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As part of the Ōtākaro Avon River Corridor (AORC) Regeneration Plan, three new pedestrian and cycle bridges – Snell, Medway and Avondale – have helped re-establish connections that were severed during the Christchurch earthquakes. The bridges are part of the green spine, a network of trails and wildlife-dense land that will hug the banks of the river between the city centre and New Brighton.
The original Snell and Medway bridges were badly damaged during the 2011 quakes. These have been replaced by new structures made from steel, concrete and timber, and are complemented by the construction of the entirely new Avondale Bridge. To contribute strength and flexibility to all three bridges, the contractors chose to use Northbeam timber posts and joists from Waipu manufacturer Northpine.
All three bridges are in the Christchurch Residential Red Zone, 602 hectares on both sides of the river that were cleared of homes after the earthquakes. This is a highly liquefiable area, and the bridges were designed by Christchurch City Council engineers to mitigate high lateral spreading under seismic loading.
With spans of 30 and 40 metres, each bridge consists of a steel superstructure set on concrete piles and integrated with the river stopbank. The structural Northbeam products used included 200x200 vertical posts anchored to the ground (dressed four sides), and Northbeam 300x75 and 300x50 SG8 H5 for landings and accessways.
‘Where most mills manufacturer up to 6 metres in length maximum, customers can access nearly all of this high-spec radiata from us and in lengths of up to 7.2 metres,’ says Bruce Larsen, CEO of Northpine.
The timber structure approaches have been designed to support flood management or rain/surface water mitigation. Timber also contributes to the bridges’ high level of seismic resilience – the bridges are able to shear off from the banks in the event of an earthquake so they can be reused.
‘The bridges are each single-span to reduce the impact on the river and embody the ideas of connection between communities and the natural environment,’ says Shaun Hung, architectural designer at Christchurch City Council’s Technical Services & Design division. ‘For Medway Bridge, the designers considered the expansive views and how the surrounding regeneration area creates a sense of space, and the bridge itself reflects the native harakeke (flax) of its surroundings.
‘The Avondale Bridge acknowledges the connection between Ngāi Tūāhuriri / Ngāi Tahu and Ōruapaeroa (Travis Wetland) which was once a settlement and food-gathering place. Timber posts have been used to reflect the surrounding vegetation and wetlands environment, as the bridge will connect to the future Eastern Reach wetland restoration area.’
The design of Snell Bridge considers the forest regeneration planned for this area, and how the bridge could create a connection between land, water and sky. The arches are designed to draw the eye upwards, with the green colour truss vertical element to represent the rhythm of the trees and future forest regeneration.
‘Though the bridges share structural fundamentals and other complementary features, they are all standalone designs, intended to work together as part of the wider OARC regeneration,’ says Shaun. After drawing up the designs in-house, Christchurch City Council engaged head contractor HEB through a tender process. The timber components were subcontracted by HEB to local firm CMT Group, and the Northbeam timber for all three projects was provided by Carters Hornby.
‘When it came to supply, we were contacted because we had virtually all the structural timber the project needed and at longer than normal lengths,’ says Bruce Larsen. ‘Our team worked with Carters Hornby to create and plan the order, as the timber required for three bridges is a massive undertaking.
‘Happening during the pandemic in 2021 – when everyone was short staffed and dealing with a protracted supply imbalance – required extra support from our team to define and compile the order. Carters was thrilled with our friendly and proactive response, and it was great to connect our northern mill with such a prestigious Christchurch project!’
Northpine timber products come only from sustainably managed, high-density Northern region pine forests which have been proven to grow stronger, harder and stiffer than anywhere else in New Zealand. Northbeam is a specialist range of large-dimension and non-standard timber beams, posts and joists tailor-made in lengths up to 7.2m. It can be ordered in small piece lots, solving problems for designers looking for cost-efficiencies and innovations in their plans.
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