Modelling the seismic response of unreinforced existing masonry buildings: a critical review of some models proposed by codes

Distinctive features which characterize the existing unreinforced masonry building (such as the presence of flexible floors and/or weak spandrels) make the possibility of simulating the actual conditions of the structure crucial: indeed, models usually employed for new constructions are not always suitable for existing ones. Different strategies of modelling can be adopted with increasing levels of accuracy and computational effort. In this paper attention will be focused on very Simplified Models, such as “strong spandrel-weak pier” or “weak spandrel-strong pier”, and much more complex models such as “Equivalent Frame Model” proposing a critical review of their use and of the reliability of the assumptions which they are founded on. Particular attention will be paid to the issues related to the assessment of the in-plane strength of URM walls distinguishing between those related to the idealisation of the masonry wall in an equivalent model (e.g. geometry and boundary conditions assumed for each panel) and those related to the proper evaluation of the strength of each structural element, as defined by the previous step, on which the non linear response is concentrated (e.g. resistance criteria to be adopted distinguishing the case of pier from that of spandrel). In particular the fundamental role played by spandrel elements will be discussed.