The replacement of Onetai Bridge, halfway between Paeroa and Kōpū on State Highway 26, marks a giant leap in bridge design. Photo / Supplied
Onetai Bridge, about halfway between Paeroa and Kōpū on State Highway 26, might reach just 9m over a tiny stream, but its upcoming replacement marks a giant leap in bridge design for Waka Kotahi NZ Transport Agency.
It will be the first state highway bridge built from timber in 50 years, representing an exciting move towards using climate-friendly materials.
It’s the timber elements of the new design that make the project more than just a run-of-the-mill replacement, says Waka Kotahi regional manager maintenance and operations Rob Campbell.
“Road bridges in Aotearoa New Zealand are generally built with concrete or steel. Onetai will be the first to mark a return to more-sustainable design, with timber beams and deck.
“We’re hoping to make this the new norm, so while it might seem like a minor project, it marks a groundbreaking shift in design for bridge replacements on state highways.”
It all comes down to the magic of layering — using an engineered timber product called glulam, which is made from layers (laminates) of wood glued together. As Campbell explains, glulam has twice the strength per kilogram compared with steel and is specially designed to be hard wearing and durable.
“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,” he says.
“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.”
Glulam is being used increasingly around the world for bridges, both on highways and on local roads.
“The first and most obvious benefit is that glulam’s main component, pine wood, is grown right here in Aotearoa and therefore does not need to be mined or go through the high-energy manufacturing processes needed to make steel and concrete,” Campbell says.
“New Zealand glulam is made from responsibly-sourced radiata pine and actually has a negative carbon footprint during the production phase. This is because the trees have absorbed carbon dioxide from the atmosphere during the 30 to 40 years they take to reach maturity, and new growth starts that process all over again.”
Glulam will last for decades with a minimum of maintenance, and can absorb energy effectively from earthquakes; it also has good fire resistance. “It might seem that wood would pose a fire risk, but large-section timber in fact performs very well in fires because timber chars at a known rate and does not deform like steel,” Campbell explains.
“One of the key benefits of a timber structure is it’s lightweight. This means we can significantly reduce the size of foundations which in turn cuts down on carbon, material costs and construction time.”
The new Onetai Bridge will be the same length as the old one, but will be 3m wider, and boasts extra safety features. These include a new type of side barrier that can be fitted right to the outside edge of the bridge, and therefore gives drivers more lane space, helping reduce the risk of head-on crashes.
The bridge will have simple concrete foundations, and new scour protection both upstream and downstream. Scour, which happens when water flows change or rivers flood, is a leading cause of bridge failure. With our climate changing rapidly, there is a renewed urgency in using scour protection to maintain the safety of bridges.
Waka Kotahi undertook a business case in 2020 for the replacement of the two-lane bridge, which, while still safe to use, is nearing the end of its life. Construction funding was approved under the 2021–24 National Land Transport Programme (NLTP), and the project has moved into the detailed design stage, with construction planned to begin in October.
With the consenting process underway, specialist structural consultants have been brought in to advise on the timber beams and deck.
During the business case, Waka Kotahi spoke with iwi partners, key stakeholders including the local council, and affected landowners to gain information for the initial concept design.
“We’ll continue these conversations throughout the design process, while also making sure the surrounding communities along SH26, from Paeroa to Kōpū, are kept informed. Once the detailed design is completed, we’ll be able to share more information about construction and timing,” says Mr Campbell.
In the meantime, Onetai Stream Bridge remains safe to use. Waka Kotahi has an active bridge maintenance, strengthening and replacement programme, which helps provide a good overview of the state of all state highway bridges. Safety is the No. 1 priority, and Waka Kotahi carries out regular inspections to ensure all state highway bridges in New Zealand remain safe.