longer bridge spans
Researchers partnered with Ian Firth on the modelling methodology used to identify optimal forms for very long-span bridges. Credit: The University of Sheffield.

Researchers from the University of Sheffield and Brunel University London in the UK have identified a bridge design based on a new mathematical modelling technique that could enable significantly longer bridge spans than currently possible.

The researchers teamed up with British bridge expert Ian Firth of engineering consultant COWI on the modelling methodology used to identify optimal forms for very long-span bridges.

As bridge spans become longer, a growing proportion of the structure is needed to carry the bridge’s own weight, instead of the vehicular traffic crossing it.

A relatively small increase in span requires use of more material, which makes the bridge structure more bulky as it needs more material to support it.

Beyond a set limit, a bridge cannot carry its own weight. So the only viable option to increase the span is to change the bridge’s design, the researchers noted.

“The mathematically optimised design for longer bridge spans features sections that are similar to a bicycle wheel, with multiple ‘spokes’ instead of a single tower.”

University of Sheffield professor Matthew Gilbert said: “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.”

The technique invented by the team uses theory developed by Professor Gilbert’s namesake, Davies Gilbert, who used mathematical theory in the early 19th century 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.

By fitting this theory into a modern mathematical optimisation model, the team identified bridge concepts that need the minimum possible volume of material, potentially making significantly longer spans feasible.

The mathematically optimised design for longer bridge spans features sections that are similar to a bicycle wheel, with multiple ‘spokes’ instead of a single tower. However, constructing the design would be difficult on a large scale.

To solve the issue, the team replaced these with split towers featuring just two or three ‘spokes’ as a compromise, which preserves most of the benefit of the ideal designs, in addition to being easier to construct.