Backbone model for displacement-based seismic design of reinforced masonry shear wall buildings

A multiphase research program has been implemented at McMaster University in order to investigate the seismic response of Reinforced Masonry Shear Walls (RMSW) on the component—and system-level. Within this program, Phase I focused on evaluating the component-level response of six reducedscale two-story RMSW tested under quasi-static cyclic loading. Phases II and III focused on evaluating the system-level response of two RMSW buildings. Based on the experimental observations and the analysis of the three research phases, a simplified mechanics-based load-displacement trilinear backbone model was proposed. However, errors of up to 40% were observed within the elastic response predictions of the proposed model. This was attributed to the fact that the first point defining the backbone model was the yield point, subsequently, all the loading points prior to the yield point were assumed to have the same stiffness as yield stiffness. This resulted in underestimating the RMSW strength at early loading stages. The current study focuses on enhancing the model predictions for the building tested in Phase III. Subsequently, an additional point was computed in the model corresponding to the initiation of masonry cracking in order to enhance the model predictions in the elastic region. After the model modification, a maximum error of 25% was recorded. Within limitations, the quad-linear model is capable of modeling the RMSW response on the system-level including the strength degradation branch. Subsequently, this model presents a simple and effective tool that can be implemented in displacement-based seismic design frameworks.