Ancient structures have been designed according to semi-empirical rules based on few simplified mechanical principles; nevertheless, their structural performance is rather good in the majority of cases. In this article, the ancient rules for masonry bridges and arches, derived from some historical manuals, are analyzed on the basis of modern mechanics. Some issues on the material mechanical characteristics and on the structural response of masonry arches allow a better understanding of the structural performance and response of old arch structures.

A stepwise iterative procedure for the nonlinear analysis of multispan arch bridges, suitable for implementation by standard programming of commercial finite element codes, is discussed. Relying on the plane section hypothesis, masonry is assumed elastic-perfectly plastic in compression and no tensile resistant; a collapse condition is found when an ultimate strain is reached. The iterative procedure is that of an elastic prevision and subsequent nonlinear correction of the nodal forces: tensile stresses are not allowed in the mortar joint by adapting the effective height of the arch to its compressed part, while the plastic response is represented by additional external fictitious forces accounting for the compressive plastic plateau. The procedure is first tested by comparison with experimental data and then applied to sample bridges, pointing out how the collapse mechanism and the ultimate load depend on the geometric and mechanical parameters.