Keywords: stone arch bridge, finite element, flood load, contacts stiffness, Drucker-Prager, Mohr-Coulomb, saturated density.

In recent years, great interest has been shown in the effective modelling of masonry arch bridges. However, the issue of an efficient model is a controversy amongst researchers with contrasting strategies. The fact that there are many stone arch bridges in Iran (about 3300) serving the railway network, makes the issue very crucial in terms of the network vulnerability due to their unknown behaviour when subject to usual and unusual loads. In this paper, a nonlinear three-dimensional finite element method is employed in order to determine the ultimate failure load of stone arch bridges. Most these bridges are composed of three structural parts: arch barrel, backfill and spandrel walls. Efficient description of the material properties of these parts has a great influence on the accuracy of the resulting ultimate failure load. The experience has shown that elastic modelling of these systems could not yield a reasonable behaviour. Also, even if only a nonlinear model of the contacts was used, in some cases the analysis results would not be satisfactory. It is understood that accurate results could be achieved even by using a simple Mohr-Coulomb model for the barrel arch and spandrel walls. At the same time a Drucker-Prager material law for the backfill, along with appropriate modelling of the contact surfaces of different materials should be used. It is shown that hardening stiffness in pressure over-closure of hard contacts should not be neglected. In addition, the important role of spandrel walls has to be accounted for in a three-dimensional analysis model. Indeed, the former improves the bridge's failure mechanism, whereas the latter restrains the bridge's lateral deformations. To validate the proposed model, the actual failure test on the Prestwood Bridge is considered. The aforementioned analysis strategy shows a similar collapse mechanism as that of the prototype, while the ultimate failure load is achieved with only 1.3 percent error compared with the experimental one. Finally, the same bridge is also analyzed subject to the action of a flood load by replacing dry unit weights of the masonry and fill material with their respective saturated weights when flood level is higher than the bridge deck. Considering fully saturated and dry regions in the fill, a stress analysis was performed. It is shown that the bridge shell permeability could lead to a considerable (here about 30%) increase in the maximum principal stress while slightly decreasing the lateral displacement of the bridge.

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