Fibre-based capacity model for urm piers subjected to combined in-plane and out-of-plane actions

Seismic performance of masonry buildings is usually ssessed at local and global levels, separately. S ismic safety against local collapse mechanisms is evaluat ed via macroblock models and linear/nonlinear kinem atic analysis methods. Conversely, safety against in-pla e f ilure modes is assessed through nonlinear stat ic/dynamic analysis of global macroelement models. Nonetheless , the in-plane seismic capacity of masonry walls ca n be strongly influenced by simultaneous response to out -of-plane actions. Although this issue has been rec ently investigated in a few experimental programs and num erical studies, the level of knowledge is still lim ited. In this study, the authors present a novel fiber-ba sed capacity model that allows performance-based se ismic design/assessment of unreinforced masonry piers sub jected to combined in-plane and out-of-plane loadin g. Based on a nonlinear incremental analysis procedure , moment–curvature diagrams are derived at differen t levels of axial load and 3D flexural strength domains are developed at five performance limit states. Nonline ar sectional capacity under biaxial bending and axial lo ding is directly governed by the macroscopic con stitutive model assigned to masonry and sectional shape. Anal ysis results show a strong interaction between bend ing moments related to in-plane and out-of-plane loadin g, which changes with the axial load level. Simplif ied biaxial interaction models are derived through nonlinear re g ession analysis for engineering practice. It is s hown that the axial load level and ratio between in-plane and out -of-plane actions has an impact on sectional ductil ity at different limit states. The capacity model allows co nsidering the softened response of masonry sections under increasing axial load levels, which also induces a reduction in ultimate axial load.