Role of diaphragm flexibility modelling in seismic analysis of existing masonry structures

Numerical modelling considering inelastic response often becomes essential for seismic analysis and assessment of existing masonry structures. Post-earthquake surveys and past experimental studies have demonstrated that flexible diaphragms significantly alter the seismic behaviour of masonry structures. The absence of a rigid diaphragm alters the seismic performance of a structure due to local mechanisms or out-of-plane actions that could compromise the global in-plane capacity. Consistent inferences from different numerical modelling approaches for masonry structures with flexible diaphragms have however been elusive in previous research. Macro-element (or equivalent frame) modelling and non-linear finite element modelling approaches have important differences, particularly in the way interactions between out-of-plane actions and in-plane shear response can be modelled, and in the way they can represent flexible diaphragms. The current paper examines the role of diaphragm flexibility modelling in the global seismic performance of existing masonry structures through a numerical study. Differences observed in the results of equivalent frame modelling versus non-linear finite element modelling, for a set of representative structural models with rigid and flexible diaphragms, under both static and dynamic analyses, are examined. The structural models examined range from single-storied to multi-storied (G + 2) and plan symmetric to asymmetric configurations. The approach to modelling diaphragm flexibility in equivalent frame models is also discussed. Significant differences between approaches imply repercussions on seismic assessment and retrofit design for existing masonry structures.