EXPERIMENTAL STUDY OF THE EFFECT OF INTERFACIAL GAPS ON THE IN-PLANE BEHAVIOUR OF MASONRY INFILLED RC FRAMES

Abstract

Previous studies on masonry infilled frames have shown that gaps between the infill and the bounding frame can significantly affect the stiffness and strength of the infilled system. However, the relationship between the magnitude and location of the gap and the stiffness and strength of the infill has not been thoroughly studied. In addition, the majority of the existing research on gap effects has been focused on infilled steel frames with only a few studies on reinforced concrete (RC) frames. For design, the American standard MSJC 2011 states that the infill can be treated as a participating infill with a top gap of less than 9.5 mm, provided that the infill strength and stiffness are reduced by 50%. The Canadian standard CSA S304.1 does not allow any gaps for design of a participating infill. However, the validity of these provisions has not been thoroughly examined. To investigate the effect of interfacial gaps on the in-plane behaviour and strength of masonry infills bounded by RC frames, five scaled masonry infilled RC frame specimens, as well as one bare RC frame specimen, were tested monotonically to failure under an in-plane lateral racking load. Parameters considered included gaps between the frame top beam and the infill, as well as between the frame columns and the infill. Two magnitudes, 7 and 12 mm, were studied for each gap situation. The load vs. lateral displacement response, failure mode, stiffness, strength, and ductility of each specimen were presented and discussed. Experimental results showed that, when compared with the bare frame, the presence of infills significantly increased the initial stiffness and ultimate strength of the infilled frames regardless of the presence of gaps. When compared with the infilled frame with no gaps, the infilled frames with gaps showed a decrease in initial stiffness regardless of the gap location. Gaps of 7 mm did not cause significant reduction in ultimate load, whereas gaps of 12 mm resulted in a more pronounced reduction. It was found that unreinforced masonry bounded by RC frames had improved ductility when compared to unreinforced masonry alone. The ductility of the masonry infilled frames was comparable to that of the RC bare frame. For the stiffness prediction, CSA S304.1 grossly overestimated the stiffness of the infilled frame without gaps. Reducing the calculated strut width by 60% resulted in a better stiffness estimate for the infilled frame without gaps. The MSJC design stiffness agreed reasonably well with the experimental stiffness. For the strength prediction, both CSA S304.1 and MSJC 2011 provided reasonable and conservative estimates of the ultimate strength for the infilled frames with no gaps. For the gapped specimens, the reduction on both the stiffness and strength due to gap effects was proposed