Bridges are key infrastructures that are built worldwide adopting different typologies, geometries, and construction materials. Everywhere is required that these important infrastructures must be checked before their opening to traffic and, more specifically, that the static proof tests are performed, verifying that the bridge behaviour subjected to exceptional load conditions complies with that expected and does not exceed limits provided by codes (in terms of stresses and deck deflections). Nowadays it is also often required to check the fundamental frequency of the real structure and to verify that it is in accordance with that obtained numerically during the design procedures. The fundamental frequency of the bridge can be experimentally identified adopting ambient vibration tests and operational modal analyses, performing the dynamic tests on the unloaded structure. In this case, it would be interesting to perform dynamic tests also during the loading phases planned for the static proof test, in a way to perform a further control of the loading procedures, and to obtain frequency benchmarks that can be used as a support for the bridge structural health monitoring during its life.

This paper discusses about the dynamic identification of bridges performed during the loading phases of the static proof test. Indeed, the paper shows that the dynamic interaction between the bridge and the stationed load (almost always provided by trucks) affects the result interpretation and, sometimes, can lead to erroneously judge the bridge performance. At first, the interaction phenomenon is exposed; then an application to a real bridge case study, dynamically tested during the proof load test day, is presented.