UBC Theses and Dissertations
Out-of-plane stability of reinforced masonry shear walls under seismic loading : in-plane reversed cyclic testing Robazza, Brook Raymond
Reinforced concrete block masonry shear walls (RMSWs) often constitute the principal seismic force resisting system in masonry structures in Canada. During an earthquake, these walls experience the combined effects of axial gravity loading and overturning moments due to inplane lateral seismic forces. This loading precipitates out-of-plane instability when the longitudinal reinforcement in the wall end zones is subjected to cycles of high tensile strain followed by compression. The Canadian masonry design standard (CSA S304.1-04) [Canadian Standard Association 2004] stipulates stringent height-to-thickness ratio limits for the seismic design of ductile RMSWs. Experimental research and earthquake evidence have demonstrated this failure mechanism in reinforced concrete shear walls loaded in-plane. However, similar evidence of the mechanism occurring in RMSWs is not available. This provided motivation for the research study described in this thesis. The research presented here represents the second phase of a comprehensive multi-phase research program. The first phase involved the experimental testing of full-scale reinforced masonry (RM) column-like specimens subjected to uniaxial cyclic tension-compression loading. The testing provided valuable insight into the out-of-plane instability as it occurs in RM. The second phase of the research program focused primarily on the lateral reversed-cyclic experimental testing of two full-scale, slender RMSWs with height-to-thickness ratios of 27, well exceeding the CSA S304.2 limits. The target failure mode was an out-of-plane failure mechanism. The results contribute unique benchmark data for the qualitative and quantitative assessment of the factors influencing out-of-plane instability of RMSWs as well providing better understanding of the mechanism itself. The effect of applied axial stress on out-of-plane instability is evaluated and possible other influential design parameters are discussed. From the results of the experimental study, it was concluded that the applied axial stress is a critical factor in the initiation of out-of-plane instability. This factor had effects on many other parameters, the most important of which was the development of tensile strain in the vertical reinforcement as well as the width and distribution of cracks over the plastic hinge height. These results indicate that the height-to-thickness ratio alone may not be an adequate factor governing the occurrence of out-of-plane instability in RMSWs.
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