Continuous maintenance

To ensure the best performance and the longest life, the Auckland Harbour Bridge is constantly maintained to keep it in good shape.   BY MARY SEARLE

Most Aucklanders wouldn’t realise that their iconic bridge is constantly being worked on to ensure it stays in top condition. In addition to a on-going general maintenace programme, a number of one-off projects are undertaken to maintain the asset.

TBS Joint Venture has the performance specified maintenance contract for the bridge, and has been taking care of it for the past nine years. The joint venture comprises TBS Farnsworth, Opus International Consultants and Fulton Hogan.

A large part of the maintenance work is inspections and painting, both of which are pretty much continuous. TBS Joint Venture undertakes inspections almost daily. In addition to this, under a separate contract, TBS and Opus conduct a detailed monthly inspection and Opus carries out a very detailed inspection annually.

The painting work is equally thorough. A continuous painting programme is in place and a series of crews operate all year round. The NZ Transport Agency has specified an average residual life of the paint of 4.3 years. However, some areas need to be painted more frequently than others – those close to the water and high on the gantry where salt laden air is trapped. Paint on the larger, flatter areas lasts longer.

TBS Joint Venture also undertakes general repair work as the contractor to the NZTA for the bridge. It is responsible for the security system in and around the bridge that prevents people walking onto it. And once a year, over the Christmas break, the surface of the bridge is resealed.

In addition to this maintenance contract, TBS Joint Venture has secured a number of additional bridge contracts by competitive tender.

It undertook strengthening work on the main truss bridge over an eight month period, completed late last year. It also carried out a stormwater containment project to stop rainwater running down the bridge and, ultimately, into the harbour.

Nine years ago Brian Perry Civil undertook a $1.7 million project to strengthen the pier brackets and trestles and install new wind braces; drill and install stress bars up to 11 metres deep from tops of existing pier walls; and cast concrete pilasters and capping beams associated with the stress bars.

Brian Perry Civil also built the materials handling system to lift equipment and steel into the bridge for the current project underway by TBS Joint Venture – the $45 million strengthening of the box girders, or clip-ons.

The areas that required the most strengthening are the top and bottom flanges of the box girders in the main navigation span. This primarily involves installing longitudinal steel plates or angle brackets, known as stiffeners, inside the box girders. Similar but lesser amounts of stiffening are required on all spans of the bridge. There’s also some stiffening being done to the pier brackets – these support the box girder trestle legs and carry their load back into main bridge piers.

Steel is transported to the workface using a special motorised trolley on rails inside the box girders, which is driven by one of the workers.

The strengthening work is adding more weight to the bridge – around 760 tonnes of steel will be used – and there is a fine balance between how much steel added to strengthen the bridge and the impact that has on the overall load of the bridge.

During the discussions about potentially adding a cycle lane to the bridge, it was this strength/weight equilibrium that was the main topic of discussion. There’s a finite point to how much more steel you can put into the bridge, after which it becomes a greater load than the strengthening it provides.

The main strengthening work began in August 2008 and is not due to be finished until next year. It’s a big job, with approximately 100 people on site, plus those involved in the steel fabrication off site. The work is largely being undertaken overnight, when the bridge can be closed down to six lanes.

The NZTA says this work is part of the ongoing maintenance regime for the bridge to ensure it stays in peak condition.

The bridge has been heavy vehicle restrictions on the outermost lanes of the bridge (lanes 1 and 8) for some time, and while the current strengthening work takes place heavy vehicles are restricted to the truss and not permitted on the clip-ons at all.

When strengthening work is finished the NZTA has the option of unrestricted use of all lane, but in terms of the asset life long term, there some benefits in keeping the restrictions on the outside lanes.