## 4 Effect of SSI on the Capacity and Demand Response of Existing Buildings
### Effect of SSI on the Capacity Curves  
All building models were analyzed to determine the capacity curves of buildings according to fixed-based assumption and SSI, and total of 160 (40 buildings \(\times\) 4 different SSI modeling) analyses were conducted via static pushover analysis. In this paper especially, the \"Collapse Prevention\" and \"Yield\" limit states are investigated since one of the purposes of the current study is to determine whether consideration of SSI alters the capacity curves of the buildings. Collapse prevention damage limits of buildings are determined by checking damage states of columns, beams and the story shear forces. To define building collapse prevention limit, it is assumed that at least 80% of beams in any story should not exceed the member collapse limit. The collapse of any column is accepted as the collapse prevention limit of the buildings. Shear force carried by the columns beyond the yield limit at both ends were checked and the contribution of these columns to the total shear capacity of each story was limited to 30%. Collapse prevention damage limit is determined according to each rule and building damage limit is attained by whichever gives the lowest drift ratio. After the determination of capacity curves via static pushover analysis, authors investigated whether capacity-related parameters were altered or not when the SSI was considered. Vibration periods (T), ductility capacities (\(\upmu\)) and proportion of drift ratios corresponding to yield, and ultimate damage levels are used to evaluate and compare the effect of SSI on building capacity curves. In order to explain the details of the applied procedure, BO20 SN6 was selected among the considered buildings. The first two letters of the building name refer to the design code of the building, and \"BO\" and \"BN\" are used to represent old (98\(-\)) and new buildings (98\(+\)), respectively. The numbers following these letters indicate the order of the building in the inventory. \"SN6\" refers to the story number of the building. In Figure 5, capacity curve, yield and ultimate damage limits of the BO20 SN6 are plotted according to each of the soil classes considered. Figure 4: Ground motion reduction factors: (**left**) embedment effects, (**right**) base slab averaging (NIST, 2012). It can be seen from the figure that yield and ultimate damage levels of the building are shifted left to right from stiff to soft soil types. A similar situation can also be observed for the initial part of the capacity curve. This situation clearly explains the period elongations from stiff to soft soil profiles. It should be stated that similar results were observed for all buildings in the inventory. In Figure 5, capacity curves of BO20 SN6 and BN19 SN6 buildings corresponding to different soil conditions are presented. Significant differences between the strength and deformation capacities of selected buildings clearly shows the effect of changing code regulations on the old (98-) and new (98+) buildings. Figure 5 clearly indicates that curves of fixed-base and stiff soil cases are almost identical and capacity curve of moderate soil case is closer to them with respect to the soft soil condition. Variation of capacity curves imply that the main difference occurs in the elastic slope of the building and drift ratios corresponding to yield and collapse limits are significantly affected from SSI in the soft soil case. This figure also indicates that plastic drift capacities of the BO20 SN6 building are almost unchanged and the movement of plastic deformation capacity, which is caused by the inertial interaction between structure and soil, can be explained by shifting instead of increasing. Previous studies have also indicated this inertial interaction, and hence elongation of the natural vibration period of the buildings [23].