Evaluation of Response Reduction Factors for Moment Resisting RC Frames

Abstract

Moment resisting frames are commonly used as the dominant mode of lateral resisting system in seismic regions for a long time. The poor performance of OMRF in past earthquakes suggested special design and detailing to warrant a ductile behaviour in seismic zones of high earthquake. Thus when a large earthquake occurs, SMRF which is specially detailed with a response reduction factor, R = 5 is expected to have superior ductility. The R factor of 5 in SMRF reduces the design base shear and in such a case these building rely greatly on their ductile performance. To ensure ductile performance, this type of frames shall be detailed in a special manner recommended by IS 13920. The objective of the study is to evaluate the R factors of these frames from their pushover curves and to check its adequacy compared to code recommended R value. The accurate estimation of strength and displacement capacity of nonlinear pushover curves requires the confinement modelling of concrete as per an accepted confinement model. A review of various concrete confinement models is carried out to select appropriate concrete confinement model. It is found that modified Kent and Park model is an appropriate model and it is incorporated in the modelling of nonlinearity in concrete sections. The frames with number of storeys 2, 4, 8, and 12 (with four bays) are designed and detailed as SMRF and OMRF as per IS code. The pushover curves of each SMRF and OMRF frames are generated and converted to a bilinear format to calculate the behaviour factors. The R factors obtained show in general that both the OMRF and SMRF frames, failed to achieve the respective target values of response reduction factors recommended by IS code marginally. The components of response reduction factors such as over-strength and ductility factors also evaluated for all the SMRF and OMRF frames. It was also found that shorter frames exhibit higher R factors and as the height of the frames increases the R factors decreases.