The project was identified as one of four critical bridges on the North West coast of Tasmania that required replacing/ rehabilitating due to their serviceability status and the subsequent impact on TasRail network operations should one of them fail - each bridge was managed as a separate contract.
The Western Line is a key strategic element of the TasRail network and the Don River Rail Bridge is a critical element of the rail asset.
Originally built in 1886, the Don River Rail Bridge had been gradually deteriorating due to exposure to wind and the saline environment. In the event of a sudden impairment affecting the structure, a solution to keep the line open could take a significant amount of time and would be high risk.
The original five-span bridge comprised a ballasted timber deck supported by two 1900mm deep riveted wrought-iron girders with lateral and diagonal cross-bracing at regular intervals. This project entailed the design and construction of a new 100 metre, 5 span steel and pre-cast concrete superstructure on existing substructure.
There were four original mass concrete piers with a more recent reinforced concrete footing extension protecting the base of the pier. The abutments were also mass concrete of similar construction.
Upon re-examination of the final concept design VEC’s Design Engineer advised that the specified strengthening work would only guarantee a maximum live load rating of 250LA. Once this constraint had been formally communicated to the client, VEC worked with the Design Engineer to develop a proposal to replace the old riveted wrought iron beams with new welded beams and a new precast concrete deck, thus replacing the entire superstructure in order to achieve the target load rating of 300LA and a 60 year design life.
This proposal also mitigated the requirement to remove the old lead paint onsite, therefore removing any potential environmental issues related to the paint removal.
When the final design was approved for construction, VEC was able to start fabricating the precast elements in our precast yard and procuring and modifying the steel beams to achieve the approved design.
After months of extensive planning and preparation the actual works occurred over an 84-hour time frame. Work commenced on Wednesday 26 December and was completed on Saturday 29 December – without incident and 6.5 hours ahead of schedule.