This paper describes the evolution of large road bridges in NSW, citing examples of various timber and iron genres in northern NSW. In particular it highlights the high proportion of iron bridges constructed in northern NSW over approximately a 25-year period from around 1870. Various postulates are canvassed as to why that might have been so. Financial astringency forced the engineering profession to account for deteriorating economic conditions and political imperatives. Typical of such major changes was a dramatic swing from substantive and expensive iron road bridges to more slender, astutely-designed and economical timber truss bridges. These colonially-designed "lean and mean" timber truss bridges were a far cry from the earlier, stockier, high maintenance versions that were inherited from British/European designs. In some respects such innovative local design was a symbolic way of releasing the restraining shackles of the colonial past and the spawning of a new nation. For over 40 years these new-style timber bridges, of successively improved forms, dominated timber truss bridge construction in NSW, to the extent that NSW was euphemistically known as the "timber bridge state". It was not until innovations and improvements were made in steel production, steel-fixing and concrete technology in the early 1930s that the newer materials started to replace timber.

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of unknown reliability, have high traffic loadings and were designed according to codes, many of which have since been superseded.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.

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.

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.

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.