites are considerably higher than that of others. Figure 24 also reveals that the devastating effects of SSI are much more evident in old buildings with respect to new ones. Higher seismic capacities of new constructions (Figure 5) suppress the deteriorating effects of SSI and the effect of interaction becomes limited on new buildings.  
Figure 24 also shows that in each soil condition seismic performance of old buildings decreases by the increasing number of stories. However, the ratio of collapsed buildings on soft soil to fixed-base case indicates that the relative effect of SSI is much more significant in lower story buildings. In three-story old buildings the ratio of collapsed buildings on soft soils (38%) is greater than two times that of the fixed-base case (17%). Differences between soft and moderate soil cases are much more apparent with respect to moderate and stiff or stiff and fixed-base cases.  
Distribution of collapsed buildings is presented in Figure 24 and these values clearly show that seismic performance of especially older constructions is significantly affected from the SSI. Results have clearly revealed that damage risk significantly increases in soft soil cases with respect to moderate and stiff cases.