Timber bridge renewal and repair: a renewed focus

Roads & Civil Works Magazine looks at the myriad benefits of timber bridge renewal and repair that are making it a cost-effective alternative to complete structural replacement with other materials.Timber plantations, together with well-managed native forests available for timber harvest, produce around 30 million cubic metres of wood per year in Australia. For the Australian construction sector, this represents a readily available resource for infrastructure projects. By 2020, the Federal Government aims to treble the area of commercial tree crops around Australia to three million hectares.

The target is outlined in Plantations for Australia: the 2020 Vision – a strategic partnership between Australian, state and territory governments and the plantation timber growing and processing industries.

This focus on creating economic opportunities and improving the proliferation of renewable timber resources boasts significant environmental benefits.

“Timber is a renewable, recyclable material with low carbon emissions compared to other building materials, such as steel and concrete. If half of all the new houses built in Queensland in any one year were timber maximised, for example, this would have avoided 600,000 tonnes of carbon emissions,” explains Timber Queensland CEO Mick Stephens.

The benefits of timber in building construction are well recognised, according to Mr. Stephens, likewise in civil bridge construction, adding that local government authorities in Queensland are progressively identifying the benefits of timber bridge structures. “In Far North Queensland (FNQ) in particular, they are prioritising the replacement or renewal of a timber bridge rather than choosing a concrete alternative.”

One major aspect of utilising timber over other materials such as steel or concrete in bridges is the potential to save costs by way of renewal or restoration rather than complete asset replacement. However, this is often seen as high-risk or laborious, and the option for full replacements with concrete or steel is seen as the easier path.

Mr. Stephens explains that some of the major setbacks that fuel this perspective are the misconceptions about the material in this application. “Part of the legacy of timber bridges in Queensland and Australia is that many were built at the turn of the century. A lot of those structures are 100 years old, and that may impact durability going forward.”

He explains that many of these timber structures, having been in service for a century, are often viewed as unreliable and old-fashioned. “But, they are missing the fact these bridges have been in place for a very long time. Part of the misconception is because a lot of those timber bridges were installed in rural areas as single-lane structures. There’s a view that these narrow old structures represent the technical limitations of using timber. This is simply not the case with modern construction methods and timber products in the market.

“The other major aspect is probably a lack of recognition of the emergence of global environmental certification schemes in the forestry industry and the highest standards of sustainability in the Australian industry,” explains Mr. Stephens. “Globally, there are concerns over deforestation and illegal logging practices with their negative consequences. The Australian timber industry is highly regulated and was one of the first industries to employ independent environmental certification schemes. We have a very high uptake of it in Australia. Aspects such as biodiversity, soil impact and water quality are fully taken into account.” Mr. Stephens adds that a carbon constrained economy is also regarded as a positive environmental driver for timber. “Timber has fewer carbon footprint impacts than concrete by far, because it relies on the sun to produce the product – with minimal fossil fuel used in its production. With replanting after harvest, it also provides an ongoing resource into the future.”

A cost-effective solution

Mareeba Shire Council, the local government authority for its namesake town on the Atherton Tableland in FNQ, has recently begun a program of works to repair and renew three timber bridges in its district.

“We’ve got a bit of a mix of timber and concrete bridges at Mareeba Shire – we have about 20 timber bridges, which isn’t as many bridges as other councils out there. But, like other councils we don’t really have the means to fund replacements,” says Glenda Kirk, Contracts & Project Management Officer at Mareeba Shire Council.

Ms. Kirk has played an important role in integrating the council’s timber bridge program, having brought experiences and learnings in this area from her previous role with the Cassowary Coast Regional Council.

“The three bridges we’re looking at in Mareeba are on low-order roads. If we were to put them under road project priorities, we’d never get them over the line,” she says.

Ms. Kirk says the focus on timber bridge renewal rather than replacement proved effective and financially beneficial for Cassowary Coast, and the goal is to achieve the same at Mareeba. “The main thing about single-lane timber bridges is that the abutments will be fine, but the girders are what will be failing or inadequate. If you put a concrete deck on abutments that are built to support timber, you may possibly ruin the substructure by overloading it.”

Rather than repairing or restoring the original timber structure, she says the common alternative here is often to replace the bridge entirely with concrete. However, for rural councils, such as Mareeba Shire Council, which have an extensive road network and low population base, it’s not a cost-effective option. “One of the things that’s driving councils back to timber is cost – the costs of renewing a timber bridge asset can be three to five times cheaper than replacing it entirely,” explains Ms. Kirk.

She says that people realise timber bridge renewal is not the silver bullet and, like other treatments, doesn’t last forever, but advancements in timber engineering are beginning to establish timber bridge renewal as a viable and cost-effective option.

Bringing timber into the spotlight

Dan Tingley, Senior Engineer and Wood Technologist at Australian timber bridge specialist Timber Restoration Systems, says a timber bridge is often overlooked as the first option for new construction and as an option in restoring timber structures.

“Far too often we visit a timber bridge to conduct an inspection and find a new steel bent has been installed mid-span, or timber piles have been encased in concrete, when light maintenance or reinforcement of the existing wood would have had a superior result and prevent further deterioration instead of encourage it,” says Mr. Tingley.

