Drew_fx is now Drew_TNBD

Winny_Pooh

stickyfiddle said:

Tits? Nope; Bums, dammit!

To Lol or Wiz, the dilemma

BigMonka

Teetonetal said:

fretmeister said:

It's named after this event

I had no idea bridges could be so bendy.

It ended like this so perhaps they aren't all that bendy!
https://upload.wikimedia.org/wikipedia/commons/4/4a/Tacoma-narrows-bridge-collapse.jpg

RocknRollDave

A troubled bridge. Over water.

ICBM

Trying Not to Be (worried by imminently becoming) a Dad.

PolarityMan

Is this because he has been acquired?

TimmyO

Drew_TNBD said:

ESchap said:

fretmeister said:

It's named after this event

Ah, that resonates ....

Actually, aeroelasticity.

aeroelasticity is just the name for the field of study

Drew_TNBD

TimmyO said:

Drew_TNBD said:

ESchap said:

fretmeister said:

It's named after this event

Ah, that resonates ....

Actually, aeroelasticity.

aeroelasticity is just the name for the field of study

Yes. The phenomenon is aeroelastic flutter.

Drew_TNBD

PolarityMan said:

Is this because he has been acquired?

Yes.

TimmyO

Drew_TNBD said:

TimmyO said:

Drew_TNBD said:

ESchap said:

fretmeister said:

It's named after this event

Ah, that resonates ....

Actually, aeroelasticity.

aeroelasticity is just the name for the field of study

Yes. The phenomenon is aeroelastic flutter.

from memory flutter is usually a reference to something constrained at one end and free at the other. 

This *is* a resonant phenomenon, but it's right to say it's caused as a result of various aeroelastic contributions - if it didn't change shape as it deflected, the resonance wouldn't start - the twisting causes a movement in the lifting profile of the bridge that moves down its length - more complex than a simple static resonance, but still a resonance effect.

bingefeller

TimmyO said:

Drew_TNBD said:

TimmyO said:

Drew_TNBD said:

ESchap said:

fretmeister said:

It's named after this event

Ah, that resonates ....

Actually, aeroelasticity.

aeroelasticity is just the name for the field of study

Yes. The phenomenon is aeroelastic flutter.

from memory flutter is usually a reference to something constrained at one end and free at the other. 

This *is* a resonant phenomenon, but it's right to say it's caused as a result of various aeroelastic contributions - if it didn't change shape as it deflected, the resonance wouldn't start - the twisting causes a movement in the lifting profile of the bridge that moves down its length - more complex than a simple static resonance, but still a resonance effect.

You sound like a professor!

TimmyO

bingefeller said:

TimmyO said:

Drew_TNBD said:

TimmyO said:

Drew_TNBD said:

ESchap said:

fretmeister said:

It's named after this event

Ah, that resonates ....

Actually, aeroelasticity.

aeroelasticity is just the name for the field of study

Yes. The phenomenon is aeroelastic flutter.

from memory flutter is usually a reference to something constrained at one end and free at the other. 

This *is* a resonant phenomenon, but it's right to say it's caused as a result of various aeroelastic contributions - if it didn't change shape as it deflected, the resonance wouldn't start - the twisting causes a movement in the lifting profile of the bridge that moves down its length - more complex than a simple static resonance, but still a resonance effect.

You sound like a professor!

nah - I didn't take my studies further than a bachelors in Aero Engineering 

Drew_TNBD

Its explicitly not a resonance phenomenon. I'll post the explanation later.

TimmyO

If you're going down the route of

"In many undergraduate physics texts the event is presented as an example of elementary forced resonance with the wind providing an external periodic frequency that matched the natural structural frequency, even though the real cause of the bridge's failure was aeroelastic flutter"  (from wikipedia) then that statement is (not deliberately) misleading in itself.

It's right to say that it's not a simple 'shake it at the right frequency and it resonates and falls apart' failure. There was a cascade effect of various failures that eventually led to the violent twisting. There were a couple (at least, iirc) of different failure modes that were specifically underdamped/resonant. There was a site that captured it all somewhere. I will see if I can dig it up.

TimmyO

This isn't the site I was looking for, but it's the same info it looks like:

https://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm

TimmyO

So stage 1:
"Meanwhile, Professor F. B. Farquharson continued wind tunnel tests. He concluded that the "cumulative effected of undampened rhythmic forces" had produced "intense resonant oscillation." In other words, the bridge's lightness, combined with an accumulation of wind pressure on the 8-foot solid plate girder and deck, caused the bridge to fail."

Then once the vertical deflection gets too much, with unequal restraint from the suspension elements, it starts to twist, this triggers the vortex shedding, and as a result of that you get the rolling flex travelling back and forth:

"(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."

Again, a resonant effect. but a 3d not a 2d one. 3D resonance is a bitch - it's still not really studied by most engineers - but those who go in to aero engines <cough> know a bit about it as there are failure modes in fast rotating, heavy machines (e.g. gas turbines) given natty names like 3D3EO (3 dimensional, 3rd engine order*) that get studies because they are night on impossible to model 

* xEO just means a frequency x times the rotating speed - so a vibrational mode that is a harmonic of the input forces and which is not static - its effect moves around itself - making the failure hard to predict or even inspect for the early stages of. It was the reason that 3 Harriers fell out of the sky in 1999 - I worked with Rolls Royce to help find out why and to fix it. 

Drew_TNBD

This is what I was referring to:

Drew_TNBD

(I'm not an expert, I just like the syllables)