Numerical Macro-Modeling of Unreinforced Masonry Structures: A Critical Appraisal

Unreinforced masonry (URM) buildings represent a high percentage of existing buildings in many seismically prone areas around the world. A rigorous simulation of the nonlinear dynamic behaviour of URM buildings requires refined nonlinear finite element numerical models and a consistent computational cost, making these methods unsuitable for practical applications. For this reason, in recent decades, many researchers have proposed simplified alternative methodologies. In this paper two different macro-element simplified models are compared and applied to a benchmark URM prototype structure. The first numerical approach is based on the discretization of the structure into piers and spandrels, which are connected by rigid nodes, hence creating an equivalent frame model. An efficient nonlinear fibre-section force-based frame element, which accounts for shear deformation and shear failure, allows a straightforward and accurate simulation of the combined axial-flexural behaviour of both piers and spandrels according to uniaxial constitutive laws. The second numerical strategy is based on a plane macro-element whose kinematics, governed by a discrete distribution of nonlinear springs mounted on an articulated quadrilateral, that allows a reasonable and accurate prediction of the in-plane behaviour of masonry walls. The paper shows that both approaches can be successfully applied for the seismic assessment of URM buildings. Benefits and drawbacks of both models are highlighted with the aim of providing recommendations for a proper use of these powerful, but not always applicable, computational strategies.