A Three Dimensional Model for Masonry Arch Bridge Behaviour

The models used for the distribution of load within masonry arch bridges are demonstrably imperfect. Using a thrust model, it is not viable for a wheel load applied to a small patch to be supported by a parallel thrust in a strip of arch. The thrust must gather at the load and is clearly likely to spread laterally towards the springing. Until now, distribution models have implied that all the distribution takes place in the fill. There is some experimental evidence, and it is reasonable to conclude, that the arch itself distributes the thrust substantially. It is also clear that the analytical models of arch bridges are inadequate. They demonstrably underestimate the strength of many bridges while some show signs of distress at loads much lower than predicted by analysis. Since the analyses are clearly sound in the two dimensional sense in which they work, any error must be a result of the input, of which distribution models are a major part. This paper presents a more rational concept of distribution, shows how the concept can be modelled economically and effectively and indicates the likely benefits. It is argued that, even though the model is not experimentally proven, it is more realistic and could be used in practice, at least for the exploration of behaviour of seemingly sound bridges which have been condemned by existing analyses.