The Tacoma Narrows Bridge Collapse
brief summary
Probably the most famous, and almost certainly the most dramatic bridge collapse ever occurred on the 7th November 1940, at about 11.00 am. The bridge had the highest span:width and span:depth ratios of any suspension bridge of its time, and was regarded as a marvel of engineering. However, wind gusts through the Narrows caused the deck to oscillate, and it became a tourist attraction, known by the locals as "Galloping Gertie". However, on the day of the collapse the wind speed was greater than normal, although still only about 42mph. This caused greater vertical oscillations, and finally a cable connection slipped, and the bridge failed progressively. This failure occurred because the technology level of the period did not understand the mode of behaviour present.
The decks were made out of solid plate girders, instead of the usual trusses, which accentuated the problem by caused by leading edge vortices, which induced oscillations. This, together with the more important fact of the technology step changes in the two basic parameters mentioned above, was the cause of the failure.
what happened?
The stronger wind caused the bridge to oscillate more than usual. The greater amplitude caused a failure of a cable connection which then caused a progressive collapse of the bridge.
7 November 1940 at about 11.00am.
Tacoma, near Seattle, Washington State, USA.
who designed the bridge?
L.S.Moisseiff, a leader in his profession.
There were no deaths or injuries due to the failure.
 | The oscillations caused
the slipping of a mid-span cable connection.
| This had served to
dampen the torsional oscillations, which now became more severe.
| The vertical hangers
began to break and the collapse of the entire structure followed. Both
towers and both side-spans were damaged beyond repair. | |
It was quite a risky design because both the span:width and span:depth ratios of the bridge were much greater than any previous suspension bridge, thus causing two step changes in technology, both in basic parameters.
There were two reasons for the failure:
| There was a step change
in two basic parameters.
| Solid plate girders
were used in the decking. | |
These two reasons were sufficient because the level technology of the time was not good enough to understand the effects these changes would have.
references
University of Washington ( Seattle ) . Structural Research Laboratory: Aerodynamic stability of suspension bridges : with special reference to the Tacoma Narrows Bridge / a report of an investigation conducted by the Structural Research Laboratory, University of Washington under the direction of the Washington Toll Bridge Authority in co-operation with the Public Roads Administration, Federal Works Agency. - Seattle : University of Washington Press , 1949-1954.
http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/DSmith/photos.html - Some photos of the Tacoma Narrows bridge before and after collapse.
http://www.ketchum.org/bridgecollapse.html - More information about the Tacoma Narrows Bridge collapse.