Posted on 11/07/2019 6:02:09 PM PST by Rebelbase
[snip] On November 7, high winds buffeted the area and the bridge swayed considerably. The first failure came at about 11 a.m., when concrete dropped from the road surface. Just minutes later, a 600-foot section of the bridge broke free. By this time, the bridge was being tossed back and forth wildly.
At one time, the elevation of the sidewalk on one side of the bridge was 28 feet above that of the sidewalk on the other side. Even though the bridge towers were made of strong structural carbon steel, the bridge proved no match for the violent movement, and collapsed.
(Excerpt) Read more at history.com ...
Cool!
Yes. The sides were an important part of the affair, and they did act as “swing-pushers,” to borrow an idea from earlier in this thread.
When the railing-sails came up, the wind forced them hard down, whence they bounced up again, and this wave cycle contributed (critically) to the acceleration of the vibratory movement and the failure of the bridge.
I remember them, along with the old manual-load slide projectors that the teacher got the slides all out of sequence on. It was a real hoot if it came with an audio tape that had to sync with it. LOL
All solid objects or structures have a natural frequency which they vibrate at when excited. Think of a tuning fork, same concept.
Complex structures have many. Each component can have it’s own natural frequency.
Excitation can come in the the form of any energy that is introduced to the structure.
When that energy source is exciting the structure at the structures natural frequency, it creates a “resonant condition”, which causes the structure to vibrate exponentially relative to the amount of energy introduced.
Think of pushing a kid on a swing. Push at just the right time, and the kid swings higher and higher with much less energy input that if you tried to push them that high in one push.
The roadway of this bridge just happened to have an natural frequency that was excited by the winds going through the “narrows”.
In my line of work (Vibration Analysis) this is one of the more common faults we diagnose on industrial equipment.
I’ve been scuba diving at the bridge and have seen the road sections. Tricky dive. Wicked currents due to the tide rushing in and out of the narrow channel.
There are two bridges there now
Interesting. So you have actually dived in the river and seen the collapsed sections of road?
Hopefully they are not just swinging in the wind.
Poor doggie.
The shape of the channels of the road deck, facing outboard, "cupped" the airflow, creating a rolling vortex that served as a leading edge. This created lift across the deck, and the upwind edge would rise until the airfoil lift could not exceed the weight of the deck and the deck would stall. At low wind speeds, the deck would just rock up and down a couple of feet.
Then, one day, it was REALLY windy...
You get the same effect in a pickup truck bed. A rolling vortex, behind the cab, allows a smooth airflow past the tail gate, at highway speeds. The drag drops per the Kamm Effect. Leaving the tail gate down kills this, and you actually have MORE drag with the gate down, one of those nylon mesh aftermarket gates, or missing...
Note how dry leaves in an empty bed are bunched up forward, against the cab.
Yes. Back in the early 2000’s
It’s actually a popular dive site. Still is from what I see.
A cocker spaniel died during this bridge collapse.
It's also how an opera singer's voice can break a glass.
Interesting. Thanks.
I was just thinking that.
i just perused the Therac 25 wiki article ... totally fascinating since i was a software engineer from that era ...
Electrical resonance occurs in an electric circuit at a particular resonant frequency when the impedance of the circuit is at a minimum in a series circuit or at maximum in a parallel circuit (usually when the transfer function peaks in absolute value). Resonance in circuits are used for both transmitting and receiving wireless communications such as television, cell phones and radio.
Animation illustrating electrical resonance in a tuned circuit, consisting of a capacitor (C) and an inductor (L) connected together. Charge flows back and forth between the capacitor plates through the inductor. Energy oscillates back and forth between the capacitor's electric field (E) and the inductor's magnetic field (B).
I did not see him down there.
It is astonishing that bridge stayed together as long as it did. Just wow.
Harmonics, right?
37 years ago. Were it today there would be lawyer commercials on TV, “Have you or a loved on been treated for cancer on a Therac 25 unit? If so call the law offices of Dewey, Cheatham and Howe”.
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