Dynamics of bridges during proof load tests and determination of mass-normalized mode shapes from OMA

Abstract

Modal shapes derived from Operational Modal Analysis (OMA) lack normalization with respect to masses, hindering direct estimation of effective mass or modal participation factors. The mass-change method is suitable to solve this issue since it proposes an experimental derivation of scaling factors for modes based on OMA results in different load conditions. However, in the case of bridges, the method application is challenging, as significant static masses are required on the deck for OMA in loaded conditions. To address this challenge, trucks can be used as added masses during standard static proof load tests for bridges, making the mass-change method feasible. However, the presence of trucks introduces dynamic coupling effects among bridge and vehicles, potentially complicating the dynamic identification. This paper proposes a methodology to exploit data obtained from OMA on bridges loaded with stationed trucks within the framework of the mass-change method in order to get the modal scaling factors for the mass normalisation of mode shapes and taking into account possible effects of the dynamic coupling between the two systems. To this purpose, the bridge dynamics with added stationed trucks is studied by introducing a finite element model to address the bridge-vehicles dynamic coupling. Then, a simple but effective 2-degree of freedom system is introduced to develop a parametric investigation in a non-dimensional manner in order to provide a practical tool for engineers to account for the bridge-truck dynamic coupling effects. Finally, numerical applications are proposed to derive some conclusions about the effectiveness of the proposed procedure and its suitability in real applications.