Bridge to engineering: Beam me up!

by John Cousins | 8th October 2009

The last really big lift in Tauranga's $131 million Harbour Link project is due to take place when a massive 67 tonne steel beam is slotted into place against the old Harbour Bridge.

"It will be quite an engineering feat," Harbour Link project manager Kevin Reid said.

The beam was due to be installed today, but this morning the second part of the beam had not yet been delivered. The two components will be bolted together. The installation of the beam was weather dependent. If the installation did not take place today, it was unlikely to happen tomorrow as the forecast was for rain and wind.

The 50-metre long beam has the distinction of being the biggest single component ever manufactured by Mount Maunganui's Jensen Steel Fabricators, taking nearly three months to make and two months to paint.

The zinc-coated beam is an integral part of the new off-ramp which will take city-bound traffic from the old Harbour Bridge down to Dive Crescent.

New Zealand Transport Agency was forced to build in steel rather than concrete because concrete would have been too heavy for the bridge's foundations - the widest end of the beam attaches to the bridge.

The box beam is 2.4 metres wide at the top, tapering down to 1.2 metres, and is 2.4m deep.

Even transporting such a huge load needed special logistics. Cranes lifted the front end on to a Rowe Motors tractor and trailer unit, while a HEB Construction motorised self-steering jinker took the weight at the back.

The shift to the old bridge began at 10pm last night, after the roads had quietened down.

The beam took up the whole length of Jensen's workshop and posed some special challenges when it came to lifts because the workshop only had five tonne cranes.

Mr Reid said lifting the beam into place would be quite a complicated operation and was the last really big lift of the project.

 Steel was chosen over concrete because they wanted to put as little extra weight onto the bridge as possible. The other advantage of a steel was that it was much quicker and far less complicated than concrete which would have needed the construction of a one-off mould. The beam comprises the top section of the ramp descending to Dive Crescent.

Mr Reid explained that construction of the off-ramp was why, when the new flyover opens on Monday, the existing ramp down to Dive Crescent will have to close. The old ramp will have to be dug up to make way for the new one.