Evaluation of Response Modification Factor in Consideration of Soil-Structure Interaction

Abstract

Since 1990s, great efforts have been made to replace the classical seismic design philosophy by the new ones. In both the equivalent linear static force and code ba sed response spectrum procedure the seismic design philosophy is incorporated using response modification factor “R”. The ratio of elastic strength demand to inelastic strength design is defined as “R” factor. During earthquake, the response of the structure is influenced by three interlinked systems i.e., the super structure, the foundation and the soil medium. This phenomenon is called soil structure interaction (SSI). In conventional seismic design philosophy structural analysis is performed assuming that the structure is fixed at the foundation level (rigid support). However, in actual the structure has foundation flexibility depending upon the type of soil medium supporting the structure. Code based values of “R” factor does not reflect the SSI problem. Thus, there is a strong need to redefine the “R” factor values considering the effect of SSI. In this study, the seismic behaviour of mid-rise mix use buildings located in seismic zone 2B and soil type “SD” and “SB” in consideration with SSI (assuming no water ground water table has encountered) has been investigated. The main objectives of this research work is to evaluate “R” factor for moment resisting frame (MRF) buildings situated on different soils with shear wave velocity 300m/sec (SD) and 1200m/s (SB)considering SSI and to compare R-factor values with SSI system to that of fixed based system. A 10 storey MRF building with two basements + ground + 7 storeys has been designed by equivalent linear static force method using structural analysis software SAP 2000 v15.0.0. Code based value of “R” factor 5.5 for building frame system has been used. Soil medium is modelled using horizontal and vertical closely spaced, linear elastic springs which are identical but mutually independent. Nonlinear static pushover analysis using guidelines of FEMA 356 and ATC 40 has been performed. The modified “R” factor values are evaluated as ratio of elastic base shear strength from linear static analysis (without using code based “R” factor) to inelastic base shear from non-linear static pushover analysis. Different design parameters such as base shear, storey shear, storey drift and displacements have also been compared with SSI system to that of fixed base system. The storey shears and moments for the soil type SD have increased whereas storey shears and moments for the soil type SB remain same. For soil type SD storey drifts and displacements with SSI xvi have negligible difference as compared to that of fixed base systems whereas for soil type SB storey drifts and displacements remain same. It can be concluded that structures placed on soil medium with shear wave velocity greater than and equal to 1200m/sec are not affected by SSI whereas, structures placed on hard soils with shear wave velocity equal 300m/s are greatly affected by SSI.