About 30% of Australia’s timber bridges are in poor condition and in dire need of repair. And despite councils’ efforts to replace timber bridges with concrete, timber can and must play a role in the future of Australian bridge design – but with this, it desperately needs a long-term commitment to hardwood supply.
Today, Wood Central spoke to Martin McCarthy, Sales Manager for Coffs Harbour Hardwoods – one of the country’s largest suppliers of hardwood used in bridges – who is working on the repair and restoration of the Pyrmont Bridge, one of Australia’s most famous bridges in the centre of Sydney.
“Economically, (clever use of) timber makes a whole lot of sense. Then there is the whole carbon debate, with timber having a far smaller footprint relative to carbon-intensive materials.”
Martin McCarthy, Sales Manager for Coffs Harbour Hardwoods, spoke to Jason Ross, Wood Central Publisher, on the value of large-section hardwood timbers in bridges like the iconic Pyrmont Bridge in Sydney.
Opened in 1902, Pyrmont Bridge is the major thoroughfare for pedestrians travelling from Pyrmont into the city. One of the world’s first electrically operated swing-span bridges, it is listed by Engineers Australia as a national engineering landmark and was officially listed on the NSW Heritage Register in 2002.
The 369-metre-long bridge handles more than 16,000 pedestrians daily and comprises 14 spans, including 12 fixed timber truss spans 25 metres long by three metres high and two steel swing spans. Each Timber fixed span consists of six parallel, simply-supported Allan timber trusses at approximately three-metre centres, totalling 72 throughout the bridge.
And whilst the shortage of large-section hardwood is a challenge for all bridges, “(Coffs Harbour Hardwoods) had to arrange a special order with Forestry Corporation of NSW (the state government’s forest manager) to access the log supply for the wooden girders,” Mr McCarthy said.
There are more than 10,000 wooden bridges across Australia.
According to John A Hilton, chair of the Bridge Design Standards AS 5100 committee, there are more than 11,000 timber road bridges spread across Australia, including 1,800 New South Wales, primarily owned by local council’s in state’s Northeast: “Most were constructed in the second half of the 1800s and the first half of the 1900s as simple girder bridges,” he told the ABC yesterday. “(However) a substantial number have fallen into disrepair.”
And despite the NSW government investing $500m in addressing wooden bridges since 2018, timber faces several ongoing challenges: “For one, local government authorities have historically been confronted with the increasing shortage of large-section hardwood. Then there is the effect of increasing axle loads, increased forces from frequent flooding, debris loading, and scour of the stream bed due to global warming.”
According to Dariusz Alterman, from the University of the Sunshine Coast, one of the challenges with timber use is the maintenance cost involved with upkeep: “But I will also say there are some very beautiful timber bridges, and you have to take a holistic approach,” Alterman told the ABC. “Deciding what type of bridge to build can be difficult, and you take into consideration a lot of factors.”
Why replace bridges when you can repair or restore them instead?
That’s where companies like Timber Restoration Services can play a key role in repairing and restoring current bridges: “When built and maintained properly, wooden bridges can easily last for 200 years,” they said, pointing to the Bodoga Wooden Bridge in Sri Lanka—which has survived since the 16th Century—and numerous 200-year-old bridges across the United States.
Armed with the rights to use 42 patents in timber restoration and reinforcement, Timber Restoration Services argues that “structures shouldn’t be destroyed just because they’re old. Like the trees from which they originated, we believe that timber structures acquire grace and story as they age.”
For Mr Hilton, bridge engineers now have the technology, knowledge and experience to construct increasingly complex bridges, but like-for-like timber bridge replacement can be a good option: “It all depends on the site characteristics.”
“Timber is a renewable resource and generally provides lower carbon emissions than other bridge materials. It’s also light and can be modularised, enabling components to be assembled off-site in a controlled, covered environment and brought to the site for expedited completion.”
