Bridges

Although many of the existing railway bridges are structurally adequate to carry the increased rail loading, they still be expected to suffer from fatigue related to the cyclical application of modern freight axle loads. Rail authorities are paying attention to extend the service life of existing ageing railway bridges to manage a cost-effective solution for the rehabilitation. Most bridges in the Victorian railway networks are steel made and have been built during the late 19th to early 20th century, mostly still in service. While the age of those is admirable, knowing how long they will continue to serve is a question without a simple response. Increasing volume of traffic and axle weight of trains mean that current loads are much higher than envisaged from original design. In this context, issues as maintenance, assessment, rehabilitation and strengthening assume a significant importance. Fatigue life prediction of existing bridges is still a debated topic. Railway bridges may be exposed to high fatigue stress ranges and number of stress cycles that may lead to fatigue damage. AS5100.6 is based on traditional S-N curve method with fatigue parameters typically meant for the design of new bridges to achieve 100 years life. Constant amplitude stress range is assumed. However, when applied to assess the decades old bridges, it leads to conservatism that encourages a deficient structure demanding great economic impact. Due to the variability of past and present loadings, variable amplitude stress ranges are found to be more appropriate with fatigue damage calculated based on Palmgren-Miner Rule. This paper presents the structural assessment of fatigue damage undertaken to some recent bridges. It also highlights that the application of new standard could be conservative at existing network demanding a costly solution even at low-risk environment and discusses the time of intervention versus the risk of fatigue damage.