KEY POINTS:
Clambering about in the belly of an Auckland Harbour Bridge "clip-on" is enough to induce thoughts of seasickness.
Although the Waitemata Harbour is invisible and more than 25m below, the thunder and shaking from traffic overhead makes inspecting the capacious steel-box interior of the clip-on reminiscent of being
in the hold of a ship - on a boisterous ocean.
And despite Transit NZ's assurances that we are perfectly safe inside one of the twin 1.1km-long boxes added to Auckland's transport lifeline in 1969, it's a little unnerving to learn we are separated from the traffic by a lid of steel and road surfacing thinner than the width of three of my fingers.
The clip-ons are designed to bend under the 80,000 or so vehicles travelling over them each day.
Transit's northern operations manager, Joseph Flanagan, promises that the 11mm of steel above us, overlaid by 32mm of polyurethane road surface material, is well up to the task of keeping the box intact - in combination with thick cross-beams and girders on the roof, floor and walls.
Transit has anticipated growing traffic loads by banning trucks weighing more than 13 tonnes from each of the outside clip-on lanes and is fast-tracking a $45 million project to stiffen the boxes and pier connections with 312 tonnes of extra beams and girders.
That follows an engineers' warning of a risk of "catastrophic failure" in an extreme traffic jam involving nose-to-tail trucks in both clip-on lanes in the same direction, a prospect Transit chief executive Rick van Barneveld declared yesterday was impossible because of continuous monitoring.
Each of the clip-ons, which are separate from the original 1959 "truss" bridge despite sharing its seven concrete piers, is a hollow box 1097m long.
They are 5.1m wide and of varying depths but with a central gang-plank running from one side of the harbour to the other.
Near the Northcote end, where Transit guided reporters inside the clip-on yesterday after an inspection by Transport Minister Annette King and local government leaders, the bending and shaking caused by the constant traffic above our heads is unsettling although not intolerable.
But in the middle of the main span above the centre of the harbour - where we are now back outside the box and standing on a more stable platform attached to the original bridge - a gauge shows the clip-on bending by up to 150mm under heavy trucks compared with 50mm under cars.
Mr van Barneveld assures us the bend would have to be more than 600mm below where we are standing, before the steelwork in the clip-on would start buckling.
But it is still a relief to be standing on terra firma again after climbing down three flights of stairs.
Clip-ons safe, cars only
Heavy trucks will remain banned from Auckland Harbour Bridge's outside clip-on lanes after Transit NZ completes a $45 million strengthening project.
The move, which affects vehicles heavier than 13 tonnes, will reduce wear and tear on the bridge.
Work upgrading the bridge may cost more than the $45 million price, with results of wind tunnel tests in Australia yet to show whether more bracing structures may be needed.
Transport Minister Annette King expressed confidence in the clip-ons yesterday, despite recent engineering warnings. Ms King said she accepted Transit's assurance they were safe and she had no qualms about driving over them.
She was advised that even in a catastrophic event such as a tsunami or earthquake, the bridge would buckle rather than collapse into the harbour.