At first glance it would appear easier to widen a bridge compared to designing it from scratch, however the process brings its own unique challenges in ensuring that a structure is strong and safe enough to withstand greater weight from additional lanes and traffic.
Such was the challenge for the Department of Planning, Transport and Infrastructure (DPTI) with the widening of Hackney Road Bridge – one part of the $160 m Adelaide O-Bahn city access project. The widening was the first time that an innovative methodology utilising Near Surface Mounted (NSM) Carbon Fibre Reinforced Polymer (CFRP) rods was employed to strengthen a bridge in the city. The approach was successful and saved the project significant time and costs and provides a strong case for future use.
The History of Hackney Bridge
The downstream Hackney Road Bridge was initially constructed in the sixties as part of the duplication of the roadway, located on the western side of an original, heritage-listed arch bridge that was built in 1885.
The widening was part of the initiative to create centrally aligned priority bus lanes along Hackney Road, therefore the existing western bridge over the River Torrens had to be modified to accommodate.
While the bridge was regularly inspected and maintained by DPTI according to its bridge asset management plan – and was in reasonably good condition for its age – certain aspects, in particular the traffic barriers, had to be upgraded to best meet current requirements. This meant removing the existing footpath, strengthening the steel girders, installing new traffic barriers and widening the concrete deck slab.
“Due to the necessary upgrade of the bridge traffic barriers, the capacity of the existing bridge deck slab was not enough to bear the required loads,” says Larry Yang, currently a Senior Structural Engineer with WSP, who was part of the DPTI bridge design team during delivery of the project.
“The bridge structure itself was also quite complex featuring a central drop-in span supported through steel hangers connected to cantilever girders with variable depths. Restrictions on construction access due to the live traffic and waterway below, along with tight turn-around times also posed a significant challenge to how the design was approached.
“After investigating various methods to strengthen the bridge’s steel girders, we addressed the deficiencies through staged strengthening based on detailed structural modelling and analysis, which resulted in a cost-effective strengthening upgrade and direct cost savings to the whole project.
“An innovative approach using NSM CFRP rods was implemented based on the significant advantages it could provide including high strength/weight ratio, good fatigue resistance, increased corrosion resistance, low installation cost and fast turn-around.
“Strengthening and preserving the concrete bridge using FRP composite dramatically simplified the construction by avoiding significant concrete demolition and deck reconstruction. It provides a good case for future use of the technology for the retrofitting of existing bridges that require increased loading or bridge deck widening for extra lanes,”
Larry Yang is a WSP senior structural engineer. He joins other WSP speakers at the Adelaide Convention Centre for the Australasian Structural Engineering Conference, 25 – 28 September 2018. Larry’s session: ‘Hackney Road Strengthening’ is at 11:45am, Wednesday 26 September 2018.
The works discussed in this article an relating to Hackney Bridge were completed by the Department of Planning, Transport and Infrastructure (DPTI).
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