Masonry arches are one of the most common and extended structural shapes present in the worldwide architectural heritage. When handling with these structures at least two demands are addressed to the scientific community: (i) development of reliable and affordable methods for documenting historic infrastructures and (ii) improvement of structural analysis for appraising the actual structural condition of the bridge. In relation to the first item, 3D laser scanning is an exponent of the evolution of the non contact techniques for built up structures survey and documentation. In relation to the structural stage, the limit analysis is still the most extended method among engineers and technicians and some of the most common software package for masonry arch bridges work within the assumptions of limit analysis. However computations should be adapted to work on the base of the real geometry instead of on idealized geometry in order to benefit from the advanced 3D surveying techniques and get more reliable results. In this article we describe a methodology based on using TLS to obtaining the real 3D geometry of the whole bridge and the arches. The collapse load value and position, as well as the hinges position and reaction forces are obtained within the assumptions of the limit analysis but on the base of the resulting survey. The methodology is applied in a mediaeval bridge.