Bridges

Existing bridges are generally assessed using a deterministic approach, employing simple computer models and AS5100:2017 methods. These methods often require conservative assumptions and may result in unrealistic outcomes. Based on AS ISO13822, the conclusions from the assessment must withstand a plausibility check, and any discrepancies between the results of structural analysis and structural performance must be explained.

To determine more realistic structural capacities of heavy haul rail bridges nearing the end of their service life, Probability-Based Bridge Assessments (PBBAs) were conducted. This risk-based approach aimed to understand the reliability of the bridges in terms of remaining life and load-carrying capacity while optimising repairs and maintaining safety.

This study assessed the structural capacity and long-term performance through PBBA using bridge monitoring and weighbridge data, comparing the outcomes against the deterministic method. This approach was not intended to reduce inspection and maintenance requirements but to provide realistic outputs based on actual data and the risk appetite of the asset owner.

The initial PBBA revealed the benefits of increased knowledge of material yield strength, prompting material tests on bridge samples. The assessment showed improved reliability indices after incorporating higher material strength values, indicating enhanced safety for the bridge's main steel girders. These indices were compared to the Target Reliability Index (βt), defined based on economic optimisation and asset owner requirements. The refined PBBA concluded that the main steel girders meet the safety criteria for midspan combined actions (i.e. biaxial bending) for the specified wagon loads per AS 5104:2017.

This technique has allowed the asset owner to manage bridge assets based on their risk appetite and ensure the ongoing safety and reliability of the bridges.