Vibration of Suspended Steel Fin
Vibration of Suspended Steel Fin
(OP)
I'm reviewing a situation that makes me a bit nervous. We've designed an exterior architectural fin that I'm worried might flutter terrifyingly in the wind. The details are as follows:
1) Fin plate is 19 mm x 500 mm corten steel.
2) The height of the fin plate is 12m (kL/r >> 300), for what that's worth.
3) The fin plate is suspended from the roof where it is fixed against translation and rotation about the plate's longitudinal axis.
4) At the base, the fin plate floats vertically but is fixed against horizontal translations and rotation about the plate's longitudinal axis.
5) The fin plate supports no load other than self weight and wind/seismic forces.
My questions are:
1) How nervous does this make you?
2) How might one come up with an appropriate, wind induced forcing function for use in a vibration analysis?
3) What would you recommend with regard to acceptability criteria in this situation?
3) Got any references that might be helpful for a vibration analysis on something like this?
1) Fin plate is 19 mm x 500 mm corten steel.
2) The height of the fin plate is 12m (kL/r >> 300), for what that's worth.
3) The fin plate is suspended from the roof where it is fixed against translation and rotation about the plate's longitudinal axis.
4) At the base, the fin plate floats vertically but is fixed against horizontal translations and rotation about the plate's longitudinal axis.
5) The fin plate supports no load other than self weight and wind/seismic forces.
My questions are:
1) How nervous does this make you?
2) How might one come up with an appropriate, wind induced forcing function for use in a vibration analysis?
3) What would you recommend with regard to acceptability criteria in this situation?
3) Got any references that might be helpful for a vibration analysis on something like this?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.






RE: Vibration of Suspended Steel Fin
2) Well, the wind code is so complex that I have trouble applying it properly it to the likes of a tool shed or dog house much less what you're describing. I think a hearty "good luck and best wishes" might be the best I can offer on that one.
3) I would want to look at overall deflection and some type of FEM vibration analysis. Timoshenko might also prove to be a resource though his "Plates and Shell's" book, or a similar resource.
4)I would think that most commercial structural analysis packages might offer vibration models at this point. Perhaps Josh Plum can weigh in? In the interim I'll provide a link to one of his competitors, which seems to have such a model. I'd be careful though, as they're showing some type of giant Budda structure on that page and I'd be hesitant to have any dealings with anyone involved in any type of giant Budda project...
RE: Vibration of Suspended Steel Fin
http://www.bentley.com/en-US/Products/STAAD.Pro/Ad...
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
And I guess I can't spell..."Budda" should be "Buddha" and "model" should be "module."
RE: Vibration of Suspended Steel Fin
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
And thanks for giving me a star for essentially admitting my ignorance. I'll gladly take it, though, since that's my specialty.
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
You don't know that was me. Just kidding. I love, love, love it when other engineers tell me what they actually do/know as opposed to what a hypothetical, all knowing engineer with infinite fee would do/know.
Care to share the number and the wind pressure used? I've got one of our designers looking at the same thing. This will be a good check. The section is 19 x 500 x 12000 simple span. Wind on the big flat face of 'er.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
Even using 20 psf (hitting 500 mm side) and a simple span (will it really have support at the tip?) I'm getting more than 50" deflection. (I may be misinterpreting something here.)
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
@WARose: There will be a bit of fixity at the top but nothing to rely on. Basically, weak axis rotation in the fin plate would be resisted a bit by torsion in a pair of supporting channels.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
I'm just curious why you can provide lateral but not vertical support at the base?
RE: Vibration of Suspended Steel Fin
I do know. Unfortunately, the electronic stuff is in a different city and I'm too lazy to get all "arts and crafts" on the hard copy residing at my desk. Additionally, I'm not sure if it would be cool for me to shame a colleague over social media.
I absolutely could provide vertical support at the base. However, the silly thing would make an even worse column than it does a sail. I doubt it could even support its own dead load.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
But that's just a thought and I might be one of the last people to ask about a plate bending problem like this.
RE: Vibration of Suspended Steel Fin
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
I'm also assuming adding some nominal flanges along each 12m side is out of the question?
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
Implement carefully positioned cut-outs to reduce vortex shedding?
Perforate the fin?
Of course, I'm talking out my rear end here...
RE: Vibration of Suspended Steel Fin
I'm curious: if one were to tension the fins, as Brad suggested, what is the physical mechanism by which an improvement is accrued? Does it just come down to extra lateral stiffness in the membrane / large deflection realm? I get it for cables or something with an anticlastic shape but for something like this, I'm not clear on how it works.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Vibration of Suspended Steel Fin
You have to love architect ideas. They can be great fun, but they never seem to appreciate the complexity of the problem they create until the first time this thing whacks a window and breaks a large piece of special order glass. Then I know who gets a call. A few pretty stainless steel standoffs would solve all of this and it would not surprise me if you couldn't use a thinner plate. I bet the reason to avoid the standoffs is to avoid the detailing for an envelope penetration. I argue with architects when they want to have their cake and eat it too.
RE: Vibration of Suspended Steel Fin
Vibration, deflection, I would add noise to the considerations. But then, I don't know how to check whether it would whistle or moan.
RE: Vibration of Suspended Steel Fin
Narrows Bridge problem. I think that some tests after this event suggested that flutter could be determined some how for different bridge shapes, I would suggest that they did a flat plate in these tests at some stage. there is numerical solutions for the bridges so you maybe able to use the flat plate solution as a check, but I am guessing.
http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin
Similar sort of situation, Arch. wants to single span some purlins, without cladding, over a plant platform to somewhat conceal the mechanical equipment inside.
Structural details: 8.6m single span @ 500 c/c, relatively low wind area Qu = 1.5 kPa, desired C purlin to be used with lower lip on an angle to stop it holding water.
I'm mostly worried about the low speed wind 'flutter' (in particular a cross wind response/vibration) as mentioned above, but have no clue how I would check this.
Any ideas?
EDIT: Running a quick number on the natural frequency of a simply supported beam with a service wind load acting in the vertical direction, the frequency comes out at around 3 Hz.
RE: Vibration of Suspended Steel Fin
RE: Vibration of Suspended Steel Fin