The safety of masonry arches

Abstract The paper describes the application of limit principles, derived from the plastic analysis of steel structures, to the masonry arch. Most voussoir arches have low stresses, and the lines of thrust lie well within the masonry, so that conventional factors of safety on stress have little relevance. However, a “geometrical” factor of safety may be defined by consideration of the arch of minimum thickness, of the same shape as the real arch, which would just contain a proper line of thrust. If the real profile of the arch is then discarded, and the calculations referred to an arch having the profile of the ideal line of thrust, it is shown that a simple analysis may be made of the effect of travelling point loads. The ideas are applied to the analysis of two actual bridges: (a) the stone arch between the western towers of Lincoln Cathedral, and (b) the Ponte Mosca in Turin. The paper is concerned only with the action of the masonry, and not with the design of the abutments to resist the thrust of the arch. However, the effect of geometry changes due to yielding of the abutments is discussed.