A small stretch of road connecting Thames and Paeroa will be closed for up to a month starting in February as construction on the first state highway bridge built from timber in 50 years is finally underway.
Known as the Onetai Bridge, the 9-metre-spanning bridge represents a major shift in bridge design with low-embodied carbon materials. And whilst small in stature, it is the first bridge built by Waka Kotahi NZ Transport Agency (NZTA) out of wood and not steel or concrete since at least the 1970s – a push that could have major implications for more than 4,200 bridges across NZ’s road network.
Speaking to the NZ Herald’s Wakito News today, NZTA acting regional manager of infrastructure Bruce Waugh said the Onetai Bridge, located halfway between Paeroa and Kopu on State Highway 26, will cause minor disruption for commuters:
“As SH26 is the only through route between Paeroa and Thames on the eastern side of the Waihou River, the detour route will be via State Highway 2, Hauraki Rd and State Highway 25 for northbound traffic and the reverse for southbound traffic,” Mr Waugh said. “We’ll be able to provide more information and timings in the new year once we finalise our construction programme; we recognise the impact this closure will have on road users, particularly those from local communities and are ensuring they are given early advice of these works.”
The new bridge is a huge step for New Zealand’s highway system
Last year, Rob Campbell, NZTA’s regional manager for maintenance and operations, said the use of timber in the new bridge distinguished it from conventional bridge replacements.
“Road bridges in New Zealand generally utilise concrete or steel. This will be the first to herald a return to a more sustainable design, incorporating timber beams and a deck.”
As it stands just 14 out of more than 4,200 New Zealand road bridges are made from timber, however acrcording to Rob Campbell, NZTA regional manager for maintennce and operations, bridges built out of mass timber could become the new norm for NZ’s highway network.
According to Mr Campbell, mass timber—and not steel or concrete—could become the new standard for bridge construction, turning an ostensibly minor project into a ground-breaking shift in design for future state highway bridge replacements. The key, he said, is in the layering—employing engineered glulam, which Campbell asserts is twice as strong per kilogram compared to steel and specifically designed for durability and resilience.
“The new Onetai Bridge is designed to last at least 100 years, linking the greater Waikato region with the Coromandel Peninsula and providing a small but vital part of our state highway network,” Mr Campbell said. “The replacement will add to the resilience and safety of this route, as the new design will be stronger and wider than the old bridge.”
Why glulam?
Glulam is increasingly used in bridge construction on highways and local roads worldwide – with “the primary component of glulam, radiata pine, grown here in NZ,” Mr Campbell said.
“Glulam has an extended lifespan with minimal maintenance needed, effectively absorbing energy from earthquakes and offering good fire resistance.”
Mr Campbell on the benefit of using glulam for highway construction in high seismic areas.
And though wood may appear to be a fire risk, Mr Campbell said that large-section, lightweight timber performs well in fires as it chars at a known rate and does not deform like steel: “This enables us to reduce the size of the foundations, which lowers carbon emissions, material costs, and construction time.”
Wood Central understands that the new bridge will retain the same length as its predecessor but will be 3 metres wider, boasting improved safety features. These include a new type of side barrier that can be fitted right on the bridge’s outer edge. This will offer more lane space for drivers and reduce the risk of head-on collisions.
The bridge will rest on simple concrete foundations and incorporate new scour protection upstream and downstream. Scour, triggered by changes in water flow or river flooding, is a leading cause of bridge failures. And with climate change, there is a renewed urgency to apply scour protection to maintain bridge safety.
- To learn more about the SCION “timber bridge focus group” and the push to switch steel-and-concrete bridges for wood, click here for Wood Central’s special feature last year.
