Nonlinear seismic evaluation of confined masonry structures using equivalent truss model

Modern Performance-Based Design (PBD) approach for seismic analysis of building structures is based on the evaluation of nonlinear response to earthquake-induced demand and assessment of performance based on the lateral displacements/drifts. One of the key challenges associated with the application of PBD for seismic evaluation of confined masonry (CM) structures is the development of an adequate numerical model for simulating nonlinear response at different seismic demands. In this paper, an Equivalent Truss Model (ETM) has been proposed for nonlinear analysis of CM structures. The model is based on a truss analogy concept, where a CM wall is idealized as a truss consisting of pin-ended elements. A masonry panel is modelled as a compression element (strut), while horizontal and vertical reinforced concrete (RC) confining elements are modelled either as struts or tension elements (ties). ETM is essentially a macro-model that simulates nonlinear behaviour of a CM wall through lumped nonlinearity of masonry struts, while RC confining elements show linear elastic behaviour. The proposed model has been validated using the results of experimental studies on CM walls subjected to in-plane reversed cyclic loading. The paper also presents a case study, in which ETM was applied for seismic evaluation of an existing three-storey CM apartment building located in an area of moderate seismic hazard in India using nonlinear static analysis. The damage states for PBD were established based on past research studies on CM structures. A comparison of the results of conventional force-based design and PBD analyses showed that ETM can be used in conjunction with the PBD approach to evaluate existing CM buildings and also optimize design solutions for new buildings, which were developed using a conventional, force-based seismic design approach.