Structural health monitoring of older timber bridge girders using laser-based techniques

A new method of measuring the mid-span deflections of older timber bridge girders is presented in this paper. There are many timber beam bridges of unknown reliability in regional Australia under high traffic service loadings that were designed according to older codes. In order to identify the current safety index and probability of failure of these girders while in service, it is necessary to measure their deflections under normal and actual loadings. Because of the large numbers of in-service girders that need to be measured, it is important to use a quick, low cost, and easy-to-setup method in the field. A laser-based method is proposed here, which is adjusted to produce an image of the laser on a graduated chart mounted at the mid-span of the bridge girder. The source is mounted on a stable support. Traffic loading deflects the girder and the chart moves up and down in unison. A high speed camera was used to record the movements of the chart relative to the image of the laser. The video recording of the chart movements relative to the laser source was analysed to identify the peak movements. The chart was inscribed so that any movement of the image could be easily read from the graduated scale. It can be inferred from the results that, when the girder is loaded by moving traffic loads, the peak dynamic deflection of a girder can be readily identified.