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Technical Excellence: Sophisticated spans

The bridge sector is mildly optimistic with the recently opened Pont Schuman in Lyon, France showing what is now possible.

This year’s Autumn Statement, with its promises of significant investment in infrastructure, has left the bridge sector mildly optimistic.

Yes some major projects have been put forward, but few are guaranteed to get the final go-ahead. Some hardy perennials, such as the Lower Thames Crossing, keep sending up green shoots, only to wither prematurely in the face of local objections and funding constraints. Those major crossing projects that do survive, however, will act as showcases for the technical skills of British bridge designers and constructors, skills that are still in demand worldwide. 

Bridge designer Flint & Neill, now part of the Cowi Group’s bridge, tunnel and marine structures division, is a case in point. Earlier this month the dramatic Pont Schuman in Lyon, France, opened with a spectacular light show that emphasised the crossing’s elegant lines. Flint & Neill won the design competition against stiff local opposition. 

But despite the complicated logistics, which involved off site erection on the barge that brought the arches into their final position, the project went smoothly. Flint & Neill chief executive officer David Mackenzie says Pont Schuman epitomises the changes in bridge design and construction over recent decades. 

“Twenty-five years ago you were restricted in what you could achieve as a designer by the fabrication techniques and erection methods available. And the software wasn’t nearly as sophisticated as it is today.” 

Pont Schuman

Pont Schuman: offsite construction played key part

Pont Schuman’s twisted triangular section steel arches illustrate what steel fabricators can achieve in 2014. 

Changing procurement patterns are also encouraging innovation and technical excellence, says Mackenzie. 

“Clients are moving away from traditional procurement. They increasingly prefer to leave things to the specialists. Hence the growth of design, build, finance operate (DBFO) and similar relationships. 

“But when you take on the entire responsibility for a significant piece of infrastructure, you have to be sure you have done everything right.” 

Maintenance

Long-term maintenance needs are now much higher on the design priorities agenda. Gone are the days when the original Forth Road Bridge could be designed and built without providing any protection to the underside of the deck for repainting purposes. The unforeseen dangers posed by ever-increasing use of de-icing salts are graphically illustrated by the ongoing saga of London’s Hammersmith Flyover, currently undergoing yet another major renovation. 

Concrete bridges acquired a bad reputation as cracking and rust staining became ever more obvious – not to mention the now largely forgotten panic over alkali-silica reactivity (ASR), demonised as “concrete cancer” at the time. Yet Mackenzie reports that concrete is making a comeback. 

“Its inherent design flexibility and potentially low maintenance costs always made it attractive,” he says. “Protecting steel is now a major factor – paint has to be applied under ideal conditions and the cost of containment of the working space is significant.” 

“Clients are moving away from traditional procurement. They increasingly prefer to leave things to the specialists” 

David MacKenzie, Flint & Neill 

Concrete producers have upped their game, however, in response to clients’ demands for much improved performance, especially much more predictable long-term durability. “And clients are increasingly prepared to invest in high quality concrete,” Mackenzie adds. 

Further reassurance comes from the range of proven structural health monitoring technologies, mostly focused on concrete structures and particularly those prestressed elements whose vital tendons have proved vulnerable in some cases to de-icing salt (chloride ion) attack. 

It was the acoustic monitoring system on the Hammersmith Flyover that detected problems with the prestressing cables in plenty of time for remedial works to be carried out. (NCE 23 February). Steel bridge enthusiasts, however, might well point to the Hogarth Flyover further up the road, where the steel substructure easily outlasted the precast concrete deck and was judged not to need repainting even after more than 40 years. (NCE 3 September). 

Durability

Long-term durability is just one aspect of the much wider view clients and designers have to take these days, says Mackenzie. 

“Whole life costing calculations are more sophisticated. We are much more aware of the environment now, and of sustainability. And then there is the fundamental question – who will actually benefit long term from a major piece of infrastructure? 

“Is it the users, the drivers no longer stuck in a traffic jam, or is it those living and working close by?” 

He cites the £590M Mersey Gateway Bridge, where Flint & Neill led the Samsung/Kier/FCC Construction joint venture design team, as one project that attracted very little local opposition. 

“Everyone benefits,” he says. “The whole area will be decongested, greatly reducing air pollution as well as traffic jams. Everyone’s quality of life will be improved.” 

The trend for ever more extreme crossings seems to have peaked, with the retrenchment on the original competition-winning design for the new Wear Crossing in Sunderland. Many clients and local residents want their new bridges to solve a congestion problem but also to become iconic structures in their own right, somehow putting their town on the map. 

In the case of the Wear Crossing this involved twin curving steel towers 187m tall, which would have made it the tallest bridge in England, competing with the Angel of the North for local landmark status. But the numbers failed to add up, and the local authority now hopes for a better balance between affordability and aesthetics (NCE 2 May). 

Mersey Gateway Bridge

Mersey Gateway Bridge: improving quality of life

“Since the Millennium there’s been less funding for extreme crossings, even footbridges,” Mackenzie reports. “There’s still a desire for more attractive footbridges, but within tight budgets.” 

London’s Nine Elms Footbridge is a prime example of this. Warning bells might sound in some ears, following the well publicised problems with the original “wobbly” Millennium Bridge, given that the new crossing will have similar restraints on headroom below and sightlines above, and a similar high profile location. 

Mackenzie is unfazed. “The wobbly bridge was a real turning point. For the first time, people, especially pedestrians, realised that a light bridge will move under loads. 

“We now understand the causes of movement, so we can preserve the light touch without resorting to a heavy handed solution.” 

High Speed 2 (HS2) will pose different challenges for bridge designers, he adds. “Long span crossings and high speed trains don’t mix. Instead, successful integration of off-site construction and building information modelling will be uppermost in the minds of the designer on HS2.” 

He also cautions for a balanced view on perceived “vanity projects”. “The Eiffel Tower struggled for acceptance –you wouldn’t take it away from Paris now, there’d be an even bigger uproar. 

“As an engineer I want to look back at the projects I worked on and be happy they are delivering what was intended.” 

This article was produced in association with Flint & Neill.

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