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.
