Seismic shear demand of ductile cantilever walls: a Canadian code perspective

During severe earthquakes, ductile flexural walls are expected to exhibit inelastic flexural behaviour while other brittle deformation mechanisms, such as shear, should remain elastic. The philosophy of the Canadian seismic provisions for flexural walls is based on the assumption that the force reduction factor is applicable to both flexure and shear. If the bending moments are limited because of the flexural strength of a wall, then the shear forces are considered to be limited by the same ratio. Recent case studies have not confirmed this philosophy. Brittle shear failures in walls are still possible even if their shear strengths are established by the Canadian standards. This paper presents an analytical investigation on the shear demand of ductile flexural walls designed for three different seismic zones in Canada. For each zone, an ensemble of code compatible historical earthquake ground motions is identified. The shear demand of each structure, under each earthquake record, is obtained by nonlinear time-history dynamic analyses. In 77% of the cases, the computed dynamic shear demand is higher than the current code shear strength. To address this issue, a force modification factor for shear, different from the one for flexure, is suggested for the Canadian code. Key words: earthquake, seismic response, shear walls.