Pushing the limits for maximum bridge span

Connected Society 27 February 2017 Randi Nøhr Møller

Ramboll engineer Randi Nøhr Møller’s industrial PhD project seeks to improve existing methods for calculating the instability limit of long-span bridges. This will enable the industry to build longer, slenderer and less expensive bridges in future.

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Wind is a key determinant in the design of long-span bridges. This is why existing methods for calculating aerodynamic stability define the length that bridges can be built today. The ability to make more precise calculations would enable bridges to be built in areas currently beyond reach. 

Construction costs are another crucial factor in designing long-span bridges. It was while working on a conceptual design for the Sulafjord Crossing in Norway that I first spotted an opportunity for improving existing calculation methods. I realised that significant construction costs would be saved if the number of potentially over-conservative assumptions used to calculate stability limits could be reduced. 

This was how I got the idea for an industrial PhD project. 

When we were working on the Sulafjord Crossing, I realised how relevant the instability calculations are to the design of long-span bridges, yet it was also clear to me that our methods could be even better.

Where no bridge has gone before 

Such improvements will do more than optimise the design of bridges and thus lower the cost of building them. Entirely new opportunities will emerge.

Martin Nymann Svendsen, my company supervisor for my industrial PhD, explains:

“There’s nothing wrong with the structure of the long-span bridges we see today – obviously, they’re still standing. But by improving the calculation methods, we can build bridges in areas that are unfeasible when the methods currently available are used.”

However, improving the existing methods would take a great deal of time, resources and dedicated expertise. Therefore, Martin was convinced of the potential of my PhD project, and together we applied for funding.

The Ramboll Foundation, Ramboll Denmark, Ramboll Norway, Innovation Fund Denmark and the Technical University of Denmark (DTU) all granted support, and we are now working closely with my university supervisor, Professor Steen Krenk, at DTU’s Department of Mechanical Engineering to push the limits of long-span bridge design.

Calculating the aerodynamics

Today, I am still at the beginning of my three-year PhD project, which will run until the summer of 2019. The overall project aim – to improve the method for calculating the aerodynamic stability limit – includes demonstrating advanced structural response effects and providing a consistent representation of wind load.

The instability limit is the wind speed that triggers the onset of flutter – the fatal vibrations of the bridge deck structure that occur when the wind and the bridge structure itself interact.

This research has the potential to change how the industry designs long-span bridges today. A higher flutter limit would make it possible to design and build slenderer bridge structures, to construct longer suspension bridges and to reduce the construction costs of already viable span lengths.