Influence of Operational Conditions on the Modal Based Damage Analysis of Pedestrian Bridges

Abstract

Modal based monitoring systems for civil engineering structures, especially for bridge constructions, receive more attention in recent years. The goal of those systems is to detect damage processes at an early stage. This enables quick reaction to damages and ultimately extends the life of existing bridges while also minimizing maintenance costs. The modal parameters, namely natural frequencies and mode shapes are dependent on the stiffness and mass distribution of the structure. Since the traffic loads represent temporally and spatially variable stiffness and mass distributions of the bridge structure, the question arises how reliably the modal parameters can be estimated, and if structural stiffness changes can be distinguished from the variable stiffness and mass distributions of traffic load. In this paper, the influence of pedestrian traffic on the estimation of the modal parameters is investigated. Spring-Mass-Damper systems represent the pedestrians to be able to take human mass, damping and stiffness effects into account. The vertical acceleration responses are considered for single pedestrians and a crowd of pedestrians using probabilistic approaches. The influence of the operating loads is first numerically investigated using an Euler-Bernoulli beam model and then compared with the results of vibration measurements on a pedestrian bridge. We can substantiate, that pedestrian volume, the ratio of the natural frequencies of the bridge superstructure to the step frequency of the pedestrians as well as the measurement length are the decisive criteria for successful damage identification. A clear distinction between the amplitude scatter of the mode shapes due to traffic loads and a structural stiffness change is possible by appropriate selection of the damage identification methods. The contribution of this paper consists in the investigation of the influence of operational loads on the estimated mode shapes and the establishment of the relationship to the modal based damage analysis.