## 2 Nonlinear Modeling of Existing Buildings  
The majority of existing residential building stock of Turkey is composed of low and mid-rise RC buildings. Therefore, in this study buildings were selected to reflect this situation and story numbers of selected existing buildings vary between three and six. In the study, buildings were grouped according to their story numbers and construction dates and each story group has five buildings. Distribution of the story numbers and construction dates of selected buildings are presented in Table 1. It should be stated that construction date of buildings has a crucial role since improvements in the workmanship and material qualities increased the safety of new buildings. In both seismic codes, a force-based design approach was used. However, capacity design principles were not considered in TEC-1975. One of the most important differences between the codes is also that TEC-1998 enforced the use of \"ultimate strength design\" for the analysis and design of RC cross-sections and this method is essential for the application of capacity design principles. In TEC-1975, design spectrum was not defined, but lateral seismic forces were calculated according to earthquake zone coefficient (C\({}_{o}\)), seismic weight (W), structural configuration system type (K), soil condition (S) and importance factor (I). Maximum lateral force demand was recommended as 10% of seismic weight for the design of structures (C\({}_{o}\) = 0.1). In both seismic codes, soil conditions were only considered to calculate corner periods of the spectrum. Minimum allowable concrete strength was equal or higher than 16 MPa in TEC-1975, but this value was increased to 20 MPa in TEC-1998. In TEC-1975, S220 steel class which has a yield strength of 220 MPa was allowed, but in TEC-1998, minimum S420 steel which has a yield strength of 420 MPa was recommended. It can be said that new buildings have a greater stiffness, strength and ductility capacity with respect to old buildings. By using such kind of classification, it is aimed to investigate the effect of SSI on the new (98+) and old buildings (98-). The structural properties of buildings, such as section dimensions, reinforcement details, material properties, dead and live loads acting on the buildings were determined according to their design projects. Investigation of RC design projects have revealed that concrete strength of old buildings is mainly 16 MPa, and design projects of these buildings have shown that S220 steel class with a yield strength (f\({}_{y}\)) of 220 MPa is used for both longitudinal and transverse reinforcement. It was also observed that stirrups were not used in the cross-section of members, and transverse reinforcement spacing of the members was mainly 200 mm and this was even inadequate according TEC-1975 design rules. Furthermore, design projects of buildings have shown that section dimensions of the columns were reduced in the upper stories. Buildings designed according to TEC-1998 or higher have identical steel class (S420) which yield strength is 420 MPa. However, concrete strengths were changed from project to project. For this reason, concrete strength of new buildings is not identical, and they were modeled according to their own concrete strengths. It was also observed that confinement details of code provisions were applied to beam and columns in new buildings. In addition, column dimensions were not reduced along the building height. In Figure 1 shows some pictures from the design projects of a sample four-story building and a photo from field investigation is shown. Strength and deformation capacities of members at critical sections were determined via moment-curvature analyses. Stress-strain behavior of confined concrete was represented by Modified Kent-Park model [16]. Since the purpose of the study is concentrated on the capacity and seismic demand estimation of buildings, intermediate damage limits (Immediate Occupancy and Life Safety) of structural members were not used.