Auckland Harbour Bridge has been at risk of "catastrophic failure", says an official report that undermines earlier assurances that the bridge's wear and tear poses no major safety problems.
Herald on Sunday inquiries have uncovered an official engineer's report, written less than a year ago, which says the risk of damage from heavy traffic loads on parts of the bridge can "no longer be accepted".
On one particular part of the bridge, and under certain circumstances - such as a traffic jam of fully laden trucks - "there would be a risk of catastrophic failure of the deck", says the Beca Infrastructure report, commissioned by roading authority Transit New Zealand.
Last week, Transit announced it was spending $45 million to urgently strengthen the bridge's clip-ons.
Transit made the announcement a day before having to release the engineering reports to the Herald on Sunday, under the Official Information Act. The authority did not mention the reports in its public statements - instead, it has repeatedly assured the public and city leaders that there were no major safety issues.
The papers reveal Transit officials had underestimated the traffic load on the clip-ons, which failed to meet standards by a "large margin" when tested last year.
"If the level of load in this assessment representing a traffic jam situation with maximum heavy vehicle concentrations at the centre of span two (between piers one and two) in both lanes were to occur, there would be a risk of catastrophic failure of the deck," the Beca report says.
To prevent further damage, engineers recommended immediate measures to reduce the number and weight of heavy vehicles on the bridge, including live video monitoring to stop long queues and banning trucks from clip-ons and shoulders.
Last July, Transit banned trucks weighing 13 tonnes or more from the outer clip-on lanes to reduce metal fatigue.
But Transit reassured motorists that the harbour crossing was safe.
"The 165,000-vehicle-per-day lifeline that is the Auckland Harbour Bridge is going to be around forever and will continue to provide service to Auckland and the rest of the country," said chief executive Rick van Barneveld.
More than 142,000 bolts, 313 tonnes of steel and 6.8km of welding will be used to fast-track work on the clip-on extensions in the next two-and-a-half years. This is half the original time scheduled. The work involves strengthening the hollow box girders directly under the traffic lanes.
The second task is to stiffen and strengthen the attachments of large steel angle brackets under the clip-ons to five of the seven concrete piers supporting the bridge across the Waitemata Harbour.
Joseph Flanagan, Transit northern operation manager, said a "catastrophic failure" was a worst-case scenario and a once-in-every-2000 years event.
In effect, "catastrophic failure" would only happen if there was a traffic jam on the bridge with fully laden trucks, Flanagan said.
He assured motorists that this "just wouldn't happen" because trucks were (now) banned from the clip-ons and live cameras meant Transit could divert traffic.
If those systems failed, Flanagan said, the new strengthening work could handle the load.
Van Barneveld denied Transit had downplayed the reports and said that Transit was vigilant in keeping the bridge safe. All of the recommendations in the reports were being implemented.
Asked when Transit planned to tell the public of the urgent upgrade, van Barneveld said there was "absolutely no alarm" as banning trucks and the 24/7 surveillance made the bridge "perfectly safe".
However, the Beca engineering report released to the Herald on Sunday found that Transit NZ had underestimated the numbers of heavy trucks on the clip-on bridges which would make them less safe - and put the clip-ons at risk of further damage.
Van Barneveld was adamant that the structure was not in worse repair than first thought.
He said Transit regarded it as good practice to complete the upgrade earlier, given that consultants had identified the need to strengthen the clip-ons because of the increased traffic load.
Van Barneveld said it was critical to stress that the bridge was safe, and the upgrade was planned preventive maintenance work similar to that carried out a number of times since the clip-ons were opened in 1969.
Additions of extra steel to the clip-ons made over the years meant they were stronger than when first opened.
More than $3 million is spent every year on bridge maintenance, including annual and three and six-monthly inspections of some parts. The most recent inspection, last January, said the overall condition of the box girder extensions was good, "with the exception of the known defects".
At a glance
* A 2006 Beca engineering report said if there was a traffic jam of trucks in both lanes at the centre of span 2 on the clip-ons, there would be "a risk of catastrophic failure of the deck".
* Last week Transit announced it would fast-track a $45 million upgrade of the bridge, $37m more than was budgeted two years ago.
* In July Transit banned trucks weighing 13 tonnes or more from the outside lanes of the clip-ons, saying it was to make the bridge last longer.