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he Tacoma Narrows Bridge |
The Tacoma Narrows Bridge collapsed on November 7 1940.
It was the third longest suspension bridge in the world, connecting Seattle
and Tacoma with the Pugent Sound Navy Yard. It had a span of 5,939 feet with
a central span of 2800 feet.
never
really right
From its opening there was clearly something not quite right about the structure.
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The alarming undulations earned the bridge
the nickname, Galloping Gertie.. |
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The alarming undulations earned the bridge the nickname, 'Galloping Gertie'.
The newsreel footage of this bucking action makes you wonder quite how the
people struggling to keep their feet, or vehicles travelling in a straight line
did not see how it was all going to end.
End it did, only 4 months after it opened. After two years of construction
it had been opened on July 4, 1940 to traffic. The wind speed on the fateful
day was only 42 miles per hour.
the
problem
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moderate winds ...were turned into something
rather more alarming.. |
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The problem was the moderate winds that travelled down the canyon were turned
into something rather more alarming by the geography of the area.
Until November 7th the bridge had gained plenty of publicity and people would
cross it just because of its fairground ride attraction.
The bridge cost a relatively small some of money for such a structure (approx.
$6.5million) and represented the end result of the development in streamlining
and graceful architecture of the 1920's and 1930's.
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| PLAYERS, CELEBRATED
GATEWAYS [1909] |
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The 'slender' appearance meant it was cheaper to build, because simply there
is less material in slimmer structures.
This was also a road bridge which was not designed with the weight of railroad
rolling stock in mind.
The bridge was not unique in its tendency to unusual movement. In 1818 the
Dryburgh Bridge in Scotland had been reported as showing signs that spans were
prone to movement, when it suffered sever damage due to wind forces.
not
uncommon
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This was by no means an isolated incident...
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This was by no means an isolated incident with accidents in France in 1852
with the Roche-Bernard Bridge and also Canada.
There were other bridges in US which were showing signs of this instability
and additional structural supports were required to dampen the effects. Including
the case with the Golden Gate Bridge which needed additional support in 1937
because of wind oscillation.
For whatever reason these warning signals were not picked up in the design
of the Tacoma Narrows Bridge which took the slender concept a step further.
When the collapse did occur it fell 200 feet into the river below. Fortunately
nobody was killed in the incident.
In 1992 the sunken remains of the bridge were put on the National Register
of Historic Places to protect it from salvagers.
the
aftermath
The disaster could well be one of the most analysed of all structural failures
and although the exact explanation for the collapse seems to be elusive the
lessons learnt have been well learnt.
Well, perhaps. there is of course the Millennium Bridge in London, England
which was to be so graceful and slender as to be a blade of light. Itturned
out to be something of a fairground ride itself. They might have learnt a few
more lessons from history.
