KEY POINTS:
Transport officials admit that allowing heavy vehicles unrestricted use of the Auckland Harbour Bridge clip-ons could knock 10 years off their life.
The Transport Agency caused an outcry before Christmas by blocking a campaign for walking and cycling links on the bridge, saying the extra "deadweight" would reduce the clip-ons' economic life by 10 years.
Its board feared that would mean hastening the advent of under-harbour tunnels, a proposal likely to cost at least $4 billion in 20 to 40 years.
But yesterday, agency regional director Wayne McDonald confirmed the walking and cycling links were only half the picture, and that unrestrained use of the clip-ons by heavy vehicles could have a similar effect on their longevity.
He said an estimate that only 10 to 20 years might be left in the clip-ons, even after adding 760 tonnes of steel in a $45 million strengthening project, was based on an assumption of unfettered heavy traffic use in combination with walking and cycling links.
Now walking and cycling were out of the equation, he was confident the clip-ons would - with careful management - last 25 to 40 years more.
Even so, he said a decision whether to allow heavy trucks unrestricted access to the clip-ons would not be made until towards the end of the strengthening project, in about the middle of next year.
Vehicles other than buses weighing 13 tonnes or more have been banned from using the outer clip-on lanes in each direction since 2007, after engineers warned of possible "catastrophic failure" in a worst-case scenario of an end-to-end jam by laden trucks.
That spurred the strengthening project, during which the ban has been extended to both northbound clip-on lanes while steel is being welded inside the box-girders holding them up.
The ban will extend to both southbound clip-on lanes in three or four months, when the strengthening work moves to that side of the bridge.
Despite vetoing a 1.2m addition to the clip-ons for walking and cycling, the agency still hopes it could lift the truck ban for several years, without overloading the strengthened bridge.
But Mr McDonald acknowledged that a point would be reached where the ban would eventually have to be reimposed, given that each laden 42-tonne truck caused as much wear and tear on the clip-ons as 10,000 cars.
He also confirmed there could be no more opportunities to strengthen the bridge, given metallurgical constraints on structures which had already been modified once before.
A clip-on built to the same design on Westgate Bridge in Melbourne collapsed during construction in 1969.
Auckland Regional Council transport chairwoman Christine Rose accused the agency of failing to cater for the needs of all road users by excluding walking and cycling while considering allowing even heavier 50-tonne trucks on.
She said if the agency were concerned about extra deadweight, it should consider giving walkers and cyclists a small share of the existing structure.
She did not believe the latest concern about loadings on the main bridge should hasten decisions about accelerating harbour tunnels.
