KEY POINTS:
Heavy trucks will be banned next month from both Auckland Harbour Bridge northbound clip-on lanes as hundreds of tonnes of steel are loaded on to safeguard the transport lifeline.
Although bans on other traffic will apply only at night, northbound trucks weighing 13 tonnes or more will be
confined to the main bridge for about six months.
Similar restrictions will apply for most of next year to southbound trucks, until Transit NZ completes its $45 million bridge strengthening.
Trucks affected by the initial ban will from early next month be restricted to the outer northbound lane of the main bridge at all times.
But because that is 700mm narrower than the clip-on lanes, and will be the central lane for northbound traffic during morning peak periods, trucks wider than 2.7m will be asked to avoid the bridge altogether between 5am and 10am each week-day.
The restrictions will extend an existing ban on the use of the outer clip-on lanes by heavy vehicles, which Transit NZ imposed in July last year.
It emerged later that Transit had been advised by engineers of a worst-case scenario of a possible "catastrophic failure" if an entire clip-on became jammed end-to-end with laden trucks, particularly during an early morning period of heavy container traffic movements from the port.
Transit's board noted in September that an analysis of growing traffic volumes suggested that earlier understanding about the structural adequacy of the box-girder clip-ons, which were added to the main bridge in 1969, was "over-optimistic."
It accepted a recommendation to strengthen the clip-ons "in the earliest possible time".
The agency now intends welding and bolting 760 tonnes of steel to the two clip-on structures to ensure the bridge remains capable of surviving anything less than a once-in-2000-years disaster.
But a staff report yesterday to the board said an extension to the truck ban was needed while extra weight from materials and work platforms were added to the bridge, which carries about 180,000 vehicles a day.
"These additional loads on the structure during this period will increase the risks associated with the critical elements identified until the additional steel is fixed and providing structural benefit," the report said.
Transit northern operations manager Joseph Flanagan assured the Herald that the bridge would remain safe for motorists, saying: "We have no structural concerns."
Asked to which risks the report was referring, he said the extended ban was aimed at ensuring the bridge remained capable at all times of withstanding anything short of a one-in-2000-years disaster.
"The issue is that we are managing a potential risk."
Although the lane to which trucks will be confined is just 2.95m wide, compared with the 3.65m clip-on lanes, Mr Flanagan noted that opposing traffic would retain the protection of the bridge's moveable median barrier.
He said Transit was working in consultation with the road freight industry to decide what message signs would be needed to urge car drivers on the clip-ons to give way to trucks needing to cross from the main bridge to the Onewa Rd off-ramp.
Auckland Road Transport Association spokesman Chris Carr said his industry agreed to the restrictions some time ago, and had no concerns about them.
He said most wide vehicles already confined their bridge crossings to nocturnal hours, but a number of other trucks already used the central structure.
Transit is planning a major public information campaign before imposing the extra restrictions.