Non-linear Dynamic Analysis for Collapse Probability Assessment of Historic Masonry Towers

Masonry towers constitute a significant part of the European built heritage that, due to their intrinsic geometric slenderness, result particularly vulnerable to horizontal loads. Their seismic assessment requires a numerical model able to reproduce the material nonlinear behavior and to account for the different sources of uncertainties. Among the uncertainties which may affect the reliability of the results, it is possible to list: (i) the building history, its evolution and the structural details, (ii) the masonry construction technique and the material properties and (iii) the boundary condition since in several cases towers are embedded in other buildings (such as churches bell-towers). This paper, to assess the seismic risk of historic masonry towers, investigates the reliability of a probabilistic approach aimed to include some of the sources of uncertainty above mentioned. The employed probabilistic framework was originally introduced in 1991 by Gusella, and it is here discussed by analyzing a representative case study: an historic masonry tower in San Gimignano (Italy). In a first part of the paper the results of ambient vibration tests are employed to tune a numerical model of the tower. The identified numerical model is subsequently employed to perform the non-linear time history analyses that are needed to express the seismic risk of the tower in terms of collapse probability.