By Charlotte Louise Ankerstjerne, February 2017
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 Ramboll engineer Randi Nøhr Møller first spotted an opportunity for improving existing calculation methods. She realised that significant construction costs would be saved if she could reduce the number of potentially over-conservative assumptions used to calculate stability limits.
This was how Randi, who is passionate about mathematics and physics, 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,” Randi says.
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, Randi’s company supervisor for her 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 quickly convinced of the potential of Randi’s PhD project, and together they applied for funding.
The Ramboll Foundation, Ramboll Denmark, Ramboll Norway, Innovation Fund Denmark and the Technical University of Denmark (DTU) all granted support, and Randi and Martin are now working closely with her 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, Randi is still at the beginning of her 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.
Randi’s 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.