Inelastic static (pushover) analysis for estimating seismic demands of rc buildings including soil-structure interaction

The earthquake occurred in the past revealed the destructive impacts of soil-structure interaction (SSI) on the seismic responses of the buildings. For example, during an earthquake occurred in Turkey (Gediz, 1970), a factory in a city located 135 km from the epicenter was destroyed, whereas the other buildings were undamaged in the town. After the investigation, it was observed that the resonance effect, where the dominant period of the building was almost equal to the vibration of the sub-soil, led to the destruction of the building. The earthquake occurred in the Adapazari region (Kocaeli, 1999), which is the other example, revealing the destructive effects of SSI. During this earthquake, numerous foundations have been failed, resulted from either uplifting in the foundation or exceedance in the soil bearting capacity. in view the above-mentioned information, this study intends to investigate how to effect the geotechnical conditions of foundation medium on the seismic performance of 3D reinforced concrete (RC) moment-resisting frame (MRFs) buildings. For this purpose, 2, 4 and 8 story RC buildings are considered as case studies and the analyses are performed by means of a single-mode pushover analysis method in view of the latest version of Turkish Earthquake Code (TEC-2018). The reference structures are considered to perform against the design earthquake level (DD-2). The seismic evaluation of the studied buildings is accomplished as a result of parametric investigations in which the soil properties, the base condition and the height of the structures vary. Two different support cases are considered as (1) fixed base and (2) flexible base conditions. In order to observe how the seismic responses of the compliant and fixed base models differ from each other, a simplified SSI model suitable for pushover analysis have been used. For what concern the soil layer under the foundation different soil properties for Adapazari regions is considered and the effects of soil-foundation interaction was modeled by means of equivalent elastic springs. The stiffness characteristics of the deformable soil under the rigid foundation corresponding to the rotational and translational degree of freedoms are formulized by impedance functions within the sub-structure method specified according to the seismic evaluation and retrofit of concrete buildings report (ATC-40). The mechanical characteristics of soil layers considered for Adapazari region are taken into account with respect to TEC-2018 local soil classification. Single-mode- pushover analyses and performance considerations are performed by SAP2000 computer package. The obtained numerical results are evaluated comparatively in terms of roof displacement-base shear force relationships, vibration periods, inter-story drifts and ground story column damage levels. The study shows that the effects of SSI on seismic demand of the structures founded on softer soil (soil type ZD and ZE) is higher, while it has negligible effects when the structures are founded on stiff soil (soil type ZA and ZB). It can be deduced from the capacity curves that when including geotechnical parameters of the flexible soil, non-conservative results of the structural response for weak ground conditions would be obtained in the SSI analysis.