Luckily, the design team had a solution in its back pocket. A series of curved, steel structures called ‘guide vanes’ were fastened to the bridge to minimise the vortices. But everyone on the team was left with a feeling that a better solution could be found.
Trimming sails, trimming bridges
Allan Larsen, a wind expert on the team, had to wait several years - and leave the confines of his COWI office - to find a solution.
One summer evening, while trimming the jib on his sailboat, he wondered whether the angle on the bottom of a bridge girder could be adjusted in a similar way to reduce vortices.
To test his hypothesis, Larsen travelled to Canada, where the aerodynamic laboratory at the National Research Council in Ottawa was interested in his idea and willing to share expenses to conduct an experiment which could test the hypothesis.
COWIfonden, the foundation which oversees COWI, also awarded Larsen a stipend worth EUR 21,500 to help cover expenses. This enabled him to build five bridge models with angles in the girder that ranged from 12 to 26 degrees and to test them in the laboratory’s wind tunnels.
Much to Larsen’s satisfaction, the experiment confirmed his suspicions. He found that the bridge model with an angle of about 15 degrees was largely free of oscillations.
Stable bridges
In addition to creating safer and more stable bridges, the discovery could have a financial benefit. Larsen calculated that designers could eventually phase out guide vanes – something which would save contractors about EUR 1.5 million per kilometre of span by requiring fewer materials and less maintenance.
Larsen meanwhile welcomed the chance to return to the lab and be creative. "That’s something we don’t get the chance to do everyday."
By Uzi Frank, uzif@cowi.com
Published: 02.07.2009
