Experimental Modal Properties of a Steel Arch Bridge During the Static Load Test

This paper deals with the identification of the dynamic properties of a steel arch bridge located on the No. 77 main road, close to Macerata in central Italy. The main span of the bridge, of about 115 m, is formed by two inclined steel tubular arches that sustain the steel-concrete composite bridge deck by means of couples of hangers. During the static load test, before opening the bridge to traffic, an accelerometer array was installed over the bridge deck to evaluate the dynamic characteristics of the structure under different load conditions, in particular for bridge unloaded (before and after the load test) and for bridge loaded according to different load configurations obtained by positioning trucks over the bridge deck. Measurements of ambient vibrations, and operational modal analysis, were performed to estimate modal properties of the deck, namely natural frequencies and mode shapes. These experimental modal parameters determined at different loading phases were compared with the numerical ones obtained with the structural finite element model of the bridge. The dynamic interaction between bridge and trucks was also investigated by modeling the trucks as dynamic systems, i.e., not only in terms of added mass but also including their vertical and transversal stiffness. The obtained results show a good agreement between theoretical and experimental results, in terms of frequencies and mode shapes, both in the unloaded and loaded configuration; the first results on how different strategies of modelling trucks influence the modal parameters of the bridge are also discussed.