Bridges

The assessment of the reliable, safe, live load structural capacity of a bridge is fundamentally different to design because design is a lower bound process due to uncertainties about loads, materials, geometry, analysis and the small marginal costs of conservatism. The determination of live load capacities for individual bridges will maximise/optimise safe access at the link level thereby maximising freight efficiency and productivity. Significant increases in capacity beyond design are possible, but it is also critical that these are based on robust investigations, analyses, and risk assessments. AS 5100.7:2017 and the NZTA Bridge Manual provide the basis for assessment of bridges in Australia and New Zealand respectively. However, due to the design basis and prescriptive nature of these standards, more detailed risk-informed assessment guidance is required. Austroads has commissioned the development of a guideline for the assessment of bridge structural capacity.

The initial phase involved an international literature review, consultation with Australian and New Zealand road agencies and documenting the findings. The interim Guideline under development, will provide guidance based on currently available information. A proposed second stage will research and develop additional key elements of the Guideline for subsequent publication. This paper outlines the key findings to date. It identifies the differences in design and assessment philosophies and how to reduce conservatism in assessments without compromising safety. It provides an overview of international assessment approaches and identifies topics requiring additional information for Australia and New Zealand practice.

The assessment and stewardship of ageing bridge infrastructure subjected to loads much larger than their design loads is discussed. Fundamental engineering principles are proposed to reduce conservatism and understand risk through understanding of potential collapse mechanisms and post-elastic behaviour. While safety considerations focus on the Ultimate Limit State, the behaviour of structures at serviceability loads, including consideration of fatigue, is also considered. The paper also includes an outline of an Interim Guideline for road agencies and discusses the gaps in knowledge and procedures that require further investigation during Stage 2.