He asserts that this circles back to a lack of understanding of wood and its potential. “Wood will last indefinitely if left alone. If designers and engineering managers took the time to understand how wood works, what causes decay and what can be done to prevent it, there would be far fewer concrete and steel structures being installed.”

He says a lack of understanding can lead to improper structural assessments on the condition of timber bridges in service.

“We’ve talked to some council representatives who went through three courses each for concrete and steel (12-13 weeks per course), compared to two weeks of timber design in a materials class. This lack of training and understanding has them discarding or passing over timber options in favour of steel and concrete as they get into the work force.”

A lack of understanding in this regard, he explains, can lead to improper structural assessments on the condition of timber bridges in service, but also design maintenance strategies that may degrade a timber bridge faster.

Mr. Tingley asserts that cost has a significant role to play here, and the primary benefit of restoring a bridge versus replacing it is exactly that – cost. “A timber bridge requires a substantial amount of restoration before replacement becomes the cost-effective option, and old timber structures usually remain in low traffic areas where it is difficult to justify the costly replacement.”

He says strengthening measures, such as the installation of high strength fibre wraps on piles or shear/tensile girder or pile cap reinforcement can take as little as one hour.

“Elements that are in very poor condition can have the deteriorated region removed and a new piece of wood installed or the individual element can be replaced in-situ, both of which are low cost options and can be done in two to three hours,” he adds.

Based on Timber Restoration Systems’ own experience, Mr. Tingley says timber bridge treatments usually cost 80 per cent less than the estimated price to replace a structure with concrete. “The cost of typical restoration strategies can range down to $600 to $1200 per square metre versus a greenfield concrete bridge, which can cost up to $6000 per square metre, with typical costs being in the $3500-per-square-metre range. Even when the super and deck are gone, if the timber substructure can be saved it will lead to costs for restoration from $1800 to $2200 per square metre. This is much less than greenfield concrete costs.

Mr. Tingley exemplifies Timber Restoration Systems’ work to restore the timber Farraday Bridge in Queensland. The bridge cost $2500 per square metre to upgrade and complete to extend the life another 100 years. He says the concrete replacement costs were several times that based on the original bridge square metre count. “The reason being is that the bridge had to be raised and lengthened,” he states. “By restoring the old bridge we were able to hold the same height with marginal increase and keep the same width and the approaches. This all kept costs to a minimum while improving the level of service to the community.”

Significant drawcards in the argument for timber bridge restoration are the advancements in the field that are making it cheaper and easier to undertake structural repairs. “The evolution of high-strength fibre into a cost-effective reinforcement material has turned the once backbreaking work of replacing girders and piles by hand into a much simpler process,” he says. “Instead of hauling in large lifting equipment or a full work crew, the reinforcement can be installed by two technicians in under an hour. That is an advance that councils have been looking for as budget restrictions cause their crews and resources to dwindle.

“The extra work involved in replacing elements kind-for-kind including full deck removal or replacement of entire supports is very time consuming, and every shire has a long list to get through each year.”

Timber preservation has come along way with both industry and firms such as Timber Restoration Systems pushing for better understanding in this area, particularly in decay prevention.

“Advances in our understanding of the effects fasteners have on structures has pushed us towards pre-treatment fabrication and drilling, and horizontal fastener usage which both prevent decay,” says Mr. Tingley. “Understanding the decay process has furthered the implementation of salt rod diffusers to neutralise decay instead of active fumigants which fight decay but deplete rapidly when exposed to moisture.

“The greatest advancement that has come in the wood industry is the improvement of our understanding and knowledge of how wood works, and how to optimise its strengths and eliminate its weaknesses,” surmises Mr. Tingley.

Mr. Stephens agrees that advancements in timber bridge construction and maintenance are helping to change that perception.

“There is significant innovation with engineered wood products and the use of pre-fabricated timber solutions, particularly in building applications, that is making timber more price competitive,” he says. “Timber is making increasing in-roads in building construction and we expect that to continue into civil construction,” says Mr. Stephens.

“An excellent example was the recent replacement of an old wooden bridge with a timber engineered bridge near Maryborough, Queensland. The bridge was made of locally plantation grown, manufactured and engineered softwood using plywood decking and laminated girders. The bridge is used primarily for trucks weighing around 67 tonnes, and was a cost-effective and strong replacement option for the client.”

Mr. Stephens says many Australian local government authorities are now identifying structural renewal and replacement as a viable cost-effective option compared to complete concrete replacement.

Mr. Stephens says there is an increasing trend by governments globally to adopt a wood encouragement policy (WEP), which requires timber to be considered the preferred construction material in projects when it is equally fit-for-purpose. “Within Australia, 12 councils across four states have already adopted a WEP, including Fraser Coast and Gympie Councils, who became the first councils in [Queensland] to do so.

“What we’re seeing in bridge renewal programs is a lot of advantages to renew the bridges with timber solutions rather than replacing them with something entirely new,” he adds. “It’s not mandated, but many councils are saying to their engineers that they want to use timber in construction.”

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