Obtaining mass normalized mode shapes of motorway bridges based on the effect of traffic movement

Abstract

Ambient vibration testing has become a promising alternative for the forced vibration method due to its well-established practical advantages on bridge structures. However, because of the unavailability of the input data in output-only vibration tests, the extracted mode shapes are not mass normalized. Mass normalized mode shapes are essential in various modal analysis applications such as flexibility-based damage detection techniques. Mass change method was proposed to resolve this issue for Operational Modal Analysis (OMA); however, there are certain practical issues that limit the applicability of this method to real-world structures. In this paper, we propose to utilize the dynamic effects of the traffic movement over bridges as the required perturbation to the structure's modal properties. Numerous traffic conditions are considered, and uncertainty analyses are performed to evaluate the suitability of the loads in different traffic scenarios. Furthermore, a numerical case study is conducted on Hafez Bridge in the city of Tehran, Iran. Actual traffic conditions are simulated using the traffic micro-simulation method, and vehicle forces and masses are applied to the finite element model of the bridge using the equivalent nodal forces method. OMA is conducted in the time domain, and modal parameters are extracted using the Stochastic Subspace Identification method. Finally, it is shown that using the proposed framework, not only the modal scaling factors are estimated with satisfying accuracy, but also the practical difficulties of the conventional mass change method such as bridge shutdowns are eliminated.