Building smarter bridges

Atkins | 10 Jul 2014 | Comments

Advanced composite materials are transforming the design of everything from aircraft to Formula 1 racing cars. Now they’re set to revolutionise the way bridges are built.

Bridges are not usually associated with high-tech materials, but that could all be about to change. Concrete and steel, the dominant bridge-building materials for more than a century, now face competition in the form of a composite material known as FRP – fibre reinforced polymer.

FRP is strong, light, resists fatigue and does not corrode – properties that make it an ideal material for building bridges. Atkins is leading one of the UK’s first FRP road bridge replacement projects in the village of Frampton Cotterell, near Bristol.

Spanning the River Frome, Frampton Cotterell’s new FRP bridge is designed to handle a full HGV loading and will replace an existing concrete and steel structure that has become damaged by corrosion. Current weight restrictions on the bridge (maximum 13T) will be removed once the work has taken place. The FRP replacement is light, meaning it can be installed in about half the time of a conventional bridge deck. And its corrosion and frost-resisting properties mean maintenance costs could be up to 50 per cent lower.

The idea of using composites in construction is not new. Wattle and daub – a mixture of mud and straw – was successfully used as a building material for centuries. Modern composites are based on the same principle: different materials are combined to create a new one with unique mechanical properties. In the case of FRP, the materials involved are high-strength glass, carbon or aramid fibres and strong polymer resins.

The emergence of composites as a viable alternative for traditional concrete and steel construction comes at a pivotal moment for the UK’s transport infrastructure. Government agencies and transport operators are increasingly seeking ways to get more out of existing assets, and to ensure that new infrastructure delivers maximum capacity from day one.

Long-spanning structures are at heart of this transformation. Smart motorways, for example, depend on gantry-mounted variable message signs spaced at regular intervals. These must span all carriageways. Rail electrification requires catenary – supporting structures to deliver power to trains.

“Transport infrastructure is undergoing enormous change,” says Professor Peter Chivers, chief executive of the government-backed National Composites Centre (NCC). “The shift towards managed motorways, the development of HS2, rail electrification, the need to eliminate level crossings and to improve safety for pedestrians and cyclists – all of these will require massive deployments of bridge and gantry structures. Composites could play huge part in achieving this.”

Building better bridges

FRP structures offer a number of decisive operational benefits. Speed of construction is one of them. With rail passenger numbers up 100 per cent since 1995 and road traffic twice the level it was 40 years ago, minimising disruption is a priority.

“If you have to close a road or railway line to install a bridge, it costs significant amounts of money,” says Mike Stephens, project chief engineer at Atkins. “Because FRP is lighter than concrete and steel, you can install it much more quickly and you can get a cost saving that way.”

The low weight of FRP bridge elements also means that transportation to site is less expensive – and greener – than it is with conventional bridges. In addition, the load on existing abutments is reduced.

“If we are replacing an existing deck with a new FRP element, we know it will be lighter overall, so less detailed work is required to prove the supporting structure,” says James Henderson, group engineer at Atkins.

There’s also the question of build quality. With FRP, the bulk of bridge deck construction is carried out in the factory. Off-site manufacture (OSM) means it’s possible to control the quality of construction closely – something that is not always easily achieved with conventional materials assembled in the field.

FRP has the advantage of being easy to mould and this, too, offers benefits. At a practical level, this makes it possible to build-in access channels for cables and pipework right from the start – eliminating the need to dig up the road. Mouldability also opens up new aesthetic possibilities and the scope for iconic designs could be significant.

“The beauty with composites is that you can create anything you want,” notes Henderson. “The only limit is the budget.”

But perhaps the biggest attraction with composites is the lower cost of ownership. FRP structures do not need painting or waterproofing and are largely immune to the ravages of the weather. And they’re not affected by salt, the scourge of conventional reinforced concrete structures.

“Composite structures offer massively improved through-life costs compared to steel,” says the NCC’s Professor Chivers. “Do it properly now using composites and you will reap the reward in future.”

Advances in FRP technology are also helping to build the case for increased deployment of composite structures.

“There have been quite significant improvements in resin formulations and manufacturability over the last decade,” says Stephens. “It’s now feasible to make large structures, which it wasn’t 10 to 15 years ago.”

In tandem with this, changes in market conditions are helping to spur interest in FRP.

“Our major clients such as the Highways Agency and Network Rail are looking to move away from a project-to-project based approach and looking at an overall programme-based approach to procuring transport infrastructure,” says Chris Hendy, Atkins’ technical director, Highways and Transportation. “The supply chain is being asked to collaborate across a series of transport projects and come up with what they think is the best solution to roll out across an entire programme for best economic impact.”

The shift from one-off bespoke projects to large-scale deployments and repeatable designs could also help to tilt the balance in favour of FRP.

“One of the challenges we’ve had in trying to introduce composites into bridges is the scale problem – with small orders, the economics have never quite stacked up,” says Hendy. “But if it’s possible to agree on a standardised design for footbridges or gantries, and roll them out in large quantities, then it starts to work not only in terms of whole life costs, but also initial costs. That’s got us interested in looking at it again.”

Government is also increasingly interested in composites and it is keen to see the industry grow. The UK Composites Strategy, launched in 2009, was followed in 2011 by the opening of the National Composites Centre (NCC).

“The government is helping to facilitate the use of composites by means of organisations such as the NCC and through grant support from the Technology Strategy Board,” says Professor Chivers. “But it is down to the industry itself to take the initiative and come up with innovative products.”

An example of such innovation is a new footbridge developed by Atkins. What makes it special is its standardised, modular design – and its adaptability.

“The pedestrian envelope – the parapets and footway – are formed as a kind of U-shaped shell,” says Atkins’ Henderson. “We can take this design and use it to create a bridge that can be anywhere between 4m and 30m long.”

As well as reducing capital costs, modular designs of this sort help to reduce risk.
“The more standardisation you can have in the construction process, the less likely we all are as an industry to get it wrong when we build things,” says Hendy. “And if we have standardised components as well, then we have standardised approaches to maintenance across the network, rather than bespoke maintenance plans for every single structure – which is one of the reasons why we have maintenance problems.”

FRP’s high strength-to-weight ratio means engineers can now contemplate schemes that would once have seemed impossible. Longer suspension bridges are a case in point.

“At the moment, the viable span is around 3km,” says Hendy. “The push to go any further than that will require some reduction in the weight of the basic deck. This is where FRP materials could come in, probably still in conjunction with more traditional steel suspension cables.”

Getting the most out of composites demands a range of engineering skills. As well as tapping in to decades of expertise in bridge construction in the road and rail sector, Atkins is also drawing on its aerospace know-how – Atkins’ engineers were involved in the design of the wing of the Airbus A350, the first Airbus with both fuselage and wings made from composites.

“Knowledge exchange and cross-sector working are vital,” says Hendy. “This is something we facilitate through the technical networks which run across our businesses.”

The future for FRP looks promising. As well as its use in bridge construction, Atkins is researching new applications for composites. Among these are overhead line structures for railway electrification, rolling stock components such as doors and train cabs, and applications in the oil & gas and nuclear industries where FRP’s corrosion resistance, low weight and high strength could pay dividends.

“It’s a very exciting time because the options we can take forward are endless,” says Atkins’ Henderson. “The full potential of FRP has yet to be realised.”

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