The previous Onetai Stream Bridge, located on State Highway 26 north of Paeroa, reached the end of its economic service life and was replaced in early 2025.
Rather than adopting a conventional steel or concrete solution, the replacement bridge was delivered as a prototype timber project under NZTA Waka Kotahi’s Engineered Timber Bridge Initiative. The new bridge provides a proof of concept demonstrating how engineered timber can provide a viable low-carbon alternative for transport infrastructure. The bridge carries state highway traffic loads and has a 100-year design life while reducing embodied carbon emissions.
The 11.2 m wide, two-lane bridge comprises 675 x 225 GL10 glued laminated timber girders spanning 9.0 m, supporting a 100 mm structural plywood deck, screw-fixed to form a structural diaphragm. Red Stag was the girder supplier, and the deck was the NiuDeck system from PNGFP. A free-standing barrier system was selected so that vehicle impact loads are not transferred through the timber superstructure, enhancing long-term durability and simplifying inspection and maintenance of both elements.
The bridge is located in a low-lying floodplain susceptible to regular inundation and potential deck submergence. In this extreme exposure environment, careful timber specification and protection were critical to achieving the required design life. Stainless steel fasteners and bracketry were used throughout to prevent corrosion-related deterioration. Attention was given to end-grain sealing and protective coatings, recognising end-grain absorption as the primary durability risk. Detailing was developed to shed water, prevent debris entrapment following flood events, and promote ventilation and drying of structural timber.
As a final safeguard, structural members were specified with H5 preservative treatment to provide resistance against decay in severe wetting conditions. Collectively, these measures demonstrate how a target design life of 100 years can be achieved through a combination of robust detailing and suitable preservative treatments, aligned with robust understanding of the materials and environmental exposure.
Timber was selected over steel and concrete for both environmental and performance reasons. Engineered timber offers a significantly reduced embodied carbon profile while utilising a renewable resource sourced within New Zealand. Its high strength-to-weight ratio enabled efficient prefabrication and rapid installation, limiting disruption on a state highway corridor to approximately three weeks. The project also supports national objectives to decarbonise infrastructure and strengthen domestic timber supply chains.
Beyond its structural advantages, the bridge was designed to integrate seamlessly within the highway environment. Its form, scale, and road-user experience intentionally mirror those of a conventional bridge, ensuring consistency across the network. From the highway, the structure reads as standard transport infrastructure, demonstrating that low-carbon timber solutions can be implemented without altering driver expectations. The restrained superstructure depth also minimises visual intrusion within the rural floodplain.
As a pilot project, the Onetai Stream Bridge provides valuable technical and durability insights that will inform the forthcoming New Zealand Timber Bridges Design Guide. It stands as a practical demonstration that, when carefully detailed and appropriately specified, engineered timber can deliver resilient, low-carbon bridge infrastructure suited to demanding environmental conditions.
