Demonstrating a structural reliability framework can accurately assess heavy vehicle access to bridges

Friday, 13 March 2020

Austroads has published a research report demonstrating that a probability-based bridge assessment (PBBA) framework can be used to assess the safety of heavy vehicle freight access to problematic bridges.

Bridge assessments under Australian Standard AS 5100: Bridge Design – Part 7: Bridge Assessment use a deterministic method to assess safe access to bridges, but bridges that fail this assessment are outside its scope, and there is little guidance on such higher-tier assessments.

“With more than 50,000 bridges across Australia and New Zealand, road freight continually increasing and maintenance budgets under pressure, bridge asset agencies have to implement different hierarchical strategies to safely manage heavy vehicle freight access to their bridge networks,” said Ross Guppy, Program Manager Assets. “We wanted to investigate whether agencies could use a PBBA for this purpose.”

“The most holistic framework that provides objective and quantifiable measures of safety on a continuum is a PBBA based on structural reliability theory,” said Colin Caprani, Project Leader.

“We used this framework to assess the implied safety levels of selected bridges in Australia. As about 70% of bridges are more than 40 years old, we examined bridges built from the 1950s to the present day. The most prevalent bridges are simply supported, made of reinforced or prestressed concrete, and comprise two design lanes so we assessed the ultimate bending and shear limit states of components of these selected bridges, such as girders, beams and planks.

We then quantified the bridges’ current safety levels under as-of-right road freight access. We used the Higher Mass Limit (HML) network, which includes general access vehicles at various traffic volumes and flows.

Applying the PBBA framework showed a safety margin that the lower-tier assessment approach under AS 5100 did not reveal. This was particularly notable for more recently designed bridges. For example, for ultimate shear, about 75% of bridges that failed a deterministic assessment were shown to be safe using the PBBA approach.”

The project demonstrates the benefits of using more advanced PBBA over conventional deterministic methods, for problematic cases, where the additional analysis expenditure is deemed worthwhile. It reveals the true levels of safety and provides quantitative data for further asset management, including risk quantification, and prioritisation of rehabilitation measures.

“PBBA frameworks can provide more fit-for-purpose assessments for bridge access to facilitate a more productive and safer road transport network,” said Ross Guppy, Austroads Program Manager Assets.

Download Bridge Assessment Beyond the AS5100 Deterministic Methodology