Evaluation of seismic performance of RC frame structures by pushover and time history analysis

Abstract

Bangladesh is situated in a seismically active region. Part of the country extended from Sylhet to Chittagong is in the high seismic zone whereas Dhaka lies in the moderate seismic zone. Major metropolitan cities of our country are under serious threat because of inadequacies in design and construction of structures. A lot of multistoried buildings have already been built in order to fulfill the ever increasing demand of urban population. Bangladesh National Building Code (BNBC) proposes equivalent static load method to design the buildings; however a seismic event will result in damaged structure, performance of which cannot be evaluated from a static analysis. Pushover analysis, a comparatively simplified nonlinear method involving certain approximations and simplifications, is capable of predicting the damage extent due to a seismic event. ATC-40 and FEMA-356 propose the pushover analysis method in detail. In this thesis, an attempt has been made to demonstrate the validity and efficiency of pushover analysis method of ATC-40 as incorporated in ETABS, SAP2000 and SeismoStruct software. Performance of building frames designed as per BNBC has also been evaluated against targeted performance levels for serviceability, design and maximum earthquakes. Nonlinear time history analysis, although complicated and time consuming, is a more rigorous method of modeling seismic response of a structure. Both linear and nonlinear time history analyses have been validated against published results using SAP2000 and SeismoStruct. The work also studied the effectiveness of pushover analysis in comparison to more rigorous nonlinear time history analysis with particular emphasis on the load pattern employed in pushover analysis. Different load patterns have been used in pushover analysis and uniform load pattern has been found to give better capacity curves that compare well with nonlinear time history analysis. Performances have also been evaluated for 2D frames designed according to BNBC using nonlinear time history. Well-known EL Centro and Kobe earthquakes have suitably been scaled as per required Z value for use in the analysis. A comparison of linear and nonlinear time history analyses have been carried out to demonstrate the damage extent caused during an earthquake. Similar pushover analyses carried out on 3-D six and twelve story building designed according to BNBC show that they easily satisfy the ATC-40 local and global seismic requirements. A building which is designed only for gravity load failed to satisfy the requirements for serviceability and maximum earthquakes.