"How could wind cause such a MACRO changes in the bridge's behavior?" Check the very deep plate girders on the edges of the deck. They used to provide very big resistance to the wind load. Therefore, after that incident the design of long and super long span bridges changed. Nowadays trusses are placed beneath the decks in order to let wind easily to pass under the deck. What uriga and allinsman wrote about the aeroelastic flutter sounds right to me so I don't have anything to comment upon that.
Resonance is a special case of aeroelastic flutter where the system actually has a negative damping mode allowing even minute forces to move bridges. Aeroelastic flutter can have positive or zero damping just as well, however these case are not nearly as dangerous as negative damping. Think of resonance as a small child on a swing. A small pump of the legs at the right timing (frequency) can eventually cause the child to swing back and forth greatly, even though only a small force is ever input.
Thanks. I follow your example of a child on a swing -- the opposite action (CG shift?) will reduce the amplitude of the swing.
It seems odd (to me) to term the amplification that destroyed the Tacoma Narrows bridge "negative damping mode". (Just because "positive feedback" is used in other disciplines to signify something that increases amplitude.) Do you know how this increasing displacement came to be termed "negative"?
Positive damping (i.e. friction) slows something down. So I guess we just call it negative since it does the opposite of slowing down. This is common in aerospace engineering at least. Idk about other fields.
What happened with the bridge wasn't positive feedback so much as resonance. Think of the positive feedback you get from holding a microphone to its speaker. The output and input get larger in time. In resonance only the output grows, the input remains constant but timed precisely.
Ah, I see it! I was focusing on the "negative" and not on the "damping". Duh!
I thought about the forces, though. As the leading edge deviates downward, the force becomes more perpendicular to the upper "road" surface, effectively increasing the force vector? Vice versa for upward deflection of the leading edge. It's like a sail that is not allowed to spill the wind. So, once the deviation begins, it will self-amplify. As you said, like the child on a swing. Fascinating!
This is what happens if something reach resonans when the wind makes the bridge move in it's "self-frequenze". this can also happen to buildings and would make a building collapse on itself too. However it's VERY unlikely to happen to buildings. When new bridges are made this resonans is takeninto account when they make them so it won't hapen.
Iirc, resonance occurs in many systems when a small force coincides with the natural frequencies of the object. I thought that resonance was tangential to forces.
I have read that aeroelastic flutter is not resonance, but I'm not an engineer. Aiui, flutter is a self-amplifying dynamic reaction to forces that exceed the structure's damping effects. This force is perpendicular.
Thank goodness that this problem is not built into skyscrapers - goodbye lunch!
"How could wind cause such a MACRO changes in the bridge's behavior?" Check the very deep plate girders on the edges of the deck. They used to provide very big resistance to the wind load. Therefore, after that incident the design of long and super long span bridges changed. Nowadays trusses are placed beneath the decks in order to let wind easily to pass under the deck. What uriga and allinsman wrote about the aeroelastic flutter sounds right to me so I don't have anything to comment upon that.
thessalonician 1 year ago
@thessalonician
You appear to have difficulty understanding the word "SATIRICAL."
If you actually watched the video, you'd see that the switch to open design is mentioned.
You could have saved yourself---and me---a lot of typing.
EvolvedAtheist 1 year ago
Resonance is a special case of aeroelastic flutter where the system actually has a negative damping mode allowing even minute forces to move bridges. Aeroelastic flutter can have positive or zero damping just as well, however these case are not nearly as dangerous as negative damping. Think of resonance as a small child on a swing. A small pump of the legs at the right timing (frequency) can eventually cause the child to swing back and forth greatly, even though only a small force is ever input.
uriga 2 years ago
Thanks. I follow your example of a child on a swing -- the opposite action (CG shift?) will reduce the amplitude of the swing.
It seems odd (to me) to term the amplification that destroyed the Tacoma Narrows bridge "negative damping mode". (Just because "positive feedback" is used in other disciplines to signify something that increases amplitude.) Do you know how this increasing displacement came to be termed "negative"?
EvolvedAtheist 2 years ago
Positive damping (i.e. friction) slows something down. So I guess we just call it negative since it does the opposite of slowing down. This is common in aerospace engineering at least. Idk about other fields.
What happened with the bridge wasn't positive feedback so much as resonance. Think of the positive feedback you get from holding a microphone to its speaker. The output and input get larger in time. In resonance only the output grows, the input remains constant but timed precisely.
uriga 2 years ago
Ah, I see it! I was focusing on the "negative" and not on the "damping". Duh!
I thought about the forces, though. As the leading edge deviates downward, the force becomes more perpendicular to the upper "road" surface, effectively increasing the force vector? Vice versa for upward deflection of the leading edge. It's like a sail that is not allowed to spill the wind. So, once the deviation begins, it will self-amplify. As you said, like the child on a swing. Fascinating!
EvolvedAtheist 2 years ago
LAWL
anthonzi 2 years ago
This is what happens if something reach resonans when the wind makes the bridge move in it's "self-frequenze". this can also happen to buildings and would make a building collapse on itself too. However it's VERY unlikely to happen to buildings. When new bridges are made this resonans is takeninto account when they make them so it won't hapen.
allinsman 2 years ago
It is a satire on ID "theory".
Iirc, resonance occurs in many systems when a small force coincides with the natural frequencies of the object. I thought that resonance was tangential to forces.
I have read that aeroelastic flutter is not resonance, but I'm not an engineer. Aiui, flutter is a self-amplifying dynamic reaction to forces that exceed the structure's damping effects. This force is perpendicular.
Thank goodness that this problem is not built into skyscrapers - goodbye lunch!
EvolvedAtheist 2 years ago
lol
Censeo 2 years ago