Monitoring the Health of Timber Bridge Beams

Abstract

There are over 2000 timber-bridges in regional New South Wales (NSW) and many more are still in use throughout Australia. Many of these bridges are of unknown structural integrity. They were built in an era when structural components were expected to survive their lifetime without failure. Many of these bridges are now degraded and need to be monitored to determine their integrity. The aim of this research was to test the hypothesis that continuous deflection monitoring can be used to assess the probability of timber-bridge girder failure. To achieve this aim, new Structural Health Monitoring (SHM) strategies were created together with new laser-based deflection measuring equipment and high speed camera recording techniques. Bridge performance was evaluated by firstly determining Modulus of Elasticity (MoE), Modulus of Rupture (MoR) and percentage loading from load-v-deflection measurements. Then the probability of girder failure and a safety index were calculated. Bridge performance benchmarks were set and structural integrity ensured by checking that limit state safety indices were not exceeded. The testing of timber-bridges, by measuring girder deflection, has historically been restricted to non-linear static proof-load testing. Strength testing with lighter, in-service loads has not been developed because of the lack of a relationship between girder deflection and girder strength. More recently, dynamic techniques have been utilised. Strain sensors have been applied to the surface of timber girders to determine the peak stress levels. This approach is limited for long term use. As the surface of the girder degrades, sensors cease to accurately measure the peak stress, unless it is continually recalibrated throughout the monitoring period. In another approach, the vibration of a complete structure is analysed. This technique works well for rigid structures, but is too complex for long term use with timber-bridges that have loosely connected girders and deck planks.