Response of slender reinforced masonry shear walls with flanged boundary elements under in-plane lateral loading: An experimental study

Abstract

Past experimental studies have demonstrated that reinforced masonry shear walls (RMSWs) can provide adequate ductility and energy dissipation capacity when acting as the seismic force-resisting system in low-rise buildings. However, slender RMSWs used for these applications, characterized by high height-to-thickness (h/t) ratios, may be vulnerable to out-of-plane instability failure modes when subjected to in-plane seismic loading. Out-of-plane instability may develop when a wall end zone undergoes cycles of high tensile strain, followed by compressive strain during in-plane seismic loading. Out-of-plane instability has the potential to cause unexpected and rapid strength degradation, which can lead to a global collapse. Provisions of the Canadian masonry design standard CSA S304 currently aim to prevent out-of-plane instability by prescribing h/t restrictions. Relaxation provisions were introduced in CSA S304-14 for walls with flanges or boundary elements and for walls subjected to low axial stress, however these provisions have not been experimentally verified. Moreover, prescriptive h/t restrictions disregard many important factors that affect failure modes for RMSWs under in-plane reversed-cyclic loading, including the type of loading protocol (symmetrical or asymmetrical) and the effect of flanges for T-shaped walls. This paper presents a detailed experimental study on the influence of these latter two factors on the out-of-plane response of slender RMSWs subjected to in-plane cyclic loading. The testing program comprised three full-scale RMSW specimens (two T-shaped and one rectangular) subjected to reversed-cyclic loading. A displacement-controlled loading protocol was applied until the lateral load capacity of the specimen was reduced to 80% of its maximum value. The test specimens were designed to be vulnerable to out-of-plane instability, however, although significant out-of-plane displacements were observed in one of the tests, failure of the test specimens was not governed by out-of-plane instability. Detailed descriptions of the specimen responses and a discussion on the key factors affecting the out-of-plane instability failure modes are provided in the paper. The test results confirm that the out-of-plane behaviour of slender RMSWs is similar to reinforced concrete shear walls (RCSWs) in that it is significantly affected by the presence of boundary elements (e.g. [18,11,31,1]) and the type of loading protocol (e.g. [23,24]). This leads to the conclusion that seismic design and detailing provisions for preventing out-of-plane instability in RMSWs and RCSWs should consider these parameters.