SEISMIC EVALUATION AND RETROFITTING OF RC FRAMES

Abstract

Recent earthquakes in many parts of the world have revealed the issues pertaining to the vulnerability of existing buildings. The existing building structures, which were designed and constructed according to earlier codal provisions, do not satisfy requirements of the current seismic code and design practices. Many reinforced concrete buildings in urban regions lying in active seismic zones, may suffer moderate to severe damages during future ground motions. Therefore it is essential to mitigate unexpected hazards to property and life of occupants, posed during future probable earthquake. The mitigation of hazards is possible by means of seismic retrofitting of inadequate existing building structures. There are a number of techniques available for enhancing earthquake resistance but how effective they are remains to be answered. The available measures have rarely been adequately verified for their effectiveness through experimental testing. In the first part of the present study an attempt has been made to verify the analytical results with the experimental results to investigate the seismic behaviour of infill wall and steel bracing as retrofitted schemes for RC frames. For this purpose three models namely bare frame, infilled frame and a steel braced (cross bracing) frame are constructed and pushover testing is carried out under quasi-static condition. The reinforcement details of these three frames are identical which does not meet the current IS: 13920 specifications for ductile frame. The models are instrumented with LVDTs to capture the deflections at the critical locations and the online data is acquired through data acquisition system. The analytical study includes a nonlinear static (pushover) analysis, in which the force-deformation relationships of individual elements are developed based on FEMA 356, have been carried out for these three frames and compared to the test results. Computer package SAP2000 is used for pushover analysis. The second part of the present study includes the seismic evaluation and retrofitting of a 4-storied 2D frame using infill wall and steel bracing. In this context firstly a 4-storied 2D RC frame structure, assumed to be located in Roorkee has been analyzed and designed using different versions of IS: 456 and IS: 1893 one by one in order of their occurrence starting from the year 1970 up to year 2002. One frame is also analyzed and designed considering without any seismic loads. Then non-linear static (pushover) analysis has been carried out using software package SAP2000 to find out their existing capacity. In the second phase the 4 storey frames (which have been iii designed as per different previous Indian codes and evaluated for strength and ductility and if found seismically deficient) are re-evaluated with masonry infill and steel bracing as retrofitting schemes using pushover analysis. The different pushover parameters of the frames before and after retrofitting have been compared. The seismic performance of the two retrofitting techniques namely infill wall and steel bracing are also relatively compared. iv