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Transit New Zealand intends putting more than twice as much steel as earlier expected on to the clip-on structures on each side of the Auckland Harbour Bridge.

The agency said yesterday that contractors were about to begin preparations for welding and bolting more than 700 tonnes of steel to the box-girder clip-ons, which each carry two traffic lanes.

The reinforcement will protect the transport lifeline from punishing fatigue stress caused by increasing numbers of heavy truck movements.

The extra steel is more than double the 313 tonnes Transit said last year would be needed to bring the bridge to modern design standards and protect it from "catastrophic failure" in the worst-case scenario of the entire length of a clip-on being jammed nose-to-tail with heavy trucks in both lanes.

Transit's move to accelerate the project followed a decision to ban trucks weighing 13 tonnes or more from the outside clip-on lanes last May to ensure that the bridge, which now carries 180,000 vehicles a day, could remain in working order for another 20 to 40 years.

But Transit's northern operations manager, Joseph Flanagan, yesterday denied that the need for extra steel meant the bridge was weaker than had been thought.

"The clip-ons are stronger today than when they opened [in 1969] - we are reinforcing them so they continue to meet the growing traffic demand," he said.

Detailed design had identified a requirement for thicker steel to be secured by longer bolts than envisaged initially, but Transit still hoped to complete the project for its initial estimate of about $45 million in 2 1/2 years, starting in July.

Preparations, to begin this month, would include improving access to the insides of the box girders - under the traffic lanes - and installing ventilation, lighting and power supply for the project's workforce.

Mr Flanagan said that although logistical details had yet to be confirmed, Transit was considering installing what might resemble a miniature railway inside each clip-on to carry heavy loads of equipment and material.

Tracks were likely to be installed over a central gang-plank running inside the 1.1km clip-ons from one side of Waitemata Harbour to the other, along which trolleys would be hauled by an electric winch.

Although teams would work round the clock to complete the project as quickly as possible, Mr Flanagan said any closures of the clip-ons to traffic would be at night only, and in one direction at a time.

Cycling and pedestrian groups are meanwhile questioning Transit's response to their "getacross" campaign for a bikeway to be added to the bridge's western flank and a walkway to the eastern.

Although Transit agreed in February to include an assessment of such facilities in a wider study of future harbour crossing options, Cycle Action Auckland and Walk Auckland believe time is running out for adding these to the bridge-strengthening project.

Cycle Action chairman Bevan Woodward said he was alarmed to learn from a disclosure of official information that a staff report to Transit's board in September, although confirming the feasibility of cycling and walking links, stated there was "no intention to progress the project further at this stage".

But Mr Flanagan, who wrote the report, said Transit's position had changed to the point that it was actively considering the feasibility of walking and cycling links.