Quantification of Higher Mode Effects in the Seismic Response of Two-Dimensional (2D) Reinforced Concrete (RC) Frames Including the Effect of Soil

Abstract

Soil-Structure Interaction (SSI) can significantly influence the dynamic characteristics of the structures and in turn affects its seismic performance. In conventional modeling practice, mostly structures have been analyzed considering the fixed based supports and neglecting the effects of soil-structure interaction (SSI) and the foundation flexibility. Higher mode effects become predominantly significant with increasing height of the building. Accordingly higher modes contribute more significantly to the response of the structure as the height of the structure is increased. The present study focuses on quantification of higher-mode effects in the seismic response of two-dimensional (2D) reinforced concrete (RC) frames located on soil Type D in Pakistan. For this purpose, six two-dimensional (2D) generic RC frames with stories number ranging from three to thirty have been taken and then analyzed for seven different ground motions. These ground motion records are accessed using the NGA-West2 coast of (PEER ground motion database). These time histories of ground motions are adjusted and scaled by spectral matching in “SeismoMatch” software to match with target response spectra, which is developed for a 5% damped design base earthquake (DBE) level for the region of Islamabad capital territory (ICT). These analyses have been done with two different conditions i.e., with taking the effect of soil (flexible base) and without taking the effect of soil (fixed base). For this purpose, a general finite element analysis program named as Response 2D has been developed for the analysis of two- dimensional frames. The main advantage of this program is that the user can also see each mode response and its contribution in the combined response for modal time history analysis which helps in investigating the higher modes effects in different responses in a much better way and with better understanding. Modal properties such as time periods, mode shapes and responses like roof displacement history, story shear, story moment etc. have been compared thoroughly for the two cases of fixed and flexible supports. Along with this, top four modes contribution in the combined response has also been studied for the two cases. It has been observed that the phenomenon of soil- structure interaction (SSI) has significantly affected the higher mode effects in responses like base moment, base shear, story moment, story shear and roof displacement.