Comparative seismic assessment of masonry towers through nonlinear analysis: The RiSEM experience.

Masonry towers are diffused through the whole European territory; these structures show unique typological and morphological features that might lead to severe effects under horizontal loads (such as the earthquake loads). Recently, the seismic assessment of historical masonry structures has been specifically taken into account by the Italian Guidelines. These recommendations propose a methodology of analysis based on three different levels of evaluation, according to an increasing knowledge of the structure. The last level of evaluation (LV3) requires a global seismic analysis of the whole structure by proper nonlinear numerical models. In general, the reliability of these tools depends on the achieved level of knowledge and, in this respect, a fundamental task is the estimation of the uncertain parameters (material properties, boundary conditions, etc.) affecting the structural behaviour. The paper aims to approach the effects of the uncertain parameters on the global structural response through the discussion of an illustrative case study: the historic masonry towers in the city centre of San Gimignano (Siena, Italy). The seismic risk of these towers was analysed in the framework of the research project RiSEM (“Seismic Risk of Monumental Buildings”, a project granted by Tuscany Regional Administration in the period 2011–2013) and the availability of experimental data on as many as fourteen towers with similar features, make the case study particularly relevant. The results of the LV3 approach carried out on some of the historic towers are summarized in the paper and useful conclusions are drawn in order to quantify the effects of the uncertainties on the seismic risk of such structural typology when performing a LV3 analysis through nonlinear models.