Recording timber bridge girder deflections using a laser reference source and a high speed camera

Abstract

This paper identifies a method of measuring the mid-span deflections of timber bridge girders when loaded by traffic. There are many short span timber beam bridges of unknown reliability in regional Australia that have high traffic loadings and many of these bridges 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 girders that need to be measured, it is important to use a low cost method that is quick and easy to set up in the field. The method proposed here involves a laser source 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 synchronously. A high speed camera is used to record the movements of the chart relative to the image of the laser. The chart was inscribed so that any movement of the image could be easily read from the graduated scale. A video recording was made of the chart movements relative to the laser source and the recording was analysed to identify the peak movements. The results show that, when the girder is loaded by moving traffic loads, the peak deflection, the dynamic resonant behaviour of girder deflection and the recovery can be readily identified.