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Composites for long spans

Main spans of cable-stayed bridges could be doubled in length to 2km using composites in the deck, a leading bridge engineer told NCE this week.

SKM rail and bridges technical director professor Sam Luke said world leading long span bridge experts are considering using carbon fibre composites to help increase cable-stayed bridge lengths.

“We could see 2km long [main spans] within five years,” said Luke.

“The biggest issue limiting cable stay spans is weight and the dynamics affecting cables. We need to look at composites to help reduce dead load,” he added.

Bridge engineers including Aecom global long span and speciality bridge director Robin Sham and Arup bridges director Naeem Hussain agreed that composites now need to be considered to help span greater lengths.

Currently the longest cablestayed bridge in the world is the Russky Island Bridge in Russia with a main span of 1.104km (NCE 5 July 2012).

But spanning greater lengths increases self-weight in the cables, causing them to sag, reducing the bearing capacity of the cables. With the cables less efficient at carrying loads, more cables are needed, further increasing the weight of the bridge and adding to its cost.

These issues can be offset if lighter, composite materials are used to replace some steel elements in bridge decks.

Carbon fibre is typically a fifth of the weight of steel. But if it was used, most of the deck would still be made of steel and concrete, for robustness and stiffness.

Luke estimates using composites on a long span bridge deck would reduce its overall weight by about 25% to 30%.

Bridges with main spans of 2km would normally be suspension bridges. But the costs of construction are almost double compared with a cable-stayed crossing - £5,100/m2 to £5,700/ m2 compared with £2,900/m2 to £3,200/m2. Luke also stressed the benefits of using composites on smaller span bridges.

“It’s not just weight reduction - it helps reduce whole life costs,” he said, citing the example of the Mersey Gateway - which currently consists of four spans, but which could be reduced to three using composites.

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