GROUP 4
TACOMA NARROWS BRIDGE
FAILURE
REBECCA DENTON
DAVID DRAPER
OMOSOLA FIFO
GERARD DUFFY
MICHEAL METAIS
Group 4 - 4A6 - Tacoma narrows collapse
Twist of the bridge
Summary
When it was inaugurated, on the 1st of July 1940, the suspended bridge of
Tacoma was the 3rd largest suspension bridge in the world. A thin and slender
structure, the bridge was very aesthetically pleasing. Four months later, on the
7th of November 1940, nothing remained of this masterpiece. Between those two
dates a wind, of about 65 km per hour, caused the bridge to vibrate and
ultimately collapse. The structure which was too light buckled and twisted, first
slightly, but then more and more furiously. The movement of the bridge,
continually amplified by the wind, caused one of the suspension cables to break.
The load became too much for the others cables, which caused them to break
also, and made the bridge collapse.
Former state description
The Tacoma Bridge was a suspended bridge of total length 1810 metres. Its
intermediate range, the most important, was 853 metres long. Designed to
support two road lanes (one in each direction), the bridge deck was only 12
metres in width, and had a height of 2.45 metres. Indeed, its slenderness was
very detrimental to its failure since the height to range ratio was 1 / 350 (2.45 /
853m) for the main span. Its main defect was its flexibility and its weakness
allowing it to twist and buckle.
Post collapse state
Nothing remains of the slender bridge.
One of the main suspension cables broke due to the increased load put on
it and this caused a 600 foot long span of the bridge to collapse into the water.
Causes
The theory is presented as an example of elementary resonance with the wind
providing an external periodic frequency that matched the natural frequency of
the structure, amplifying continually the movement of the bridge, until its
collapse. However it has since been proven that the bridge failed due to
aerostatic flutter. Flutter is when aerodynamic forces coupled with an objects
natural mode of vibration act to cause periodic vibrations of the object.
The wind was not that excessive; it was because the structure was not
designed to resist them the created the problem. Written reports, by the
Department of highways, advised to make the bridge deck bigger but the
engineer Moiseiff was able to justify using the shallower girders by claiming
that the main cables would be able to take half the static wind load. This was
favourable for the project as they had financial troubles and these shallower
girders significantly reduced the cost. Another problem was that rigid plate
girders were used to support the road bed as opposed to the common practice of
a truss design. The problem is that for a truss design the wind passes through the
deck whereas the plate girder method provides resistance to the wind and the
wind was designed to travel above and below the bridge deck.
Finally, engineers at this time were ignorant in their knowledge of
aerodynamics, which prevented them from seeing how the problems related to
the wind would have acted, in spite of the fact that they tried to consolidate the
bridge when they noticed oscillations by tying the trusses to 50 tons blocs
located on the riversides and making holes in the steel trusses to make the wind
resistance smaller.
Consequences
There are several consequences to this accident.
 Most importantly there were no human casualties since the phenomenon
began one hour before the collapse, so that the area was able to be
evacuated.
 On the economic and material side however, the bridge had to be rebuilt
parallel and slightly south of the original bridge. During the 2 nd world war
there was a shortage of both steel and wire so the decision was made to
try and salvage as much of the material that collapsed from the first bridge
but ironically this ended up costing the state $350,000 more.
 On the knowledge side, the failure impeded the ardour of the engineers
who wanted to build thinner and thinner, more and more slender
structures. One of the most important consequences was to focus
engineers on aerodynamic phenomenon. Since this event, computer
simulations and aerodynamic tests are carried out in wind tunnels before
the erection of suspension bridges throughout the world.
 Engineers understood their mistakes since when the new bridge was built
at the same place; the deck was changed to a deeper deck as originally
advised (see illustration below). Therefore, rigidity of the new bridge is
greater than before. Furthermore, the new bridge has a truss deck which,
explained before has less resistance to wind.
Before
After