Photo: Helen E. Fairclough et al.

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)

Exploring the optimal form for very long-span bridges

25.09.2018

Academics have developed a new mathematical modelling technique to identify theoretically optimal forms for very long-span bridges.

A new study on the most efficient structural design of long-span bridges has just been published.

COWI consultant and structural engineer Ian Firth, who reviewed the research and provided input on aspects such as loading and material grades, explains the reasons for conducting the thought experiment:

"Long-span suspension bridges use large volumes of material simply to hold themselves up. As the span gets longer and the self-weight increases, the problem worsens, and the structure is no longer viable because the weight can no longer support things like imposed loads from vehicle traffic."

The research, published last week, explores whether there is a different structural form that could be more efficient, and whether spans of longer distances could be designed using less material.

The authors outline a radically new layout optimisation formulation which identifies a bridge form that would require the minimum volume of material for a given span, as well as simplified variants that utilise split-pylons, which would be more materially efficient for spans over 2 km.

The appearance of the bridge form has been compared to that of a bicycle wheel, with the fan shaped towers resembling the spokes. The authors acknowledge that this design would be impractical to construct at large scale, but suggest a simplified split-pylon concept, as shown in the example image below, could enable the development of effective construction methods.

Ian recognises that further research is needed to make the designs practical: “The research is at an early stage, and more work is needed to extend the theory beyond the simple consideration of gravity loads to include other aspects relevant to practical long span bridges, and to consider the practical implications of constructing such structures. Maybe one day we will see these new forms taking shape across some wide estuary or sea crossing."

Theoretically optimal forms for very long-span bridges under gravity loading was published in the Proceedings of the Royal Society A on 19 September 2018. The team included researchers from the University of Sheffield and Brunel University London, and COWI consultant Ian Firth.