The significance of the dynamic interaction between bridge and trucks in the evaluation of the resonance structural frequencies

Bridges are key infrastructures that are built worldwide adopting different typologies, geometries, and construction materials. For these important infrastructures experimental proof tests before their opening to traffic are mandatory, as prescribed by many codes. In detail, static proof tests are performed to verify that the bridge behavior subjected to the design characteristic 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 in many countries 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 is usually experimentally identified from ambient vibration tests on the unloaded structure through the operational modal analyses. This paper discusses about the dynamic identification of bridges performed during the loading phases of the static proof test, which is of interest to improve the 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. In detail, the paper addresses the problem of the dynamic interaction between the bridge and the stationed load (almost always provided by trucks) that affects the results interpretation and, sometimes, leads to erroneous conclusions on the bridge performance. Firstly, the bridge-truck interaction phenomenon is briefly exposed referring to a 2-dof dynamic system, then, an application to a real bridge case study dynamically tested during the proof load test day, is presented.