MASONRY INFILL REINFORCED CONCRETE FRAMES UNDER CYCLIC LOADING

Abstract

This study involves experimental and analytical investigations on the seismic performance of masonry infill non-ductile reinforced concrete (RC) frames. An innovative retrofit scheme is proposed for enhancing seismic performance of such structures. Four ¾ scaled models of single bay, single story, non-ductile RC frames infilled with masonry panels were tested under lateral cyclic load and constant vertical load. The aspect ratio of the masonry infill panel (width to height) in the un-retrofitted specimen is 2.0. As for the analytical investigation, nonlinear static pushover analyses of the tested specimens are performed based on an equivalent three diagonal struts approach. The surrounding frame is modelled using discrete elements with flexural and shear plastic hinges lumped at the ends, and the infill panel modelled using compression-only bar elements. Experimental results indicate that shear failure occurs prematurely in the columns of the un-retrofitted specimen at a very small drift ratio of 0.30% due to the large strut force exerted by the unreinforced masonry (URM) infill panel on those columns following corner crushing of the URM panel. Guided by such failure, the URM panel is separated from the columns and steel brackets are provided to transfer the interactive forces between the panel and the beams, thereby totally eliminating transfer of large strut force to the columns. The proposed scheme results in much enhanced performance of the retrofitted structure over the un-retrofitted one, with the drift capacity increased 5 folds while retaining most of the strength and stiffness. Moreover, premature severe shear damage in the columns was totally eliminated. With slight steel wire mesh reinforcement (reinforcement ratio of 0.11%) provided in the infill panel, the test specimen could attain a drift capacity of 8 times that of the un-retrofitted one. The analytical study of masonry infill RC frames reveals that the ASCE 41-06 specified cohesive capacity of masonry infill is too conservative for the specimens tested, and the drift associated with the peak lateral capacity of the retrofitted masonry infill should be taken as twice the value specified in the standard. With proper adjustments made for specifying the lateral load-deformation of the URM infill, reasonable agreement in the simulated lateral load-displacement relation of the URM infilled RC frame with the experimental results is obtained.