Keywords: masonry bridge, assessment, load carrying capacity, service limit state, structural model, finite element analysis, limit analysis..

Even though masonry bridges are the first structures to have been designed, according to some rational criterion, their mechanical response still remains partially unknown. Essentially, masonry bridges are not "arch" bridges, in which all the elements but for the arch barrel are non-structural, but are complex structures in which any bridge element takes part in the load bearing structure.

The scientific and technical literature provides few data on real bridges due to the difficulty in performing tests on in service structures of such large dimensions. Laboratory testing asks for reduced scale models to be tested and this asks that the model-to-prototype similarity be conserved to extend the results of the models to real bridges. Dimensional analysis shows that the model-to-prototype similarity asks to increase the masses or decrease the material strength along with the reduction in geometric scale. If only the geometry is retained, the similarity is lost (in fact, over-strong materials for the prototype are represented) and a bias is introduced in the tests. Some laboratory tests on masonry bridges suffer from this circumstance.

The assessment procedures rely on some commonly accepted assumptions and largely make use of Limit Analysis. Its assumptions, i.e. ductility of the materials, are not fulfilled by brickwork, which is a well known quasi-brittle material with limited strain capacity after the peak load. This raises a fog on Limit Analysis procedures.

Recent reduced scale tests showed that only deep arches can be reasonable dealt with by Limit Analysis procedures while the collapse of shallow arches is not a bending collapse, that is typical of collapse mechanisms, but is ruled by the axial force, collapse being attained because of compressive crushing of some sections.

The alternative to Limit Analysis nowadays are finite element procedures. Even though they seem to be capable of representing the whole bridge in its three-dimensional shape, practical difficulties and the impossibility of defining all the mechanical parameters needed for complex constitutive models make finite element models approximated approaches, too.

In this chapter a comprehensive view of the assessment procedures is given underlying the pros and limits of every approach. Each procedure allows some aspect of the bridge mechanical response to be discovered and it is only a wide and rational use of all the available procedures that may give satisfactory knowledge of the structural response. An example of the issues discussed is provided.

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