DETERMINATION OF MOMENT RESISTING RC FRAMES OF RESPONSE REDUCTION FACTORS

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 Ordinary Moment Resisting Frame (OMRF) in past earthquakes suggested special design and detailing to warrant a ductile behavior in seismic zones of high earthquake (zone III, IV & V). Thus when a large earthquake occurs, Special Moment Resisting Frame (SMRF) which is speciallydetailed with a response reduction factor, R = 5 is expected to have superior ductility. The response reduction 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 objectiveof the present study is to evaluate the R factors of these frames from their nonlinear base shear versus roof displacement curves (pushover curves) and to check its adequacy comparedto 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 1893 (2002). The pushover curves of each SMRF and OMRF frames are generated and converted to a bilinear format to calculate the behavior factors. The response reduction 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 1893 (2002) 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.