An atypical flat arch in Prato Cathedral: some remarks on the non-standard plastic behaviour of stone structures

In this contribution the equilibrium of an intriguing case study is studied, which was inspired by the flat<br/>arch that crowns the transept portal in the Prato Cathedral. The ‘wrong’ shape of some voussoirs invites to reflect<br/>on the difficulties for the application of limit analysis techniques to determine the collapse conditions.<br/>Investigations on the minimum friction coefficient compatible with an equilibrium condition are conducted by<br/>means of different methods.<br/>At first, a semi-analytical approach combines static and kinematic analyses exploiting both a modern version<br/>of the ‘stability area method’ and the virtual work theorem. Then, the results thus obtained are compared with<br/>those derived from a FEA, which aims at simulating the construction process of the flat arch, and from DEM,<br/>which capture the collapse mechanisms; results are in a very good agreement. Finally, inconsistencies in adopting<br/>a limit state analysis based only on the lower limit theorem are highlighted.<br/>This research stresses the need for a critical viewpoint in the examination of the collapse behavior of masonry<br/>structures