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Vibration of Suspended Steel Fin
4

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?

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

1) Yeah, it would make me nervous. Not saying it can't be done, just agreeing with you that it warrants further investigation.

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...wink

RE: Vibration of Suspended Steel Fin

I think the question is one of vortex shedding. I've seen plenty of formulas for the frequency for round/tubular structures, but I'm not sure I've seen any for such a plate (I'd check Roark's). A STAAD model can give you the various (natural) frequencies for the fin.

RE: Vibration of Suspended Steel Fin

(OP)
Oh my... it really is the Buddha again! Thanks for the feedback Archie.

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

Same Buddha? You do seem to get interesting projects.

And I guess I can't spell..."Budda" should be "Buddha" and "model" should be "module."

RE: Vibration of Suspended Steel Fin

(OP)
Culture-tard. My Buddha is the standing model.

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

Ah? So there are many large Buddha's being built. There's an opportunity there for someone ought to specialize, perhaps.

And thanks for giving me a star for essentially admitting my ignorance. I'll gladly take it, though, since that's my specialty.jester

RE: Vibration of Suspended Steel Fin

Actually (running some rough numbers) I wonder about this thing's stiffness (just running a static wind load hitting the weak axis). I get a tremendous amount of deflection when I do that (but then again: I may be misinterpreting it).

RE: Vibration of Suspended Steel Fin

(OP)

Quote (Archie264)

And thanks for giving me a star for essentially admitting my ignorance.

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.

Quote (WARose)

I get a tremendous amount of deflection when I do that (but then again: I may be misinterpreting it).

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

Quote:

Care to share the number and the wind pressure used?

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

I did a quick back of the napkin type calc and have over half a meter of deflection with 5 psf of wind pressure. Just using the standard 5/384 * w * l^4 / E / I. Likely need to consider membrane stress to get an accurate model of deflection, or I made some mistake somewhere. But my gut instinct says building a big steel sail like this is not a good idea.

RE: Vibration of Suspended Steel Fin

(OP)
50"? 0.5m? Is that bad?? Just foolin'. We're getting similar numbers. We'll have to get that down to L/250 or so and then revisit the vibration issues. The question still stands though so, if anybody has additional vibration information to share, I'd love to hear about it. Apparently, there used to be a perpendicular plate turning the section into a oddly proportioned WT shape.

@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

(OP)
@Canwest: with the bottom of the fin floating vertically, I don't think that membrane stresses would come into play. Your napkin calcs stand.

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 know you are obligated to post a sketch of this thing so we can see just how ridiculous it's supposed to look :)

I'm just curious why you can provide lateral but not vertical support at the base?

RE: Vibration of Suspended Steel Fin

(OP)

Quote (Jayrod)

You know you are obligated to post a sketch of this thing so we can see just how ridiculous it's supposed to look :)

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.

Quote (Jayrod)

I'm just curious why you can provide lateral but not vertical support at the base?

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

I'm thinking more of providing a vertical resistance to enact some form of membrane action. Keep the weight hanging, but stop it from pulling out of the base.

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

(OP)
Ah.. I see. My gut tells me that, if we're relying on membrane action here, a serious vibration check is surely in order.

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

Presumably this is some sort of sunshade or merely an architectural detail. Will they allow you to roll the shape to improve the stiffness? Most of the shades we work with from Dexone are built like an air foil. No chance of tensioning this to a grade slab somehow? If not, this seems like a long term problem that is magnitudes worse than typical lamp standard movement in a big windstorm.

RE: Vibration of Suspended Steel Fin

Without a doubt. I don't believe you would get around a serious vibration check regardless of the method of resistance.

I'm also assuming adding some nominal flanges along each 12m side is out of the question?

RE: Vibration of Suspended Steel Fin

without an restraint in the axial direction there is no way it works.....with a restraint to develop axial tension as jayrod mentioned there is some hope.....but, the problem is so complex and has so many variables that to me it would be a real guessing game....in situations like this I would try and bound the problem.....for steady-state, one could use wind perpindicular with a drag coeff of approx 2.0 to get an overall strength capacity....the difficult part is the votex shedding which could lead to vibration, torsion , fatigue etc......at first glance, one would think 19mm thickeness could withstand quite alot of vibration etc. but this is not based on any scientific data and is quite a bold ass-umption...

RE: Vibration of Suspended Steel Fin

Make the plate thicker?

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

(OP)
It is an architectural sunshade. I'll pass the various excellent solutions on to our project manager. Construction is already under way and I don't have a good sense for what would be acceptable to the design team at this stage. Our project manager is leaning towards doubling the plate thickness as Trenno has suggested.

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

Koot, I am not sure how I would analyze the pre-stressed option. Thinking about it a little more one problem might be achieving sufficient pre-stress in the plate for it to be effective. If you were to install two cables from the base to the roof level you could likely analyze the cable deformation assuming a uniform load applied to each. A non-linear analysis will reveal the base reactions and then it is simple. Is the roof element sufficiently stiff or would you expect a deflection? None of this would work if the roof element deflects or the base element is likely to heave or settle. This is likely more complicated than what you and the contractor can come up with. In any event, it is just a random idea that popped into my head.

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

A solid plate seems a very strange solution for a long sunshade, but then I realize that a lot of strange solutions come from architects.

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

I would say that your going to be more concerned about low speed wind flutter than high speed vibration, to me it is like Old Tacoma
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

Lol..

Quote (hokie)

...But then, I don't know how to check whether it would whistle or moan.

RE: Vibration of Suspended Steel Fin

If you have access to ESDU, the report no. 90036, 'Structures of non-circular cross section: dynamic response due to vortex shedding' might be of interest.

RE: Vibration of Suspended Steel Fin

For similar structures in water I've had some of our mechanical engineers run CFD models to determine the vortex shedding. We then adjust the geometry to reduce vortex shedding. You should be able to run a similar, simple CFD analysis (if there is such a thing).

RE: Vibration of Suspended Steel Fin

And.... we're back.

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

I would put some bridging in, say at third points, just to make them all act together. Other than that, I wouldn't worry too much about flutter.

RE: Vibration of Suspended Steel Fin

An architect did some decorative fins on a structure here in San Diego and when he got wind vibrations, he put spotlights on them and called it a shimmering light feature.

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