Auckland's harbour bridge clip-ons could be candidates for "orthopaedic surgery" which would enable walking and cycling paths to be added and could improve their longevity.
The Transport Agency has been given a proposal by engineering consultants working with the Getacross campaign to find ways of adding walking and cycling links without shortening the life of the 40-year-old clip-ons.
The proposal from the award-winning Holmes Consulting Group - for two "orthopaedic diaphragm" beams to be slung under the main arch section of the bridge to support the clip-ons running along both sides of the structure - is being examined by the agency's bridge engineering specialists in New Zealand and Britain.
It features a load-sharing mechanism by which the main truss bridge would support proportions of peak loads on the clip-ons, reducing metal fatigue by limiting their vertical movements.
Getacross spokesman Bevan Woodward said last night that he understood the diaphragm beams would cost no more than about $10 million to install, although he acknowledged that would be in addition to the price of adding extra structures on both edges of the bridge for walking and cycling.
The Transport Agency last year rejected walking and cycling links estimated to cost up to $43 million, saying the extra "dead weight" of such structures could take 10 years off the economic life of the clip-ons, which it hopes will last for 30 to 40 years.
Mr Woodward said that cost estimate was "grossly inflated".
Agency regional director Wayne McDonald said the new concept was being thoroughly assessed but it was too soon to comment on its viability.
Neither would it prompt the agency to delay publication next month of a report on the structural capabilities of the clip-ons following a $45 million strengthening project already more than half completed.
He said that was because the Auckland Regional Transport Committee needed guidance by then on when an additional Waitemata Harbour crossing would be needed, so it could decide whether to include that project in its new 30-year transport strategy.
Mr Woodward said his organisation of more than 11,000 supporters welcomed the agency's consideration of the new proposal, which it had referred to its bridge specialists, Beca and Hyder UK.
But he could not understand a need to rush into decisions about another harbour crossing when a solution might be at hand to extend the life of the clip-ons indefinitely for all users.
He understood Hyder had indicated support in principle for the concept, which was used in overseas bridges.
He said Holmes Consulting had been involved in a wide range of civil engineering projects including motorway bridges and large buildings.
Mr McDonald said the effect of extra loads on the main truss bridge, which he expected to last for at least another 50 years, would also have to be examined.
The Government is waiting for advice from Transport Agency by the end of the year on whether to add the next harbour crossing to its 20-year national infrastructure plan.
Consultants in a study last year for the former Transit NZ and Auckland councils recommended four bored tunnels - two for motorway traffic and two for passenger trains - as the main crossing of the future for up to $4.1 billion.