Keywords: bridge, masonry, scale, model, dynamics, frequencies, modes.

In spite of the large number of static tests performed on masonry bridges and on reduced scale models, the mechanical response of these structures is not yet fully understood for static loads and the dynamic response remains still largely unknown. Dynamic identification procedures have been recently applied to masonry arch bridges showing that the two main difficulties come from the large number of accelerometers needed and the low signal to noise ratio.

In this paper two arch models have been considered with a rise-to-span ratio of 0.2 and 0.3 respectively, spanning 4m, 25cm thick and 45cm deep. These two geometries are intended to be 1:2 scaled models and similarity to the prototype bridge has been retained reducing also the material strength and stiffness. The prototypes consist of three geometries and were tested in the following conditions: bare arch; arch + fill, arch + spandrel. Static and dynamic tests at different levels of load-induced damage allow some useful information to be derived on the contribution to the bridge stiffness of "non structural" elements such as the fill and the spandrels, that could be useful for monitoring the condition state of a bridge and to detect severe load-induced damage.

Even though some aspects of the experimental outcomes are still not clear, we can say that the bare arch and the arch with the fill are substantially different structures. Also from the dynamic point of view, as already conjectured from static load tests and from theoretical work, the fill changes the response of the arch in a fundamental way increasing the stiffness and the natural frequencies. This latter outcome, which is somehow unexpected since the fill increases the structural masses; shows that the increase in stiffness is much more important than the increase in mass.

The effect of damage can be addressed generally as a reduction of stiffness and, therefore, as a reduction of the natural frequencies. Nevertheless, only severe damage can be noticed from dynamic identification since damage is a typical local phenomenon while natural frequencies and models are global quantities that are affected by local damage only if the damage is severe. Moderate damage will probably induce reduced change in structural frequencies that could be overshadowed by the errors in dynamic identification.

In the case of arches with fill the natural modes appear to be non symmetric. This is due to the lateral structures used for containing the fill that are not rigid and, as such, affect the dynamic properties of the arch. Nevertheless, it appears that this effect plays a minor role on the overall dynamic response of the model.

The research is still going on and several other arches, also with spandrel, are being tested so that more general conclusions will be derived in the near future.

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