Estimation of Seismic Response of R/C Frame Structures to Vertical Earthquake Motion Considering Fixed Support and Soil-Structure-Interaction (SSI)

Abstract

The present study aims to demonstrate the effects of vertical earthquake excitation on reinforced concrete frame structures (R/C) incorporating the approaches of Soil-Structure-Interaction (SSI) and rigid foundation. This effect is intended to be determined by making comparative study according to the engineering parameters of the base shear force, base axial force and overturning moment. Three earthquake time-histories currently used for the earthquake analysis of structure in literature are regarded to properly elucidate this effect. For the linear time-history analysis (LMTHA), finite element model (FEM) of a high-rise existing R/C structure designed according Turkish Seismic Code (TSC) is developed through frame, shell, solid, gap, link, linear damper elements. Similar modeling considerations are adopted for infinite soil zone. For the SSI analysis, the Direct Method (DM) is utilized, which enables to make an earthquake analysis of combined FEM (soil model+structure model) instead of separated FEM (the Substructure Method). The comparative study is carried out between the SSI and the fixed support (FS) model with fixed-supports under both only horizontal (H) and horizontal+vertical (HV) seismic motions. The difference between only H and H+V load cases for the engineering parameters that vertical earthquake motion is determined not to be effective on the base shear force, and the SSI model is recommended to be used for analysis due to resulting higher change than the FS. Therefore, no damages/failure resulting from the shear force is estimated under the vertical earthquake motion. Instead, the overturning moment is obtained with great change under H+V for all models; however, the SSI model yields to higher increase than the FS model; so, the SSI model is offered for more reliable analysis. Considerable increase in the overturning moment is predicted that vertical load bearing elements such as, columns and piers need to more strength capacity in terms of bending moment. The highest increase among the other parameters is obtained in the base axial force. However, the percentage increase is resulted as higher under H load than H+V, which means that vertical seismic motion treated as balancing effect. This lower change under H+V load case is directly pertinent to damping properties of the soil media. Therefore, design of columns and piers against axial force is recommended to be made for more safety according to the outcomes from the FS model.