Some Considerations on the Expected Resonance Frequencies of Bridges During Proof Load Tests

In case of bridges, static proof tests are mandatory to certify the compliance of the structural performance with that expected from the design model before the opening to traffic. Tests consist in progressively increase the load (by using fully loaded trucks) on one or more bridge spans and to monitor the structural response in terms of displacements, often obtained through topographic measurements. For new bridges, a linear structural response is expected while for existing ones some nonlinearities may also arise, for example in case of prestressed simply supported decks with I shaped or box girders of the 60s, for which the concrete cracking at midspan can be achieved, depending on the amount of prestress long term losses of tendons. The monitoring of the bridge resonance frequencies during the test may contribute to the interpretation of the overall test results, for both new and existing bridges. However, the estimation of the expected bridge dynamics during the proof load is not a simple task since it may be affected by bridge-truck interaction phenomena. To this purpose, a simple 2 degree of freedom model is adopted to qualitatively address the bridge-truck interaction phenomenon in the case of simply supported bridge decks. The model is adopted to perform a parametric investigation aimed to identify situations for which above interactions may be significant. The basic idea is to provide indications about the expected frequency changes during the tests with respect to the unloaded condition.