On the morning of the 7th of November, 1940, travelers on the Tacoma Narrows bridge in the
state of Washington noticed something strange: the roadway suspended between the bridge's pillars
was oscillating and twisting, gently at first and then more and more dramatically.
Soon enough almost the entire central span of the bridge was in motion, rippling wildly. At around
11am whatever force was causing this movement proved too powerful, and the bridge collapsed
into the water below. What exactly had happened was, at that stage, a mystery to almost everyone.
Construction of the Tacoma Narrows bridge had been completed just a few months before its collapse,
in July of 1940. At the time it was the third-longest suspension bridge in existence,
beaten only by the Golden Gate bridge in California and the George Washington bridge in
New Jersey. Compared to these bridges the Tacoma Narrows bridge was to be exceptionally narrow
and lightweight. The original design proposed had featured very deep trusses underneath the
roadway - a feature that would provide rigidity and weight. but which would also drive up costs.
Ultimately this proposal was set aside in favor of a cheaper one.
Instead of deep trusses, shallow plate girders would be used, saving millions of dollars.
Cheap as it might be, this design choice quickly proved problematic.
Even during construction the bridge displayed a great deal of vertical movement during high winds.
This excess motion earned the bridge a nickname: "Galloping Gertie".
While alarming, it was not considered to be dangerous enough to halt construction
or to forestall the opening of the bridge, which took place on the 1st of July, 1940. A number of
dampening measures were experimented with in order to reduce the movement of the roadway.
For example, hydraulic buffers were installed between the roadway and the towers,
and the bridge was also anchored by long cables to massive weights on the shore.
Both of these measures came to nothing however the seals on the hydraulic
buffers were damaged when the bridge was painted, rendering them ineffective,
and the cables leading to the anchor weights simply snapped not long after installation.
The Washington Toll Bridge Authority was actively looking for other solutions to Galloping Gertie's
excess movement, but as it would transpire we're not doing so with sufficient urgency.
The first of several studies which they commissioned into the bridge wound up just
five days before the collapse - not soon enough for any of its proposals to be implemented.
On the day of the disaster there was a fairly normal level of traffic across the bridge.
This swiftly cleared as the eccentric movement began, and the bridge was closed to drivers...
leaving just one car stranded on the roadway. This was the vehicle of Leonard Coatsworth,
the editor of a local newspaper, who gave this account of the disaster:
"I drove on the bridge and started across. In the car with me was my daughter's cocker spaniel,
Tubby. Just as I drove past the towers the bridge began to sway violently from side to side.
Before I realized it the tilt became so violent that I lost control of the car.
I jammed on the brakes and got out... only to be thrown onto my face against the curb.
Around me I could hear concrete cracking. I started back to the car to get the dog but
was thrown before I could reach it. The car itself began to slide from side to side on the roadway.
I decided the bridge was breaking up and my only hope was to get back to shore on hands and knees
most of the time I crawled 500 yards or more to the towers. My breath was coming in gasps,
my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb.
Toward the last I risked rising to my feet and running a few yards at a time.
Safely back at the toll plaza I saw the bridge in its final collapse,
and saw my car plunge into the Narrows. With real tragedy disaster and blasted dreams all around me
I believe that right at this minute what appalls me most is that
within a few hours I must tell my daughter that her dog is dead, when I might have saved him."
Coatsworth wasn't the only one who tried to save Tubby the cocker spaniel.
During a brief lull when the bridge's movements were not so extreme two other men ran out and made
another attempt to save the dog, only for Tubby to bite one of them and refuse to leave the car.
Unfortunately then, Tubby was still on the bridge when it finally collapsed. His body was never
found. Investigation was launched to pinpoint the reasons behind the collapse of the bridge.
Numerous theories were proposed and numerous contributing factors ultimately found.
First and foremost the structure was far too flexible.
The shallow roadway with its plate girders rather than trusses
was prone to flexing. This was exacerbated by the slippage of a cable band midway along the bridge
which allowed the roadway to twist rather than just moving up and down..
On the day of the disaster the bridge was subject to a strong and sustained current of wind. When
this wind hit the bridge it separated and flowed around it, creating swirling air currents that,
after building for some time, eventually became powerful enough to move the deck of the bridge.
As this movement became more pronounced the bridge became subject to "aeroelastic flutter" - a
complex phenomenon whereby the elastic quality of the bridge deck combined with the effects
of the wind to make the motion of the bridge worse still, to the point that the movement was
self-generating. What had been started by the wind was now driven by the weight and momentum
of the bridge itself. The end result (total collapse) was, at this point, inevitable.
In the aftermath of the disaster some efforts were made to salvage what was left of the structure.
The eccentric movements had caused significant damage
and most of the metal components could not be saved. The concrete pedestals, however,
survived intact and were eventually incorporated into a new bridge across the Tacoma Narrows.
The wreckage of the roadway which fell into the water was left there,
and eventually colonised by aquatic life, becoming a huge and unintended artificial reef.
Bridges elsewhere in the country were reinforced and stiffened to prevent a similar incident
from taking place again, and going forward wind tunnel testing of all new bridge designs became
mandatory. The Tacoma Narrows bridge collapse remains, thankfully, a unique case study
demonstrating the importance of aerodynamics in bridge design to engineers around the world.
The replacement bridge, delayed by the advent of world war II, was eventually opened in 1950,
and operated without major incident for many decades. So popular did the route become,
in fact, that a second bridge was built alongside it to increase capacity.
This new bridge, similar in design to its neighbor, opened to traffic in 2007.
Thanks to lessons learned after the catastrophic collapse of its predecessor
it is likely that this new bridge will remain standing for many, many years to come.