Measuring Dynamic Impaction Aged Timber Bridges: Some Experimental Options
There are thousands of aged timber beam bridges on local roads in New South Wales (NSW) and because of deterioration their safety levels are unknown. To identify a bridge safety level requires a structural performance measurement, preferably with a Structural Health Monitoring (SHM) system, so that any significant temporal change can be quickly identified. There is a need, however, to identify sensors and systems that can be used to monitor the dynamic impact of loads moving at highway speeds that are of adequate performance and of a cost that is a small fraction of the structures' value. Three measurement systems are considered: a high speed camera system to enable the establishment of base-line performance; a laser sensor system to enable accurate validation of other measurement systems on in-service structures; and a system comprising accelerometers to provide a relative motion record of components compared to the motion of a main girder.
Structural Health Monitoring of Older Bridges: Current Studies in Australia and Worldwide
Iron, steel and timber bridges were built extensively in the last half of the 19th and early part of the 20th Century. Many of these bridges are now deteriorating and becoming potentially unsafe. There is a need and an opportunity for engineers to analyse the methods and procedures necessary to ensure a responsible balance between structural safety of these bridges, their economy and forward planning for rehabilitation of these bridges. It is difficult to determine the load-carrying capacity of degraded bridges in order to subsequently assess suitable designs for more economic and efficient repairs and maintenance versus replacement. Static load testing has often been the main method to quantify the acceptable load capacity of structures such as timber bridge girders, but it can be expensive. Therefore, there is justification to develop a low-cost, non-destructive evaluation (NDE) that can be used to identify the load-carrying capacity of older bridges in order to identify alternative maintenance and repair strategies. Some attempts in this direction have been made in the past. One of the objectives of Structural Health Monitoring (SHM) is to ensure the safety and reliability of an engineering system for specified functions and loading conditions over a given period of time. Structural safety and reliability become the key issues in the whole process of evaluation. Reliability assessment of the design of structural members and systems has received a great deal of attention by researchers in the last decade and many methods have been developed. A number of selected studies on older bridges in terms of structural health monitoring in Australia and worldwide are presented.
Monitoring timber beam bridge structural reliability in regional Australia
There are many thousands of timber beam bridges throughout regional Australia, which are monitored primarily by visual inspection. Experience gained from historical failures has led to the clear realisation that visual inspection at intervals of many months or years is insufficient to identify potential failure caused by overloading and biological degradation. A bridge overloaded today can fail tomorrow and there is a need to implement structural health monitoring (SHM) so that the incidence of overloading can be identified soon after it occurs. This need is becoming more vital with the increased expectation to cater for the increased loads during periods of transporting seasonal produce. The measurement mid-span displacement of girders can be used to determine safety indices for the evaluation of structural safety. The detection of real-time damage in timber girder bridges by the use of high-speed camera and laser-based methods offer unique advantages and can lead to low cost measurement techniques. This work reports on the use of continuous monitoring methods for determining the structural reliability of timber-beam bridge girders. Some applications of the use of laser-based displacement sensing systems are discussed in relation to the monitoring of the structural reliability of two older timber beam bridges in regional New South Wales, Australia. Experimental and analytical approaches are presented and used to demonstrate that the probability of failure can be readily determined on a continuous basis using an SHM system.
Monitoring the Structural Integrity of Timber Bridges under Dynamic Traffic Loads using a High Speed Camera
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.