This paper presents the results of an investigation of a prototype system for determining the daily structural performance of timber beam bridges. Local Government authorities in New South Wales are reported to be responsible for 2500 timber bridges. The lack of appropriate resources thwarts satisfactory maintenance, thus the need to identify a low cost method of continuously measuring the health of individual spans. Measurement of bridge girder deflection has been variously reported as an effective indicator of performance. Many existing measurement techniques apply to bridge measurement, but few can be applied to short span timber bridges with appropriate accuracy or suitable mounting method. One technique, adapted to gather typical data has been applied to a sample bridge. Data, representative of the normal traffic flow over the sample structure, were used to develop a method of identifying structural health. The data comprised deflection measurements for a typical 24-hour period with daily reports interpreted for their temporal behaviour pattern. This monitoring technique will lead to the identification of component lifetime in the presence of degradation and enable the replacement of components prior to wear out. It will also allow for more precision in setting particular load limits for bridges while enabling a more effective prioritisation of remedial work.

Timber bridges were built extensively throughout Australia, in the 19th and 20th centuries and over 2500 are still in use on rural roads in New South Wales. For many of these bridges degradation may have occurred and it is difficult to determine the carrying capacity of an apparently degraded structural timber element and the subsequent need for maintenance or replacement. This review examined commercially viable non-destructive methods of evaluation. Although some techniques hold promise for future application, such as optical fibre techniques, none was currently found to be entirely suitable for use with both new and aged timber beam bridge girders. A novel laser based technique was developed to provide a low cost measurement system that could be easily retrofitted to both old and new structures to provide a continuous indication of timber bridge girder performance. Extensions of this technique are also applicable to other large beams constructed from a variety of materials such as concrete, steel and engineered 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.

In 2006 the NSW Government announced a Timber Bridge Partnership program to upgrade timber bridges on regional roads. However, limited guidance was available to identify the most cost effective method of upgrading and at the time the most common construction methods available, involved concrete and steel. Hence, despite some timber bridges been replaced by new structures involving timber beams, many engineers and asset managers chose not to use timber. Two reasons for disregarding timber are the lack of adequate data about bridge reliability and lifetime cost. Other reasons relate to lack of knowledge, understanding, skill and confidence in working with timber. Examples are provided, firstly of some NSW bridges that have been part of the NSW Timber Bridge Partnership program and secondly of some overseas structures that have been cited in papers at recent international conferences. The outcomes address some of the research required to improve understanding of how to best upgrade the Australian bridge infrastructure. This paper provides an update and comparison of the state of the art timber bridge design and construction. Novel timber bridges have been recently constructed overseas, but many Australian designs are over 100 years old.

A new method of measuring the mid-span deflections of older timber bridge girders is presented in this paper. There are many timber beam bridges of unknown reliability in regional Australia under high traffic service loadings that 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 in-service girders that need to be measured, it is important to use a quick, low cost, and easy-to-setup method in the field. A laser-based method is proposed here, 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 in unison. A high speed camera was used to record the movements of the chart relative to the image of the laser. The video recording of the chart movements relative to the laser source was analysed to identify the peak movements. The chart was inscribed so that any movement of the image could be easily read from the graduated scale. It can be inferred from the results that, when the girder is loaded by moving traffic loads, the peak dynamic deflection of a girder can be readily identified.

This paper links past and current research to demonstrate how the structural integrity of timber beam bridge girders can be managed and maintained more effectively. There are a large number of timber beam bridge girders in use on local roads throughout Australia, for which the condition is not well known. For the last decade or so there has been a lack of funding to enable these girders to be maintained in an 'as new' status. The condition of some timber bridges is such that low load limits have been applied because of the difficulty to quantify both the current traffic loads and the girder strengths and not necessarily because the structure is known to be unsafe. Previously measured data, published by others, are evaluated and comparisons made of aged girders comprising a variety of conditions and ages. The wide variation in the values for Modulus of Rupture (MoR) and Modulus of Elasticity (MoE) among the pooled results of previous measurements has not enabled the variation of these parameters to be readily identified for a particular girder. It is inferred from this study that MoE can be used as an indicator of a particular girder's potential MoR and thereby enable useful in-service measurement systems to be created.

Any assessment document pertaining to existing bridge infrastructure requires an accurate record of each individual bridge in service, the history of repairs and modification as well as the current state of structural health after each inspection. Bridge inspections need not only be regularly documented, but compared with previous inspections and the probability of ongoing performance assessed. Such knowledge allows the planning of regional sustainability of rural bridges over major and minor transport corridors. This paper examines the variety of timber bridges on rural NSW roads with the data that describe the likely limitations to normal loading. The discussion outlines the level of measurement accuracy required for documenting bridge health and experimental evidence verifying the level of accuracy achievable. Because many timber bridges have had a variety of owners, and society has for many years restricted the funds available for infrastructure maintenance, bridge structural health is poorly understood at any level of quantifiable predictability. Alternative methods of monitoring heavy traffic on rural roads have not been well examined and bridge load limits may often not reflect actual bridge carrying capacity. In the absence of suitable data, some of the structures being replaced may not be the ones at most risk of failure. This is not a new issue and has changed little in the last twenty years. To extend the serviceable life of bridges and to sustain a low probability of structural failure, new low cost measurement systems are required. This paper discusses one such method of measuring mid-span deflection that can be readily used by bridge maintenance crews after short periods of training.

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.