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MAKING THE MODERN WORLD
Stories about the lives we've made

module:Bridges

page:Are there any disadvantages to the suspension bridge?

The first Tacoma Narrows Bridge, near Seattle in the United States, was a cutting-edge design that was hailed as a masterpiece when it was finished in July 1940. It was the ultimate example of a trend in suspension-bridge engineering towards maximum lightness, grace and flexibility. The deck was only 2.4 metres deep and 12 metres wide.


The Tacoma Narrows disaster, 1940. picture zoom © Topham

Unfortunately, it displayed rather too much grace and flexibility, becoming known as 'Galloping Gertie' for the wave-like motion of its deck, even in light winds. It became something of a local dare to drive across Gertie when she was in motion. Thousands of people drove hundreds of miles to experience the sensation of crossing the rolling centre span.

However, on 7 November 1940, moderate 40 mph winds set up strong sideways twisting in addition to the usual lengthways waves. Once this new feature appeared, it took only 45 minutes for the steel road-deck to break apart and send sections plunging into the river below.

There the huge chunks of steel still lie, 60 years later - the biggest sea-based wreck in history, they are more than five times the size of the Titanic. Remember: this was a comparatively small suspension bridge compared to present-day record holders!

So, yes, there is a disadvantage to the suspension bridge design – namely its lack of the rigidity implicit in both beam and arch designs. This makes suspension bridges susceptible to the phenomenon of 'resonance', where vibrations build in magnitude due to some regular energy input.

After the Tacoma Narrows Bridge disaster, wind tunnels were used to test a bridge design for the first time. The deck of the replacement bridge that was developed using this technique had a very different appearance to that of its predecessor.

Since then such 'modelling' has been compulsory, and advanced computer simulations have become vital tools in designing the deck-stiffening trusses and 'damping' features that keep our giant suspended spans safe and stable.


Millennium Bridge across the Thames, London. picture zoom © structurae.de (Guido Morgenthal)

But even today it is possible to build a bridge that behaves in an unpredictable way. Perhaps you remember the reports about the strikingly unusual Millennium Bridge across the Thames in London. It was shut down just days after opening due to unexpected wobbling caused by the footsteps of 200 people all trying simultaneously to compensate for the bridge's vibrations.

Six months, several Cambridge University professors and £5 million later, the bridge reopened - without the wobbles.


The Akashi-Kaikyo Bridge, Japan. picture zoom © structurae.de (John Ochsendorf)

The Akashi-Kaikyo Bridge in Japan is currently the longest bridge in the world, being some four kilometres in total length with a centre span of 1990 metres. For several years there have also been detailed plans in existence for a bridge to join Sicily to mainland Italy with a single 3.3 kilometre span. Both of these enormous bridges are designed to withstand Force 8 gales and major earthquakes. They are designed as potent symbols of the might of civilisation, standing against the forces of destructive nature.

Resource Descriptions

The Tacoma Narrows disaster, 1940.
Millennium Bridge across the Thames, London.
The Akashi-Kaikyo Bridge, Japan.
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