In your opinion, what caused the tacoma narrows collapse?

Question

Heres my rough opinion only based on limited research. As the cable tie snapped, a small amount of torsional movement was created due to vortex shedding and wind eddies. The shedding frequency did not match that of the bridge so it was not the FINAL cause of collapse. However, it did allow some torsional movement. The cross-section of the bridge acted as an airfoil. As it tilted up on the windward side, the velocity of the wind was greater on the top (since a greater distance had to be covered in the same time) than the bottom. This created a greater pressure (P=F/A) on the bottom edge, thus creating a force perpendicular to the cross-section of the bridge. THis motion was repeated until the bridge finally collapsed. As the width to length ratio of the bridge was very small causing the bridge to be too flexable.

Someone said that it was resonance. "Resonance requires the frequency of the driving force to be close to a natural frequency of the structure. This was clearly not the case in the motion of the bridge at failure.... It was NOT resonance" (Engineering Disasters -Lessons to be Learned by Don Lawson) The bridge was well-designed for static loads (translational) but not static wind loads which caused torstional movement.

Yes, I understand that the bridge design did not consider aerodynamic forces. The collapse of the bridge was pretty much the point where engineers started making models of bridges and testing them in windtunnels.

But HOW did the torsional osciallation begin?

Some say vortex shedding and wind eddies did not match the required frequency. Others say that vortex shedding did cause the initial torsional movement, but do not comment on its required frequency. Of course there are the details that the depth of the supports were alot less in Moisseiffs design than in Eldridge's design. Also the width to length ratio was quite small and the side of the bridge acted as a sail.

Answer 1

I live in Silverdale, 35 miles north of the bridge. The bridge collapsed for exactly one reason, flexing beyond the limits of the structure, in a very similar way to a paper clip unfolded and flexed. The reason was the air currents which caused an oscillation in the roadway, which flexed the roadway to the point of failure, exactly like a paper clip bent one too many times. There is no mystery or conspiracy here, just a simple mechanical failure because of flexing beyond the structural limits. Today, suspension bridges are gauged for reactions to wind, unlike the Tacoma bridge which failed. Before the Tacoma Narrows bridge failed, NO ONE considered aerodynamic forces on the road deck. Now, everyone takes aerodynamic forces into account, so the failure of the Narrows bridge, resulted in a GOOD thing. Fortunately, no blood was shed in learning this lesson, unlike may other engineering disasters in which people died to teach an important engineering lesson... One last thing, the only loss of life MIGHT have been a dog which belonged to a reporter whose car ended up in the middle of the bridge with the dog left in his car. The car of the reporter is the one in the middle of the bridge roadway you see in the film clips in which the roadway is oscillating wildly just before it failed. No one knows if the dog survived the collapse into the Narrows but it is likely the dog died when the roadway structure failed and fell into the Narrows.

Edit: The torsional motion of the roadway was caused by aerodynamic forces similar to what makes a flag wave in a steady breeze. The forces on each side are slightly unequal, which cause the oscillation, which is a positive feedback effect, which makes a small movement amplified by the unequal forces larger the more the flag waves, or in the case of the roadway, the bridge deck. Look at a flag waving for a very clear example of positive feedback amplifying the motion. Even if the breeze is very slight and very steady and constant, the flag flaps to the maximum it can. In essence, the roadway, the bridge deck, "flapped in the breeze" to the point of mechanical failure of the structure when stressed beyond design limits. The was later proven to be fact, not speculation or fiction when the original design was tested in a wind tunnel at Harvard or Yale or some other well known university with an engineering section. I forget the name of the testing university, but it was a well known and respected institution. The current bridge (and the new span recently completed) has trusses and things suspended below the roadway deck which reduce the aerodynamic forces so it will not "flap in the breeze" which is why the current bridge (and the new one) will not flex as the failed bridge did.

Answer 2

You said what caused the collapse. You are just dwelling on the first structural failure of the bridge.

The collapse of the Tacoma Narrows bridge was due to the design not taking into account of the force of the wind across the bridge. The wind made the bridge go into a standing wave with increasing oscillations until it broke apart.

The cause was bad design.

Answer 3

It sounds to me like it might have been work-hardening of the cables. As metal is repeatedly stretched and flexed it "work-hardens" and becomes brittle. The Tacoma Narrows bridge (or "Galloping Gertie" as it was known to the locals) was an example of resonant frequency. The bridge's natural frequency just happened to neatly coincide with the speed of the wind coming through the narrows and it made for some great film footage.

Answer 4

Hold up... why did the cable snap in the first place? That already should not have happened.