Effect of foundation soil stiffness on dynamic behavior of RC frame building

Abstract

This study was to investigate the effect of foundation soil stiffness on dynamic behavior of a 5-story (low-rise), a 10-story (medium-rise) and a 20-story (high-rise) Reinforced Concrete (RC) frame building. Each building was supported on soft clay, stiff clay and very stiff clay with shear wave velocity of 100 m/s, 300 m/s and 600 m/s respectively. The soil and foundation were modeled as equivalent spring of specified stiffness along with various degrees of freedom. The Special Moment Resisting Frame (SMRF) system was considered for all example buildings as lateral forceresisting system. The dynamic analysis was carried out using SAP2000 software. The normalized response spectra for 5% damping ratio was used as ground motion as mentioned in Bangladesh National Building Code revised in 2014. From this study it was found that there was no significant variation of natural period due to variation of stiffness of foundation soil. However, past research works showed that there is significant variation of natural period due to variation of stiffness of foundation soil. This is because of using same foundation design for different types of soil. In this study foundation sizes were changed with the variation of soil stiffness. Lateral deflection and base shear decreased with the increase of stiffness of foundation soil, because response spectra suggested by BNBC-2014 for stiffer soil is less than that of soft soil condition. Drift ratio and percent base shear decreases with the increase of number of story for same soil condition. This is because of change in natural period with the increase of number of story. From the response spectra it is seen that value of response spectra decreases with the increase of natural period after peak value. The most important conclusion is that there is no significant variation of lateral deflection and base shear for same building and soil condition if soil structure interaction is considered. This means that if response spectrum for different soil types given in BNBC (2014) is used and foundation design is done properly, reasonably accurate result of analysis can be done without considering soil-structure interaction.