Tacoma Bridge collapsing

This video shows how Tacoma Bridge Caollapsed...



Taken from: http://www.youtube.com/watch?v=hBxQCvVykRE

Why did Tacoma Narrows Bridge collapse?

The collapse of the 1940 Tacoma Narrows Bridge stunned everyone, especially engineers. How could the most "modern" suspension bridge, with the most advanced design, suffer catastrophic failure in a relatively light wind?

For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge's failure remains a mystery.

In general, the 1940 Narrows Bridge had relatively little resistance to torsional (twisting) forces. That was because it had such a large depth-to-width ratio, 1 to 72. Gertie's long, narrow, and shallow stiffening girder made the structure extremely flexible.

On the morning of November 7, 1940 shortly after 10 a.m., a critical event occurred. The cable band at mid-span on the north cable slipped. This allowed the cable to separate into two unequal segments. That contributed to the change from vertical (up-and-down) to torsional (twisting) movement of the bridge deck.

Also contributing to the torsional motion of the bridge deck was "vortex shedding." In brief, vortex shedding occurred in the Narrows Bridge as follows:
(1) Wind separated as it struck the side of Galloping Gertie's deck, the 8-foot solid plate girder. A small amount twisting occurred in the bridge deck, because even steel is elastic and changes form under high stress.
(2) The twisting bridge deck caused the wind flow separation to increase. This formed a vortex, or swirling wind force, which further lifted and twisted the deck.
(3) The deck structure resisted this lifting and twisting. It had a natural tendency to return to its previous position. As it returned, its speed and direction matched the lifting force. In other words, it moved " in phase" with the vortex. Then, the wind reinforced that motion. This produced a "lock-on" event.

But, the external force of the wind alone was not sufficient to cause the severe twisting that led the Narrows Bridge to fail and collapse.

Taken from: http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm#6

Tacoma Bridge collapsing

This video shows how Tacoma Bridge Caollapsed...



Taken from: http://www.youtube.com/watch?v=hBxQCvVykRE

Why did Tacoma Narrows Bridge collapse?

The collapse of the 1940 Tacoma Narrows Bridge stunned everyone, especially engineers. How could the most "modern" suspension bridge, with the most advanced design, suffer catastrophic failure in a relatively light wind?

For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge's failure remains a mystery.

In general, the 1940 Narrows Bridge had relatively little resistance to torsional (twisting) forces. That was because it had such a large depth-to-width ratio, 1 to 72. Gertie's long, narrow, and shallow stiffening girder made the structure extremely flexible.

On the morning of November 7, 1940 shortly after 10 a.m., a critical event occurred. The cable band at mid-span on the north cable slipped. This allowed the cable to separate into two unequal segments. That contributed to the change from vertical (up-and-down) to torsional (twisting) movement of the bridge deck.

Also contributing to the torsional motion of the bridge deck was "vortex shedding." In brief, vortex shedding occurred in the Narrows Bridge as follows:
(1) Wind separated as it struck the side of Galloping Gertie's deck, the 8-foot solid plate girder. A small amount twisting occurred in the bridge deck, because even steel is elastic and changes form under high stress.
(2) The twisting bridge deck caused the wind flow separation to increase. This formed a vortex, or swirling wind force, which further lifted and twisted the deck.
(3) The deck structure resisted this lifting and twisting. It had a natural tendency to return to its previous position. As it returned, its speed and direction matched the lifting force. In other words, it moved " in phase" with the vortex. Then, the wind reinforced that motion. This produced a "lock-on" event.

But, the external force of the wind alone was not sufficient to cause the severe twisting that led the Narrows Bridge to fail and collapse.

Taken from: http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm#6