Collapse analysis of masonry bridges taking into account arch–fill interaction

Experimental collapse tests on full and model scale masonry bridges have shown that fill and spandrels can strongly affect the collapse behaviour and increase the load carrying capacity. To provide a structural description of the arch–fill interaction effects, a two-dimensional model of multi-span masonry bridges is developed in which arches and piers are described as beams made up of elastic, non-tensile resistant (NTR), ductile in compression material, and the fill as a Mohr–Coulomb material modified by a tension cut-off under plane strain conditions. The load carrying capacity is evaluated by a finite element limit analysis procedure based on the kinematic theorem. The fill domain is discretized by triangular elements connected by interface elements in order to allow possible velocity discontinuities at common sides of adjacent triangular elements; arches and piers are discretized by two-noded straight beam elements. By linearization of the limit domains in the generalized stress space, a linear programming problem is formulated and upper bounds of the collapse loads are obtained. Two examples are discussed, concerning a real single-span bridge, tested up to collapse, and a multi-span bridge. The ideal ductility assumption implicit in limit analysis is discussed by comparing the upper bound evaluations to the results obtained by incremental analysis in order to obtain the validity limits of the upper bound limit analysis for the proposed model.