New bridge forms could allow for longer spans

Researchers from the UK have identified new bridge forms that use a new mathematical modelling technique, which could enable significantly longer bridge spans to be achieved in the future.
Split-pylon concept bridge to cross Strait of Gibraltar, with two 5 km main spans (not showing additional measures likely to be needed to counteract unbalanced live load effects)

Researchers from the UK have identified new bridge forms that use a new mathematical modelling technique, which could enable significantly longer bridge spans to be achieved in the future.

A team from the University of Sheffield and Brunel University London worked with long span bridge expert Ian Firth from engineering consultants COWI on the modelling method used to identify optimal forms for very long-span bridges other than traditional forms.

“The suspension bridge has been around for hundreds of years and while we’ve been able to build longer spans through incremental improvements, we’ve never stopped to look to see if it’s actually the best form to use, Professor Matthew Gilbert from the University of Sheffield, who led the research, said.

“Our research has shown that more structurally efficient forms do exist, which might open the door to significantly longer bridge spans in the future.”

The technique devised by the team draws on theory developed by Davies Gilbert, who in the early 19th Century used mathematical theory to persuade Thomas Telford that the suspension cables in his original design for the Menai Strait bridge in North Wales followed too shallow a curve.

He also proposed a ‘catenary of equal stress’ showing the optimal shape of a cable accounting for the presence of gravity loads.

By incorporating this early 19th century theory into a modern mathematical optimisation model, the team have identified bridge concepts that require the minimum possible volume of material, potentially making significantly longer spans feasible.

The mathematically optimal designs contain regions which resemble a bicycle wheel, with multiple ‘spokes’ in place of a single tower, which the team identified would be difficult to build in practice on a large scale.

The team would then replace these with split towers comprising just two or three ‘spokes’ as a compromise that retains most of the benefit of the optimal designs, while being a little easier to construct.

According to the researchers, the new bridge forms require less material principally because the forces from the deck are transmitted more efficiently through the bridge superstructure to the foundations. This is achieved by keeping the load paths short, and avoiding sharp corners between tensile and compressive elements.

For a 5km span, which is likely to be required to build the 14km Strait of Gibraltar crossing, a traditional suspension bridge design would require far more material, making it at least 73 per cent heavier than the optimal design. In contrast, the proposed two- and three-spoke designs would be just 12 and 6 percent heavier, making them potentially much more economical to build.

The team emphasise that their research is just the first step and that the ideas cannot be developed immediately for construction of a mega span bridge.

The current model considers only gravity loads and does not yet consider dynamic forces arising from traffic or wind loading. Further work is also required to address construction and maintenance issues.

Co-author Ian Firth said the research findings were an interesting development in the search for greater material efficiency in the design of super-long span bridges.

“There is much more work to do, notably in devising effective and economic construction methods, but maybe one day we will see these new forms taking shape across some wide estuary or sea crossing,” he said.

The research, funded by the Engineering and Physical Sciences Research Council (EPSRC), was published this September in the Proceedings of the Royal Society.

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