'More than safe passage, a destination' - Manganui Gorge bridge closer to finish

Specialist builders working from ropes and in sync with helicopters have erected two bridge masts that now tower over the Manganui Gorge on Taranaki Maunga.

The 110-metre Manganui Gorge suspension bridge will be a major feature of the $13.4 million Taranaki Crossing.

Site manager Matthew Thom of Abseil Access had 30 bridge builds under his belt, but said constructing one on a mountain came with its own peculiar challenges.

"Obviously as you can see it's a steep craggy gorge and so getting into some of the locations where anchors are or to get up to the top of the towers to rig the main cables or to fit components to the main cables all of that requires rope work and abseiling skills."

One of the masts was 25 metres tall.

"Basically, there's a big tower that's on a nine-degree list so we had to rig the nine-degree list as it was flown in under the chopper," Thom said.

"So, there was some precise rigging involved there to get it all flying nicely, so that it would come in and we'd be able to place a one-tonne object onto 16 bolts just nicely."

He reckoned nailing it was not a given.

"It was on the trickier end of things, but it all went smoothly."

Dean Arthur had a background in search and rescue and mountaineering

He worked out on the ropes.

"The next stage of this is receiving things out on the cable from a helicopter. So, receiving the hangers that the decking will be supported on.

"And, so they'll be two rope guys out there working with the helicopter pilot to receive those hangers as we sort of clip them on to the main cables."

Arthur said the work was amazing.

"It's spectacular. I feel like I'm mountaineering everyday and it's spectacular."

But not really for adrenaline junkies.

"I think that if you are experiencing too much adrenaline you need to question what's going on, but it's spectacular."

Taranaki Crossing project infrastructure lead Wayne Boness liked to keep his feet on the ground.

"This is one of the wind sway anchors. So, if you think you've got your main cables going across your bridge what these do is they take out the sideways movement because visitor comfort is quite important.

"When you think people are going to be walking across 100 metres they want to be doing it without being thrown around."

A suspension bridge without wind sway anchors did not bear thinking about.

"Probably like a mechanical bull is probably the best way of describing it because with the bridge as you walk on it, it starts to get some movement and it will be a bit like a seasick feeling for some people," Boness said.

"So, a lot of work has been done through the design phase to make sure that user comfort has been captured and delivered."

Department of Conservation Taranaki operations manager Gareth Hopkins had big ambitions for the bridge.

"This is a great way of providing safe passage of Manganui Gorge, but at the same time, it's a bit of a statement.

"I think it will be more than just safe passage, I think it will become a destination. People will come to see the bridge because of its uniqueness."

The Manganui Gorge Bridge was being built in partnership between DOC, Ngā Iwi o Taranaki and the regional economic development fund, Kānoa.

It was on schedule to be completed in late April 2024.

Manganui Gorge Bridge facts

• 100-year design life
• Iwi-designed laser-cut balustrade infill panels which depict the various phases of avalanches
• 49.5m above the valley floor, and is 1.2m wide
• 109m long
• 850m of main and support cables
• 1700m of 5mm stainless steel handrail infill cables
• 10.5cum of concrete supporting mast towers
• Designed for 210 km/h wind speed
• 430 clips to hold down custom-coloured deck material
• nine-degree inward leaning masts
• 168m of inground drilled rock anchors
• Unrestricted person limit
• Designed to resist a one in 250-year avalanche and one in a 1000-year seismic event
• Backstay cables and anchors carry up to 33 tonnes of tension fully loaded
• Main bridge cross section was modelled in a computerised wind tunnel to determine aerodynamic effects to achieve a stiff deck to avoid bounce for visitors when crossing
• 4 tonnes of steelwork
• Steelwork mast coated in thermally sprayed zinc for long-term durability
• Maximum walkway gradient of 6 percent (one in 17)

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