A progressive design approach and a collaborative construction methodology helped ensure a new bridge officially opened six months ahead of its scheduled completion date.
On December 10, 2023, more than six months ahead of schedule and only 11 months after a catastrophic flood, the New Fitzroy River Bridge, in the Kimberley region of Western Australia, was officially opened.
BlueScope’s National Engineering Manager – Infrastructure, Rob Danis, says that this was a phenomenal achievement and a testament to the unprecedented collaboration between the local Indigenous community, local workers, alliance partners, suppliers, manufacturers, and fabricators.
Crucially, Danis explains, the decision to adopt a progressive construction methodology incorporating a prefabricated composite REDCOR® weathering steel girder superstructure, constructed using an incremental launch method, contributed to the outcome.
In late December 2022, ex-tropical cyclone Ellie triggered a once-in-100-year flood event that saw the Fitzroy River rise an unprecedented 15.8 metres. As flood waters subsided, irreparable damage to the Fitzroy River Bridge and 500 metres of the Great Northern Highway became apparent.
This vital infrastructure is a key commercial transit route for the East Kimberley and Northern Territory, and a crucial link for local, regional and remote Indigenous communities.
Its reconstruction was an economic and social imperative, prioritised by State and Federal Government with funding through the Disaster Recovery Funding Arrangements.
The contract to design and construct the new bridge was awarded to the Fitzroy Bridge Alliance, comprising Main Roads WA, Georgiou Group, BMD Constructions, and engineering company BG&E Group.
Bridge design – concrete vs steel
Typically, in Australia, road bridge sub- and superstructures are constructed using concrete. For the New Fitzroy River Bridge, the design and construction teams identified early that, given the region’s climatic conditions and isolation, there were impediments to using concrete, including the timely availability of prestressed beams and the high cost of freighting heavy concrete beams over long distances.
The team concluded that a composite steel superstructure was the best option. The Kimberley is a semitropical region with distinct wet and dry seasons, so the construction methodology needed to enable substructure works during the dry season while allowing superstructure works to continue through the wet season.
An incremental launch method using steel in the superstructure made this possible, and was a key factor in the bridge’s early completion.
BlueScope’s REDCOR weathering steel
Alliance partner and client, Main Roads WA, raised concerns about the ongoing maintenance of a steel superstructure, particularly given the remote location. To address this, Danis recommended REDCOR weathering steel.
REDCOR weathering steel is a high-strength structural steel that develops a stable oxide layer on the surface known as the ‘patina’. When used in appropriate environments, the patina enhances corrosion resistance and does not require any protective coating during the bridge’s expected 100-year lifespan. It is also known for its distinctive evolving appearance brought about by the patina that develops over time when exposed to alternating weather cycles. With time, the protective layer’s appearance evolves from deep orange to dark brown and a rich purple, complementing the surrounding Kimberley landscape.
Danis worked with the BG&E team to provide technical guidance on material selection, beam sizing, connections, flange and web details.
The use of REDCOR weathering steel as sacrificial formwork within the bridge’s road deck provided a stable platform for rebar placement in preparation for the deck’s concrete pour, and further helped to reduce the project’s overall timeline.
Delivering project efficiency
While substructure works at Fitzroy Crossing progressed at pace, BlueScope’s Welded Products facility in Unanderra, New South Wales, simultaneously began processing the 1200 tonnes of REDCOR weathering steel plate for the prefabricated superstructure.
The structure comprised 100 welded I-beams made from WR350B-graded weathering steel plate, with 40-millimetre (mm) Base Metal Thickness (BMT) flanges and a 25mm (BMT) web. Each assembled beam was 18 metres long, 1.2 metres deep, 0.5 metres wide, and weighed approximately nine tonnes.
Assembling these components to create the finished I-beams required more than 6400 lineal metres of sub-arc welding which took just over two months to complete and stringently test to ensure compliance with Australian standards.
The finished I-beams were then transported by rail to Civmec, a fabrication and construction contractor in Henderson, Western Australia, where they were transformed into prefabricated modules.
This phase involved pairing the beams with support and intermediate braces, fabricating hundreds of connecting plates, pre-drilling precision holes to support site assembly, and welding brackets for additional lateral, shear, and transverse restraint.
The modules were capped with a 10 millimetre REDCOR weathering steel plate to provide the sacrificial formwork, before being trucked to the site and craned onto the concrete casting bed for the superstructure’s final assembly.
Danis says the Fitzroy River Bridge is the longest road bridge in Australia to incorporate welded ‘I’ girder beams made from REDCOR weathering steel.
Its official opening on 10 December 2023 – more than six months ahead of schedule and only 11 months after the flood – was an outstanding achievement, given the complexity of the build, the challenging weather conditions, and the remote location.
As a further endorsement of its success, Main Roads WA commissioned the rebuilding of the nearby Brooking Channel Bridge, replicating the basic design and construction methodology used on the New Fitzroy River Bridge.
To find out more, visit steel.com.au
