In an EPSRC-funded project, researchers at Sheffield University and Brunel University London, alongside long span bridge expert Ian Firth of engineering consultants COWI, used a mathematical modelling technique to identify new bridge forms.
Most existing long span bridges are either suspension bridges, such as the Humber Bridge, or cable-stay, such as the Queensferry Crossing.
However, as these bridge spans increase in length, more and more of their structure is needed just to carry their own weight, according to project leader Prof Matthew Gilbert at Sheffield University.
“There is a theoretical limit on how long a bridge span can be before the material fails,” said Gilbert.
So instead the researchers set out to investigate whether new designs could lead to more structurally efficient forms, allowing longer bridge spans to be built.
The researchers developed a mathematical optimisation model, in which they incorporated a nineteenth century mathematical theory by Davies Gilbert, who advised Thomas Telford on the design for the Menai Suspension Bridge in North Wales.
They found that the most mathematically optimal designs would contain regions resembling a bicycle wheel, with multiple spokes in place of a single tower.
However, since this would be difficult to replicate on a large scale, the team replaced these wheels with simpler split towers comprising just two or three spokes each.
“Very often there are slight variations on [the mathematically optimal] forms, which are only very slightly less efficient, in that there is very little extra weight, but they are much easier to build,” said Gilbert.
In the design, the forces from the deck are transmitted more efficiently through the bridge superstructure to the foundations, meaning less material is needed. This is done by keeping the load paths short, and avoiding sharp corners between tensile and compressive elements.
More work is needed to ensure the new designs are practical, including how easy they will be to construct, said Gilbert.
“We haven’t yet looked at the lateral loading from wind,” he said. “So we’d like to work with experts in that field, to see whether [the design] is a practical solution for very long spans.”
The research is published in the Proceedings of the Royal Society.