Porta san Giorgio in Florence. Rigid block model analysis for the crack pattern interpretation

A distinctive element in the medieval defense architecture is the gate-tower type which was introduced in Florence in the eighties of the 13th century when the second municipal circle of the city walls was built to enclose the suburbs that had radially developed outside the perimeter of the original Roman plan. Furthermore, a common feature of the Florentine city gates is the presence of two overlapping arches in the external façade, in such a way as to provide a lunette surface for decorative and representative elements. However, the few city gates that were not knocked down together with the city walls in the 19th century to create, for Florence the new capital of Italy, a ring road surrounding the city, following the model of Paris or Vienna, have currently a reduced height. It was a defense strategy adopted in the 16th century in order to make them less vulnerable to cannon fire. Porta San Giorgio (hereafter, Saint George Gate), which is located along the Oltrarno city walls, underwent changes that have radically changed its appearance as well. However, its current configuration is also the consequence of more recent interventions consisting in the knocking down of the sixteenthcentury ashlar inferior gate and the wooden door to widen the opening and allow the passage of the cars. Currently, the masonry structure of the gate, made of “pietra forte” blocks, highlights a rather pronounced crack pattern, consisting of a clear sign of sliding blocks of the lower arch. In order to identify the possible causes of these cracks, the authors have used the method they have conceived to perform the nonlinear analysis of a rigid block model. In the paper an historical overview of Saint George Gate and the city walls together with the former results of the investigations on the mechanical behaviour of the gate is provided.