- Publication no: ABC2017-069-17
- Published: 21 April 2017
Australia still has thousands of timber bridges in service which were designed and constructed long ago to carry lighter loads than they now carry. The Queensland Department of Transport and Main Roads manages in excess of 300 timber bridges, of which more than 100 are on the major freight network. These bridges carry current loadings in practice, though not in accordance with current code assessment methods.
Various methods for the assessment of timber bridges were investigated, with reference to the practices of several state road authorities, the Timber Engineering code, AS 1720.1, international codes, and research. Aspects of particular interest included the choice between working stress and ultimate strength methods, the load duration factor, the dynamic load allowance, the effective span length, the distribution of loads between girders, and the treatment of snipes.
Inspection records of every bridge were interrogated for span arrangements and component cross section dimensions, including loss of section from rot. The most heavily loaded components in each span were identified, and every span was analysed to determine the load rating of the bridge. This process was automated so that the performance of the bridge stock could be analysed under various scenarios with differing operational parameters.
The common causes of low capacity were identified, along with the bridges to be prioritised for strengthening or replacement. The paper makes recommendations for assessment of timber bridges and for improving inspection records, especially with respect to snipes.