## 6 Conclusions  
Forty existing buildings constructed before and after in 1998 when the modern seismic design code of Turkey was implemented were investigated by considering soil-structure interaction. Seismic response of buildings was determined by using non-linear time history analyses and 20 strong ground motions. Interaction between soil and the structure was modeled by applying the substructure method and four different cases classified according to shear wave velocities were considered. Seismic displacement demands at roof and story levels were calculated for all buildings under fixed-base, stiff, moderate and soft soil conditions. In addition to demand calculations by using non-linear time history analyses, capacity curves of the buildings were obtained through static pushover analyses. By this method, drift ratios corresponding to yield and ultimate levels were obtained for each soil case and for each building. Then, displacement demands and building capacities were investigated, the effects of soil-structure interaction on existing buildings under various soil conditions were compared and the results are summarized below. Figure 24: Distribution of number of collapse events for the existing buildings (%). The capacity curves obtained from the static pushover analyses of old and new buildings show that the interaction between soil and the structure shifts the total displacement capacity of buildings instead of increasing it. Depending on the weak soil conditions, rotations at base increase and this situation increases the elastic drift ratios corresponding to the yield. In other words, plastic deformation capacity of buildings remains almost constant while yield displacements increase. Decreasing displacement ductilities are the result of this situation. Changes in the elastic slope of the buildings also increase the vibration periods and then the displacement demands of the buildings. Changes in the seismic demands were investigated by comparing the roof drift ratios corresponding to exceedance probabilities of 50%, 10% and maximum. Distribution of values clearly indicated that most critical results were obtained under soft soil conditions, and results obtained from the soft soil cases were significantly separated from the others. Drift demands of moderate soil cases are closer to fixed and stiff soil cases with respect to soft soil conditions. Relative demand increase under soft soil case is much more evident in low story buildings with respect to higher ones. Investigation of inter-story drift demands showed that first stories are mostly affected from the weak soil conditions and in all cases most significant demand increase occurs at the first stories. Increase in the maximum inter-story drift demand is much more evident with respect to ones corresponding to 50% and 10% exceedance probabilities. Effect of soil conditions on the number of collapsed buildings is also investigated and compared. Distribution of results have shown that a number of collapsed buildings are almost similar in fixed-base, stiff and even in moderate soil cases. However, under soft soil conditions the number of collapsed buildings increase and the level of increment is much more significant in old and low story buildings. A distinct increase in the number of collapsed three- and four-story buildings indicates that the fragility of old and low story buildings on weak soil conditions is much higher. Performance of new buildings, on the other hand, is not as critical as the old ones. Higher seismic capacities of new constructions suppress the deteriorating effects of soft soil conditions and reduce the collapse risk of new buildings. [PERSON] conceived of the presented idea, supervised the work, and drafted the manuscript. [PERSON] contributed to method of analysis and supervised the analytical computations. [PERSON] and [PERSON] designed the nonlinear structural models and analyzed the buildings. All authors have read and agreed to the published version of the manuscript. This research received no external funding.