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Severn Bridge


This was the world's first modern suspension bridge, where the deep stiffening truss deck, so universally adopted for suspension bridges after the Tacoma Narrows collapse, was abandoned in favour of a sleek aerodynamic box girder design. Tacoma Narrows' slim, flexible, plate stiffened deck failed after a gusting, 70km/h wind set up uncontrollable oscillations in the 840m span, sending it crashing into Puget Sound.

Severn's designer Freeman Fox had no previous experience in long span suspension bridge engineering and consulted the world's leading suspension bridge engineer Othmar Amman in the US. Amman was responsible for the George Washington Bridge and Verazzano Narrows crossings in New York and suggested that they stick with a deep stiffening truss deck.

What the Freeman Fox design team under Gilbert Roberts and Oleg Kerensky had discovered when analysing the structure as a whole, was that the stability of the deck could be enhanced if it was built aerodynamically using an enclosed plate girder box. They rejected Amman's suggestion of a deep stiffening truss after extensive wind tunnel and dynamic load testing of their own aerofoil shaped design.

Wind tunnel and dynamic load testing at the National Physical Laboratory in London, in Seattle US and the Building Research Establishment concluded that a continuous welded aerofoil box construction would overcome the basic problems of uncontrolled oscillations under wind loading.

The incorporation of inclined suspension hangers also provided additional damping against potential wind oscillations, and the innovative steel box sections for the twin towers helped reduce the overall weight of the superstructure.

Design innovations on the Severn Bridge reduced the cost of suspension bridge construction by a staggering 25%, thanks to the large amounts of steel that was saved on the bridge deck and the twin towers. It revolutionised long span bridge design and helped establish Freeman Fox as the leading bridge engineers in the world.

David Bennett's book The creation of bridges, price £20, has just been published by Aurum Press

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