Allan Larsen, Denmark

On the occasion of the Annual Meetings of IABSE on March 21, 2001 in Malta, Mr Klaus Ostenfeld, President of IABSE, presented the 2000 Outstanding Paper Award to Dr Allan Larsen for his Paper

'Aerodynamics of the Tacoma Narrows Bridge - 60 Years Later'

published in Structural Engineering International, November 2000.

Dr. Allan Larsen has specialised in Wind Engineering and Dynamic Analysis. Since 1983 Dr Larsen has been working in Consulting Engineering Companies and research Laboratories, specialising in wind/structure interaction.

On 7 November 1940, the newly built Tacoma Narrows suspension bridge in Washington State, USA, was wrecked by a gale with wind speeds reaching only 19 m/s. The collapse of the bridge followed 70 minutes of violent oscillatory motion of the deck. What remained obscure until now was a thorough understanding of the fluid dynamics mechanism responsible for the wind-excited twisting motion and thus the ability to predict and design against this kind of instability. The article presents a detailed account of this phenomenon and proposes an empirical model of the vortex formation and drifting process that allows the critical wind speed for onset of the instability to be estimated. Design modifications that would have rendered the original Tacoma Narrows Bridge more aerodynamically stable are mentioned.

The Outstanding Paper was selected by an international committee, chaired by Professor Bo Edlund, Sweden. The Award is presented each year by the International Association for Bridge and Structural Engineering in recognition of one article published in Strucutural Engineering International, thus encouraging and rewarding contributions of highest quality.