The performance of unreinforced masonry walls subjected to low-velocity impacts: mechanism analysis

The development of a rigid-body mechanism analysis capable of estimating the likely response of an unreinforced masonry wall subjected to an out-of-plane impact loading is described. The method automatically identifies the out-of-plane sliding mechanism likely to be most critical for a particular impact location, from a library of five possible mechanisms. Initially, a fracture-line analysis is performed to estimate the out-of-plane force required to crack the wall. Subsequent gross displacements of the wall panels formed are calculated using an iterative dynamic analysis. The fracture energy associated with initial crack opening is included in the calculations and checks are also conducted to determine whether the mode of failure changes during the impact event. Additionally, the possibility that wall rocking can occur, either on its own or simultaneously with the out-of-plane sliding mechanisms is considered. Results from the mechanism analysis model are compared with the experimentally observed results described in a companion paper. It was found that reasonable agreement was obtained for most of the walls subjected to mid-wall impacts, provided ‘dynamic’ rather than quasi-static values for the masonry flexural strength were used in the analysis. Sensitivity studies indicated that wall thickness and the coefficient of base friction were particularly important parameters. The analysis can readily be modified to approximately simulate the interaction between a wall and moving vehicle and is currently being extended to deal with reinforced masonry walls.