Bridging the Strait of Gibraltar may now be possible using a mathematical modelling technique to identify designs for structures that can span ever further distances.
Like Brunel’s famous Clifton suspension bridge near Bristol, most long span bridges are suspension bridges or cable-stay, like Edinburgh’s 1.7 mile Queensferry Crossing.
But the longer bridge spans become, the more their structure works simply to support their own weight rather than the traffic crossing it.
Researchers at Brunel University London, the University of Sheffield and engineering consultants COWI developed a mathematical optimisation model to rethink how long span bridges can work.
“The crucial novelty of the model is that it takes into account self-weight of the structural elements” said Brunel maths lecturer Dr Aleksey Pichugin.
“Self-weight is generally not a major concern in many engineering applications. However, for long spanning structures it becomes fundamentally important because it restricts how long a span can be.”
Rather than using pillars, the novel designs use a set of spokes like in a bicycle wheel, connected by cable instead of a single tower. This in theory, can spread force along the cables more efficiently, which means less material, less weight and a longer span. These would be impractical to build, but the study suggests a cable-stayed-like bridge with two or three split towers would be nearly as efficient as the radical optimal design. The team reckon this could support a 5km span. This backs 1996 plans for a Europe-Africa bridge with two 5km spans and two 2km on each side, which would cover the shortest gap between the continents – 14km.
The study, in Proceedings of The Royal Society A, shows how the volume of material needed for the bridge span changes as the span lengthens. This highlights how novel structures can carry self-weight more efficiently than most traditional ones.
“The suspension bridge has been around for hundreds of years,” said project leader Prof Matthew Gilbert at Sheffield. “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. Our research has shown that more structurally efficient forms do exist, which might open the door to significantly longer spans in future.”
The team said more research is needed to allow for complicating factors such as wind loads and buildability.
Hayley Jarvis, Media Relations
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