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Timber Design Awards

Post Tension Timber Bridge

Access was needed over a small environmentally sensitive river for commercial logging of a radiata pine forest.

The harvesting of this forest was the culmination of a 30 year investment into small-scale commercial forestry. With approximately 8000 tonnes of logs nearing maturity, a commercially viable bridge had to be constructed in a short period of time.

Alternative solutions:

– A ford through the river was unacceptable on environmental grounds, – and provided no long-term security of access.
– A concrete or steel culvert would have required unacceptable earthworks in river.
– A steel truss bridge or a precast concrete slab bridge considerably more expensive.

The prestressed timber design solution:
The chosen design is a post-tensioned timber box-girder bridge. This type of bridge construction is used in the USA and Australia, but this is the first use in New Zealand and demonstrates enormous opportunities for future timber bridges.

The advantages of timber:
– Low cost
– Environmentally friendly
– Easily built with volunteer labour
– Long lasting
– Re-locateable if necessary

Technical details:
– Full highway design loading.
– Span: 6.0m, width 4.0m, depth of bridge deck: 610mm (to suit LVL sheet size).
– Top and bottom chords: 80 pieces of 150×50 timber (pre-drilled with holes for posttensioning).
– Webs: 10 sheets of 610 x 36 cross-banded LVL. Pre-drilled holes.
– Total volume of wood: 8.0 m3.
– Timber treatment: Sawn timber-H4. LVL-H5 custom treated.
– Post-tensioning: 7 tendons, 12mm diameter, each stressed to 10 tonnes.
– Corrosion protection: Tendons delivered in pre-greased sleeves.
– Expected life: 50 years.
– Regular checking and re-stressing of tendons to allow for shrinkage and creep.
– Foundations/ abutments: reinforced concrete.
– Constructed in one day by volunteer labour.