Diverted trucks damage centre of harbour bridge

Trucks may be permanently banned from the clip-on lanes. Photo / Herald on Sunday

A ban on trucks on the outer lanes of the Auckland Harbour Bridge has resulted in damage to the main bridge, which will need repairs and strengthening to cope with the increased load.

Trucks weighing more than 13 tonnes have been banned from the clip-on lanes since 2007, after engineers warned of possible "catastrophic failure" in a worst-case scenario of an end-to-end jam by laden trucks.

Urgent repairs on the steel-box girders will cost $45 million and were supposed to make the outer lanes safe for another 30 years. But the ban may become permanent after engineering reports showed the box-girders would last as little as 15 more years if trucks were allowed back.

Now new documents have revealed that forcing heavy traffic on to the middle lanes is accelerating damage to the most important stretch of State Highway 1.

"If more heavy vehicles are diverted to the truss bridge lanes then the fatigue lives of truss bridge components will reduce and there may be significant repairs to localised areas needed in the medium term," say New Zealand Transport Agency board papers.

"Similarly, an increase in load will require further strengthening of the truss bridge to maintain its safe load capacity."

Transport Agency regional manager Wayne McDonald confirmed the contents of the reports but said the truss bridge was much stronger than the clip-on lanes.

"The main bridge was built in a different time, a different era, an era of the old English bridges. And they are still there," said Mr McDonald.

"We're expecting the centre bridge to have a a life of 100 years plus. The damage from trucks was minimal."

Even if trucks were permanently banned from the clip-on lanes, the outer bridges will have still need to be replaced eventually - even after the $45 million upgrade.

That would cost between $250 million and $400 million, not including the "considerable economic loss" of being unable to use the outer lanes for a substantial period.

"However, it is assumed that the need to replace the box-girder lanes and the associated cost and disruption that this would involve will be a major determinant in its timing," the board papers say.

The papers, obtained under the Official Information Act, say the path of a new harbour crossing needs to be protected so nothing can be built there.

Mr McDonald said funding had been approved for the Transport Agency to prepare resource consent submissions. He would report back to the board in November for permission to seek consent.

No timeframe has been set for the tunnel crossing. The proposed route would stretch from under the Tank Farm area on the harbour front to the east of the harbour bridge.