Operational viability of a deteriorating bridge – structural assessment based on Engineering Policy 171

This is a case study about two 43-year-old deteriorating bridges on the national land transport network serving as critical link access to port facilitating trade and economic development in the region. Bridges are designed under the 1976 NAASRA standards for T44 design class. In 2019, severe cracking reported in pre-stressed concrete underwater piles, one of 8 piers was strengthened with post tensioned pile cap and cast in place piles during 2011-12. In 2019, cracks up to 15 mm wide along some piles, likely due to alkali silica reaction, triggered access restriction to oversize over mass vehicle access. Post elastic behaviour was analysed for pile collapse mechanism with certain operational and in-service structural assumptions to understand available structural capacity of pile system and available operational margins as per AS 5100.5 2017. The assumptions were confirmed through pile extractions from partially redundant piles and material testing, and a novel underwater pile survey to measure pile out of straightness to be less than 50mm. Findings revealed cracks did not advance piles beyond the cover concrete; reinforcement bars are unaffected by ASR. Piles are in better condition than expected. Analytical study identifies the structure has unacceptable margins for as of right vehicles where structure has been carrying B doubles for large portion of its life and so it is an unacceptable benchmark to understand access for this deteriorating bridge structure. Study outcomes has provided confidence to uplift heavy vehicle access under certain travel conditions while the bridges remain as “operational bridge” as per Engineering Policy 171 Management of Queensland’s State-Controlled Road Network Bridges. This study exemplifies how bridge asset owners can be enabled in making a risk informed decision whether to replace, rehabilitate, strengthen, or invest in preserving the asset to keep the bridges operational until their full potential is utilised