Efficient Finite Elements Model Updating for Damage Detection in Bridges

Abstract

The study presented in this paper is part of a global research project aiming to detect damage in bridges with modal analyses of traffic-induced ambient vibrations by updating a detailed 3D finite element (FE) model. The commercial software FEMtools is used to solve the resulting largely underdetermined problem. The paper presents the case study of the Rivière-aux-Mulets Bridge located on Highway 15 in Ste-Adèle north-west of Montreal (QC, Canada). The reference 3D FE model of the bridge is first briefly presented. Then, four numerically-simulated damage cases to deck regions, piers or bearings were considered by modifying the stiffness of the elements in the model. The effects of these damages on the vibration frequencies and mode shapes were studied. It appeared that the mode shapes are very poor indices for local damages and that severe damages are not necessarily linked to high reductions of the vibration frequencies. The basic application of the FE model updating algorithms did not provide good estimates of the simulated damage. Thus, an improved procedure is proposed that has three main elements: (i) to control the unknowns-to-equations ratio in the inverse problem within a recommended range; (2) to use an iterative procedure in which update parameters are progressively discarded; (3) to prevent the algorithms from increasing the stiffness of concrete elements. The results obtained on all cases showed good accuracy in localizing and quantifying the simulated damage. The high sensitivity of vibration modes to the bearings parameters seems to be the source of most detection errors. Finally, the proposed methodology is shown to be robust with the accurate detection of combined damage with opposite effects on the natural frequencies of the bridge.