DECOMPRESSION, PASSIVE HINGES AND ADAPTED EQUILIBRIUM OF MASONRY ARCHES

Abstract

Masonry arches are among the most widespread structural systems in the built architectural heritage, and their mechanical behavior is governed by the interaction between geometry, masonry properties, and the redistribution of internal forces. This paper proposes a hypothesis describing the mechanical behavior of masonry arches based on the sequential
development of decompression, the formation of passive hinges, and the establishment of a new equilibrium state. It is assumed that the formation of three passive hinges transforms the arch into an adapted three-hinged system capable of sustaining the modified loading conditions, whereas the appearance of a fourth hinge results from local disturbances such as differential settlements, localized damage, or exceptional loading. The proposed mechanical model is illustrated through analytical interpretation and verified by means of documented case studies of historic masonry arch bridges exhibiting characteristic crack patterns and decompression zones. The results indicate that localized damage does not necessarily signify an imminent loss of load-bearing capacity but frequently represents a natural stage in the redistribution of internal forces and the structural adaptation of the arch to altered service conditions. The proposed approach provides a physically based framework for interpreting damage mechanisms, assessing the structural condition, and selecting appropriate conservation and strengthening measures for historic masonry structures.