Monitoring the Structural Integrity of Timber Bridges under Dynamic Traffic Loads using a High Speed Camera

Abstract

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.