Evolutionary strength domains of unreinforced masonry spandrel panels including strain softening

Post-earthquake reconnaissance reports and experimental programs have shown that spandrel panels, i.e. the horizontal structural components between consecutive piers, play a key role in the in-plane nonlinear response of unreinforced masonry (URM) walls with openings to lateral seismic actions. This paper investigates the bending moment capacity of URM cross-sections through the development of simplified moment - axial force interaction equations including the strain ductility of masonry to capture strain softening effects. A constitutive model for masonry subjected to uniaxial compression parallel to mortar bed joints was processed to get flexural strength domains at both elastic and ultimate limit states. Evolutionary strength domains were also developed to be used in seismic assessment of masonry buildings based on static pushover analysis. They allow to account for moment capacity increase/degradation under increasing strain ductility demand on the URM cross-section. The proposed limit strength domains are compared to those corresponding to other constitutive models typically used for masonry structures. Ultimate strength domains are compared to experimental data available in the literature. It is shown that more conservative estimations of ultimate moment capacity are obtained if the proposed equations and those corresponding to the elastic-perfectly brittle constitutive model are used.