Extending the life of a historic transit corridor

Grafton Bridge is a crucial link in Auckland’s passenger transport network. To improve public transport usage the bridge was upgraded to carry a dedicated busway, using innovative strengthening to bring its load-carrying capacity to 40 tonne.

When it first opened in 1910, Grafton Bridge was the longest reinforced concrete bridge in the world and has remained a historic landmark for Auckland ever since. Spanning Grafton Gully, the bridge provides a public transport corridor, connecting the CBD to Newmarket.

The upgrade commissioned by Auckland City Council in 2007 required minimal intervention where possible to preserve the heritage of the listed historic structure.

Our structural team conducted in-depth research and extensive testing of the bridge’s existing structural capacity, including laboratory testing of reinforcement anchorages and verification of assessment findings by physical load tests on bridge beams in situ. These investigations succeeded in minimising the need for invasive strengthening works.

To strengthen the bridge, our innovative design incorporated the most extensive use of carbon fibre reinforced polymer (FRP) materials in New Zealand. Advanced drilling techniques were applied to reinforce approach span piers internally. Ground anchors and new piles were built into the old main span piers to provide resistance to earthquakes without affecting their aesthetics.

Careful detailing minimised impacts on the appearance of the historic landmark: FRP was slotted into beams, keyhole surgery was employed to strengthen columns, and piers were retrofitted internally.

Now strengthened and preserved for the future, Grafton Bridge carries over 65,000 bus passengers each weekday; has boosted accessibility to major destinations such as the University of Auckland, the Domain, and Auckland City Hospital; and reduced bus travel times by up to 14 minutes each way. 

Awards

• Grafton Bridge Strengthening – ACENZ Award of Excellence (Silver), 2010