Strength, grace and economy

It’s the first bridge of its type built in New Zealand, and it saved a bundle of dollars on a conventional structure. HUGH DE LACY reports.

The flood that wiped out the Mangamahu Number 47 bridge in July 2006 was always going to cost the Wanganui District Council a packet to replace, but thanks to a Norwegian design never employed in this country before, the cost was only two-thirds of what it might have been.

The network arch bridge design was pioneered by Dr Per Tviet in the 1950s, and when the man himself came to New Zealand in October 2007 for a seminar hosted by the Heavy Engineering Research Association (HERA), Phil Gaby of the Wellington branch of the Holmes Consulting Group was there to hear what he had to say.

What Tviet described was an arched bridge in which the hangers supporting the lower tie were not strung vertically, but at an angle of between 50 and 70 degrees. They crossed each other at least twice,  distributing the bending moments and shear forces to the arch at the top, and to the tie – the roadway, railway or footpath – at the bottom.

The parallels with a truss structure are obvious, and the visual impact is of something like a large, light and elegant arched trestle.

The first network arch bridge was built at Steinkjer, Norway, in 1963, and quite a few have been built in Japan, but uptake of the design has been relatively slow elsewhere.

Gaby told Contractor he had no New Zealand application in mind for a network arch bridge when he attended the HERA conference, but its relevance to the Mangamahu situation struck him when he first began to look at that project.

“I expected neither the contractor nor the client to want to take the risk [of a new design], but they were both all go,” he says.

The $2 million contract went to the Rotorua company Concrete Structures, whose general manager and owner, Mike Romanes, was undaunted by a design requiring only 150 cubic metres of concrete to go with the 80 tonnes of structural steel.

The single-lane bridge crosses the Mangawhero River in a single 85 metre span, with the elimination of a central pier, and the shallower superstructure maintaining flood clearances while minimising road approaches, allowing Holmes Consulting and Concrete Structures to present the district council with easily its the lowest tender, cutting 30 percent from the projected cost.

The council’s specimen design had envisaged two 42.5 metre spans with a central pier in the middle of the river, and a deeper superstructure that would have required the approach roadway to be raised to keep the soffit of the replacement bridge at the same level as the previous one. Significant flood protection works would also have been required for both the riverbank and the pier.

The 47-year-old original bridge, which collapsed two days after the flood, had three spans supported on raked piles, with wall-type piers and twin steel I-girders made composite with a concrete slab deck.

Construction of the new bridge, whose 3.5 metre-wide single lane has shoulders between the side barriers that take the total width to 6.6 metres, began in September last year and took only three months. This meant the district council was able to remove the Bailey bridge that gave temporary access to the residents of the Mangamahu Valley four months earlier than scheduled.

The key to the rapid construction, which features a steel-weight to bridge-area ratio of just 150 kilograms per square metre, was that much of it was conducted off-site.

“It was more of an assembly operation than a construction,” Romanes says.

The steel ladder deck was supported on falsework at spans of about 30 metres, while the arch was split into halves assembled on either side of the river. The 32 parallel hangers were angled at 60 degrees and constructed of standard off-the-shelf 32mm diameter Reid bars.

Each half of the arch, with bracing elements attached, was lifted into position by one of two 85-tonne capacity cranes, and then bolted at the knee-joint. A third mobile crane then lifted two men in a cage to bolt the arch crown splice, with the whole job of erecting the arch taking less than a day.

Once the hangers had been installed, the decking falsework was removed and the pre-cast decking panels lifted into place to provide a permanent formwork for the in situ concrete topping.

The design might have been a New Zealand first, but Romanes says it was simplicity itself to execute.

“We never even used a gas-set on the site, and every bolt fitted in every hole.”

He had just two men working on the foundations and the abutments, and six more assembling the arch and constructing the deck.   

Holmes Consulting’s Gaby said the weight of steel for a single-span arch bridge was the same as it would have been for a two-span steel truss structure, “but the advantage is no pier in the river and that’s where the huge saving is. If you tried to span that distance with a single-span truss, the weight of steel would have been over three times greater.”

Introducing the new construction system to New Zealand required extensive pre-design analysis, including building a three-dimensional model.

“It was a traditional design process but it was quite sophisticated because this hadn’t been done before,” Gaby says. “We analysed each hanger, each arch segment, bottom cords and transfer transoms. Their load paths all interconnected so we had miles and miles of load cases for miles and miles of different members, but we set the thing up and automated it as much as we could with spread-sheets connected to the analysis.”

As a result Holmes Consulting developed a set of design tools specifically for the network arch bridge that should make such structures, which are capable of forming a single span over 250 metres, a more common feature of New Zealand roads.

In fact the country may not have to wait long for a second such bridge: a Holmes Consulting Group design for a two-lane 100-metre network arch bridge is presently being constructed for the Taupo arterial route.