Overview
Entrant:
Ruamoko Solutions
Category:
14. Innovative Timber Engineering Award
Photographed by:
Ruamoko Solutions, Kirk Hargreaves
Key team members:
Architect: Isthmus Group Limited
Structural Engineer: Ruamoko Solutions
Geotechnical Engineer: Tonkin & Taylor
Structural Engineering Peer Review: PTL
Ecology: Instream Ecology
Main Contractor: HEB
Engineered Timber Supplier: Red Stag TimberLab
Spanning 30 metres across the Ōtākaro Avon River, the Dallington Bridge demonstrates how advanced timber engineering can deliver resilient and beautiful public infrastructure in demanding environments.
As part of Christchurch's City to Sea Pathway, the bridge sits within the regenerating Ōtākaro Avon River Corridor, an area severely impacted by liquefaction and lateral spreading during the Canterbury Earthquakes. Where the three preceding pathway bridges defaulted to steel and concrete, this project placed GL10 glulam timber front and centre, its warm tones evoking the Mōkihi vessels that once traversed this landscape, delivering an elegant and iconic mass timber bridge with a 50-year design-life in a challenging environment.
Rather than using timber as a secondary or architectural material, the bridge is fundamentally engineered around it. Two 2.0m deep, 450mm wide primary GL10 glulam girders span between single pile caps on 30m deep bored piles. Secondary GL10 approach beams, GL8 restraint struts, and timber cladding trusses extend timber's role throughout. Timber's low self-weight enabled the full superstructure to be pre-fabricated offsite and assembled on the riverbank before being craned into place, ensuring quality control and minimising in-river work.
The lightweight, permeable FRP deck allows light and water to reach the river below, benefiting ecology and eliminating drainage infrastructure. Fabrication engagement with New Zealand's leading mass timber suppliers informed the decision to mechanically laminate each girder from two vertical billets, improving transportability, enabling inspectable connections, and reducing cost. The result is a low-carbon, sculptural landmark that creates a genuinely positive experience for river users.
The mechanical lamination configuration created a design challenge with no ready-made code solution. No provisions within NZS-AS 1720.1, or international equivalents, adequately address flexural lateral torsional buckling of vertically laminated members or quantify restraint system stiffness effects. The design team developed an original analytical framework, deriving an effective composite width two-thirds of gross width by capturing partial interlayer slip through refined kinematics. This was validated using a 3D geometric non-linear FEA model incorporating out-of-straightness imperfections and P-Delta effects. The numerical analysis revealed critical buckling moments approximately 8% below analytical predictions, confirming that restraint system stiffness measurably influences capacity, a finding that led directly to refinement of fastener density. The methodology was published and presented at the 5th International Conference on Timber Bridges, contributing new knowledge toward future design standard provisions.
The design innovation extends beyond the girders: a yielding foundation fuse system protects the glulam girders from compression-induced buckling during lateral spreading events, while embedded wifi moisture sensors provide real-time long-term performance data on timber behaviour in exposed environments.
The Dallington Bridge proves that engineered timber can deliver long-span public infrastructure cost-effectively and beautifully in conditions where it is rarely considered. The central innovation is a purpose-built design methodology for mechanically laminated glulam girders, filling a gap in current standards and contributing to their future advancement, making a structurally ambitious, architecturally striking timber bridge viable in a seismically extreme environment.