EFFECT OF SOIL ON SEISMIC PERFORMANCE OF RC BUILDINGS

Abstract

In earthquake resistant design of structures, usually the designs are carried out based on the results of dynamic analysis considering fixed-base condition. The effect of the supporting soils in a structural model is generally neglected. Two important issues regarding the seismic response of structures are soil amplification and soil-structure interaction. The soft soils can change the peak ground acceleration by amplifying the ground motions. The physical basis of amplification in soft soils is filtering action of the soil media and conservation of energy. In the design codes, soil classification is adopted to consider the effect of soil on ground motion. The classification of soil in Indian seismic code (IS 1893: 2002) has been considered through response spectra i.e. "Rock or Hard Soil", "Medium Soil" and "Soft Soil", which is very simple and general and therefore the actual property of the subsurface soil profile is not adequately represented. In the present study soil data from different places in India, viz. Delhi, Dehradun, Kolkata, and Banglore have been considered for ground response analysis. To consider the effect of depth, the soil profiles have been classified into four categories of <10m, 11-20m, 21-30m and greater than 30m. The soil profiles are also classified according to the average shear wave velocity. In this classification three categories are considered i.e. C (>365 m/s), D (180m/s to 365 m/s) and E (<I80m/s). Since D category covers a large variation of the shear wave velocity it is further sub divided into three classes namely D 1(181-240m/s), D2(241-300m/s) and D3(301-365m/s).To classify the profiles according to NEHRP only the profiles having depth greater than 30m are considered since the criteria of classification is Vs(30). To compute the one dimensional site effect, recorded earthquakes and spectrum compatible earthquakes have been applied as an outcrop motion. The peak ground acceleration for the real earthquakes varies from 0.08g to 0.37g. The spectrum compatible ground motion was scaled to 0.05g, 0.12g, 0.18g, 0.24g and 0.36g. Thus the analysis was carried out for 11 recorded earthquakes and 25 spectrum compatible earthquakes. Ground motion amplification factor reduces with increasing shear wave velocity but it first increases and then decreases with the increasing depth of soil. The period corresponding to peak amplification elongates with decreasing shear wave velocity and increasing depth of soil. For deep alluvial deposits (Site Class-E with more than 60 m depth) there is significant amplification at period around 1-1.5 sec. As most of the structures have periods of vibration in this range, this may have quite iii adverse effect on seismic performance. Accordingly, this profile has been chosen for studying the seismic performance of a 4 storied building. Foundation flexibility has been considered for the same building and compared. It is observed that there is no significant effect on the pushover curve of the rigid and flexible base building, but the performance of the building differed due to amplification in the long period range. iv