Bridges

The paper discusses the wholistic, risk-based assessment and asset management of a 43-year-old bridge, observed to have longitudinal cracks up to 15 mm wide in the supporting piles. The bridge is on a key road link between a port and major industry in central Queensland and crosses an estuarine waterway. One of eight piers was strengthened with a post tensioned pile cap and cast-in-place piles in 2011-12 due to concerns about the original piles.

In 2019, cracks in piles supporting other piers were observed and triggered access restrictions for oversize over-mass vehicle access. The likely cause was determined to be alkali silica reaction (ASR). Previous assessments in accordance with AS 5100.7:2017, indicated that the bridge had unacceptable margins for regulation mass vehicles such as B doubles, even though the bridge had been carrying such vehicles for a large portion of its life. Detailed investigations included material testing of redundant piles and first principles analysis of second order effects in piles.

The results of these investigations indicated that the bridge was not in immediate danger of failure and could continue to carry the existing vehicle loads. However, the investigation indicated that all piles would continue to deteriorate and eventually required rehabilitation. An analysis of rehabilitation versus replacement of the whole bridge indicated that replacement offered the more cost-effective way forward. Changes to the management of the bridge were made based on a risk assessment using the results of the investigation. Consequently, piles were wrapped to slow the deterioration of the piles and instrumentation installed to monitor structurally behaviour and vehicle compliance while design of a new bridge was progressed.

The study outcomes provided confidence to reinstate heavy vehicle access under certain travel conditions and demonstrated how bridge asset owners can make risk-informed decisions concerning bridges.