Revamping Grafton Bridge to cost $7.3m

Strengthening work on Grafton Bridge will take about nine months. Picture / Paul Estcourt

Elderly beams in Auckland's historic Grafton Bridge have been declared safe to carry large buses along a new transit corridor, but other parts will cost more than $7 million to strengthen.

The 96-year-old bridge was closed at the weekend for load tests on five sample beams in preparation for Auckland City Council's development of the 4km central transit corridor between Britomart and Newmarket - at a cost of $32 million - for buses and possibly light rail.

Council project manager Graham Long said yesterday that replacing the type of beams tested would have added another $1 million to the bill.

A scheme assessment report in 2003 estimated that strengthening the bridge next year to carry vehicles of up to 40 tonnes - compared with a current 13-tonne weight limit - would cost $2.3 million.

Mr Long said detailed design work had pushed the estimate to $8.3 million to cover a worst-case scenario of having to replace the beams.

But it would still cost $7.3 million to put down new anchors for the bridge's two main piers, wrap carbon fibre around steelwork, encase that in a protective coating, and undertake tasks such as improving footpaths on each side of the road.

Scaffolding will have to be erected above motorway traffic passing under the bridge, and access to the site will be through the Symonds St Cemetery.

The work is expected to take about nine months from the middle of next year and involve a period of full closure to traffic.

When the transit corridor opens in 2008, the bridge will be closed to general traffic from 7am to 7pm each weekday, to make way for up to 1500 daily bus trips over it.

When the bridge was opened in 1910 to horse-and-cart traffic and early cars, it was the longest reinforced concrete span in the world, reaching 98m across Grafton Gully to a maximum height of 43m.

Many Aucklanders were outraged when an initial construction cost estimate of £30,000 ballooned to £40,000 and were suspicious of expansionist ambitions of then Mayor [later Sir] Arthur Myers in promoting the east-west link across the gully.

According to an account of the project sent to Auckland City Mayor Dick Hubbard, they were also suspicious of the use of such a comparatively untried material as ferro-concrete.

But the city council of the day was confident that the resulting bridge would be a permanent structure "of almost indestructible material" providing better access to the district's main hospital as well as Auckland Domain.

Mr Long said traffic around the bridge was much lighter than expected during its weekend closure.

However, he said, the big test would come next year when week-day traffic across the bridge is restricted.