IN-PLANE BEHAVIOUR OF MASONRY INFILLED RC FRAMES WITH INTERFACIAL GAPS SUBJECTED TO QUASI-STATIC LOADING

Abstract

Previous research on masonry infilled frames has shown that the presence of infill-to-frame interfacial gaps and infill openings can affect the in-plane strength and stiffness of the infilled frame system. However, most of these studies used infilled steel frames, and subjected to monotonic loading. The parameters in terms of gap magnitude and location were also limited. For design, the American Standard, MSJC 2013, allows for infilled frames with a gap of less than 9.5 mm located at the frame top beam and infill interface to be considered as participating infills in lateral load resistance, but in this case, a 0.5 reduction factor shall be applied to the design strength and stiffness of the infill. The Canadian standard, CSA S304-14, requires that no gaps be present at the infill to bounding frame interface for the infill to be considered participating in lateral load resistance. This study was motivated to further investigate the effects of interfacial gaps and infill openings on the in-plane behaviour of masonry infilled frames. To augment the existing database, this study focused on reinforced concrete (RC) frames and adopted cyclic loading scheme. Four scaled masonry infilled RC frame specimens, as well as one bare RC frame specimen, were subject to quasi-static cyclic loading to failure. The four infilled frame specimens had the following interfacial gap scenarios: 1) a gap at the top beam-infill interface of 12 and 25 mm, respectively; 2) a gap at the column-infill interfaces of 12 mm (6 mm gap on each side); and 3) a full separation gap of 12 mm with 12 mm gap at the beam-infill and 12 mm at column-infill interfaces (6 mm gap on each side). Of the infilled specimens, two specimens also had a window opening accounting for 16% of the infill area. Quasi-static loading followed the ATC-24 loading protocol in order to measure cyclic response. Results from a previous study (Hu 2015) completed in the same experimental program were used for comparison purpose. Experimental results showed that compared to the bare frame specimen, a noticeable increase in strength and stiffness was observed with presence of infill regardless of the presence of gaps or infill openings. When compared to a specimen with no gaps, a reduction in ultimate strength and stiffness was observed with the presences of gaps. As the beam-to-infill gap size increased, the reduction in ultimate strength also increased; however, the initial stiffness remained virtually the same when gap increased from 12 mm to 25 mm. The presence of openings resulted in significant reduction in ultimate strength and stiffness, however, when both openings and gaps are present, the strength and stiffness reductions as a result of gaps is not as significant. The cyclic loading scheme revealed dynamic characteristics of masonry infilled frames. It was found that masonry infilled frames had greater ductility than that recommended by NBCC 2015 for unreinforced masonry. The ductility of the infilled frame specimens is comparable to that of a RC bare frame. The 0.5 reduction proposed by MSJC 2013 proved to be overly conservative for both ultimate strength and stiffness of infilled frames with gaps.