Many short span timber beam bridges in regional New South Wales are of unknown reliability, have high traffic loadings and were designed according to codes, many of which have since been superseded. Because asset managers are delaying their maintenance for fiscal reasons, a high proportion of these bridges are structurally degraded and potentially unsafe when excessively loaded. In regional areas, a prioritised maintenance program can be a cost effective alternative to bridge replacement. Such older bridges will require continuous monitoring by low cost methods to assess the temporal probability of their failure. This paper examines the potential for measuring the mid-span deflections of girders caused by high traffic loads to obtain continually updated indicators of the structural health of girders. The mid-span deflection data of a case study bridge were continuously measured using a laser based measuring system, recently developed by the first author. An analysis of the deflection data is used to obtain a reliability index and the probability of bridge failure. Reliability indicators such as these can be used, in conjunction with continuous deflection monitoring, to prioritise cost effective maintenance of older timber bridges in regional New South Wales.

This paper reports on the novel use of a high-speed camera to record dynamic movements of a structure under in-service loading without the need for disruptive dedicated proof-loading. For local and state road authorities this represents a significant reduction in resources needed and avoids disruption to existing traffic flow. In regional Australia there are many short span timber beam bridges of unknown reliability. A case study of one multiple span bridge is examined in this paper. Many timber beam bridges were built in the 19th and 20th centuries and were designed to codes that have since been extensively revised. The original design factor of safety for these structures, with new timber, was anticipated to be about five, but full size element testing has historically been used to show that some in-service aged girders have had a factor of safety of about two. Uniform gross vehicle loads have increased and can have significant impact on multiple span bridges. To determine the level of safety for these bridges requires the application of new measurement techniques. The technique used involved a staff, a vernier and a high speed camera. A staff was attached to the mid-span of each girder and its movement monitored with a vernier at ground level. Dynamic movement was recorded with the camera as a vehicle crossed the test-case multi-span bridge at Gostwyck, NSW. The mid-span deflections caused by the test vehicle were compared to data obtained using a simplified SAP2000 model of the bridge and the mid-span influence line inferred.

About 2500 timber bridges are on local and regional roads in NSW and many of these bridges were built forty or more years ago. Regular inspections are required to ensure that they have a low probability of structural failure. The aim of this research was to determine if the health of such bridges could be continuously monitored. To test the feasibility of this aim, literature was searched to determine: • The affect that component lifetime has on structure lifetime. • The typical lifetime of a timber component. • How to determine the lifetime of timber components. • The factors that degrade timber beams. • Typical inspection methods and periods as applied to timber bridges. • Current structural measurement techniques for bridges. • Measurement techniques that could be adapted to measure bridge deflection continuously. The performance characteristics of a continuous bridge deflection measurement system was tested for accuracy and calibrated in the laboratory. This measurement system was then applied to a timber beam bridge and the peak deflections caused by normal traffic continuously recorded for a 24 hour period. A method of identifying girder lifetime and structural health is proposed using the deflections produced by light and medium weight traffic, thus precluding the need to proof-load and full-load test timber bridges.

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.