steel frame oscillating

[TLycan]

Dear All,
I am facing a structural problem right now. I’ve designed a very simple steel portal frame 8meters height and 5meters span which is strength/deflection very safe by the way. When someone rides the roof near to it and pushes it with his feat it vibrates/ sway/ oscillate, whatever you want to call it. I check it according to the AISC Vibration Design guide, It acceded tenfold the allowable. The question is I want to go the other way around, that is using Fundamental period, mass, I want to get the DEFLECTION or SWAY due to vibration. Does anyone have an idea or someplace to search?

 

[JedClampett]

Lycan, I'm not sure what you mean by this: "When someone rides the roof near to it and pushes it with his feat it vibrates/ sway/ oscillate...." Does that mean when someone is walking on the roof and pushes it with his feat (feet?), which way is he pushing? And why is he pushing?
Portal frames 24 ft. high are not easy to design. The details need to carefully follow your design assumptions. I've had to increase column sizes beyond what I expected for 12 ft. high shade structures. And then the contractor fought me on my fixed connections. My suspicions are that your columns are too small and/or the connections are not rigid.

 

[TLycan]

Jed Clampett

attached is a video showing the frame, sections and the movement.

the frame only support wood cladding and resist wind loads

you can say that the sections are welded RHS100x250x5mm for the small frame
regards

 

[TLycan]

Sorry in this video he is using his hands not legs

 

[Ingenuity]

What applied loads was the frame designed for, and what is the frames intended use?

You indeed have a deflection/sway problem. The member sizes are exceptionally slender.

Was the exterior frame properly engineered such that you can brace this interior frame to the exterior frame, what appears to be of larger section sizes?

Or install 'knee' or 'diagonal' bracing, but I would assume this will change the functionality of the frames use.

 

[Ron]

I agree with JC....your columns are too spindly and your connections are not rigid. It looks like the larger problem is the column size.

 

[TLycan]

Ingenuity, Ron
The frame will carry only a wood cladding and say someone of 100kg on the top. The stresses In the frame are very low. And the Frame when I applied 50kg laterally, representing the pushing force, a deflection of 1cm occurred. I even modeled the frame as hinge supported,actually The base is semi-rigid.
Unfortunately I cannot Add any knees as that will obstacle the cladding. But I want to add something. I tried adding,just as a trial, a box of 50*50mm at a distance. Of 2m from bottom Between the external and internal frame to see if the deflection will stop it did not

 

[CELinOttawa]

This is an SLS failure. You need to have this frame deflect less than 10mm under a 1kN load to take glass, say 50mm otherwise, to be confirmed with the requirements of the cladding... Never mind the stresses, they were never going to be the problem.

 

[Ron]

CEL is correct. This is a serviceability issue, not a strength issue.

 

[TLycan]

Dear all

please note the following:

I've done:
1- a FEM and added 100Kg at the top, representing the the pushing force which I think 100kg is very exaggerating,
2- the frame is modeled hinged at support, while the frame base actually is partially fixed ;
all this gives me a deflection of 1.6cm on the top (from the analysis I mean).

What is making me nuts where the hell is this deflection, the actual one, coming from; and how is the FEA deflections is not concurrent to the reality.

so I had to go for that the cycling loading, shown in the video first posted, is making the vibration resonance; especially the check by the AISC guide for vibration exceeded the allowable 9 or 10fold

dear all

first, FEA is not giving me this deflection secondly, I don't think that Even if the SLS is under design no man can sway the frame that way except if .... I don't know!!??!!

 

[CELinOttawa]

FEM is not going to help you here... YOU ARE NOT LOOKING FOR A STRESS.

Your frame is too soft. The force I am talking about is a HORIZONTAL force applied without factors to check how much the frame is likely to deflect to the side. It is a relative measure of stiffness to prevent EXACTLY the type of problem you've found yourself with.

I do not mean to be cruel, but you need someone's help and that's not the internet.

 

[XR250]

What if you just leave it be? Seems flimsy, but if the strength numbers work out and there is no chance of it swaying enough to hit the outer frame, what is the issue other than the perception that it is weak?
I have seen numerous inverted pendulum style carports that were easy to shake to resonance but have held up fine for decades.

 

[IRstuff]

While there may not be any problems immediately, one would think that such motion can result in degradation of fastener joints and possibly material fatigue, if the structure is constantly swaying. Occasional swaying is probably tolerable. Just consider that constant or chronic vibration in structure can result in bolts undoing themselves.

TTFN
faq731-376

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[civeng80]

I designed a portal frame about 30 years ago and it was a bit skimpy but still within codes limits. The steel erectors told me that it was a bit skimpy and showed me by swaying at the knee but they told me they had seen worse.
I put it down to a form of resonance. The human body can get a fairly good idea of the forces necessary for resonance to occur.
Incidently the structure is still there today and with all the cladding and roofing stiffening it, I think it will be there for a long time to come.

 

[TLycan]

Thank you all for your response. My greatest mystery here was the non conformance of the deflection from Sap2000 with the actual deflection. I like to note that the deflection from analysis is according to codes.

I have done analysis using time history nonlinear analysis(to represent the cycling loading of push and pull); the deflection pop-Up from 8mm to 30mm. The problem I think is either poor construction in base plate,as a space exists between the plate and footing but usually this not a problem as the plate rests on the nuts Belo the plate , or dynamics; But a normal sls I doubt it

 

[CELinOttawa]

Well there's an important point... Have them drypack the space under the base plate. I think you'll still have a slenderness issue, but one small fraction of a percent rotation under the base plate translates to inches twenty feet away!

 

[JoshPlumSE]

Dynamic deflection is approximately equal to Static deflection * {1/(1-r^2)}, where r is the ratio of the frequency of the applied load to the natural frequency of the structure.

If your static analysis says that less 1 cm then you can an amplification of 10 or 20 times if the load is applied at approximately the natural frequency of the structure (which is what the guy was doing in that video).

The question then becomes "will the structure actually experience loading at this frequency consistently during its lifetime". If there is a crane or equipment or such that will excite that frequency then you have a bit of a problem. If not, then you might be okay as is.

 

[TLycan]

JoshPlum

Excellent I've been searching for such an equation or paper to calculate this dynamic effect.

Can you suggest any source so I can read about the relation between loading/dynamicloading/frequency/vibration

Thanks in advance

and thank you all for your help and suggestions

 

[kingnero]

cannot you add gussets in the top four corners?

 

[TLycan]

I can't ;it will obstacle the wood cladding