×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

steel frame oscillating

steel frame oscillating

steel frame oscillating

(OP)
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?

RE: steel frame oscillating

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.

RE: steel frame oscillating

(OP)
Sorry in this video he is using his hands not legs

RE: steel frame oscillating

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.

RE: steel frame oscillating

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.

RE: steel frame oscillating

(OP)
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

RE: steel frame oscillating

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.

RE: steel frame oscillating

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

RE: steel frame oscillating

(OP)
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!!??!!

RE: steel frame oscillating

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.

RE: steel frame oscillating

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.

RE: steel frame oscillating

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: Eng-Tips.com Forum Policies

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: steel frame oscillating

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.

RE: steel frame oscillating

(OP)
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

RE: steel frame oscillating

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!

RE: steel frame oscillating

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.

RE: steel frame oscillating

(OP)
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

RE: steel frame oscillating

cannot you add gussets in the top four corners?

RE: steel frame oscillating

(OP)
I can't ;it will obstacle the wood cladding

RE: steel frame oscillating

To limit deflection one needs at least two things:

1) a natural frequency higher than expected inputs to avoid storing energy in elastic deflection.
a) smaller mass = less energy stored
b) higher stiffness = less easy to store energy

2) damping to remove energy as fast as it is put in.

After more mass is added to it, the natural frequency will get lower and the potential deflection will be higher.
It doesn't look like it is well fixed to the ground - there is poor moment carrying capacity at the ground for some reason. Perhaps the legs need bigger feet.

If you can, get a copy of Design of Welded Structures by Blodget. It costs less than $50 and provides a great many example frame designs.

You can do a search on "damping of welded steel structures"

RE: steel frame oscillating

definite dry pack under that base plate. It will help some. I'm sure once the wood cladding is on there it should help as well (provided it is fastened at a reasonable spacing to provide some diaphragm action).

RE: steel frame oscillating

Specify that the wood cladding be glued & closely screwed. 75mm is generally considered as stiff as the real world will get... That is without specialist detailing, staggered fastening, etc.

RE: steel frame oscillating

I don't know of a short reference for vibrations. I pulled that equation from my undergraduate text on vibrations:

Rao's Mechanical Vibrations 2nd edition.

Most texts on vibrations should have something similar. Though not all texts on dynamics will have it. Vibrations text should have some more details regarding the effects of damping and such.


While this equation theoretically explains the reasons why you are seeing the greater deflection, it doesn't tell you what you should do about it at all. Internet is wonky for me right now, so I can't see that last video. But, certainly if you are getting a lot of movement at the base plate, then you should grout it to reduce movement and stiffen up the base. That will certainly stiffen up the structure some.

RE: steel frame oscillating

Does your FEA model include the threaded studs and the gap beneath the feet? Because of camera movement it is hard to see. I also didn't see any concrete and would expect a pad at least as large as the foot. If the studs are cast into concrete, can you see if the concrete is moving? What footers/foundation is there?

Put another way, there is an 8 meter pry bar attached to some bolts and maybe some concrete. What should happen?

You might add a tripod to the camera. It is very hard to see what is moving in the structure when everything in the picture is moving.

"the client will not wait till I read a 300 pages book"

If you get it and read it, the next client will not have to wait. Resolving your present emergency requires understanding why you have the problem and what is available to fix it.

RE: steel frame oscillating

Are you also considering P-Delta?

RE: steel frame oscillating

Even with grout, the base plate appears to be too thin to provide a good moment resistance.

Dik

RE: steel frame oscillating

Can you do a computer run with a 3'x3' dogleg HSS 10x4x0.25 welded into the haunch and see what that has on the vibration effect. Your max moments will be at the haunch and this will stiffen the system dramatically.

Dik

RE: steel frame oscillating

Just to put 'intentional' human vibration of a structure into perspective, which is what you are showing in your video...There is a rather large cable stay pedestrian bridge near where I live. It spans about 150 feet. Out of curiosity I shook one of the cables trying to find the natural frequency. I don't believe the structure to be unsafe in any way but I am convinced that I could bring the entire structure down with one hand if I wanted to. This is why there are restrictions against marching across bridges. They simply are not designed for that in many cases, nor must they be.

In your case I second the suggestion to grout the base plates. The simplest solution to reduce vibration, if you must, is to add a dampener. The most effective would be a linkage between the frames near the top. Perhaps a threaded rod and a shock absorber. It would not be visually obtrusive and might be considered an engineering necessity given the architects slender members. Filling the HSS sections with some damping material might also help but I've never thought about what that could be.

RE: steel frame oscillating

I am curious where this is going to go,

Drypack the plates seems to be a popular option, as well from your video it appears that your base plate will be burried in concrete - Is this the case?

This could be an excellent opportunity for a heavy weight elastomeric. I would recommend NOT simply abutting the concrete in a monolithic pour around the column, as it will likely crack, but rather leaving a 15mm gap and inserting a heavy elastomeric to act as a dampener.

RE: steel frame oscillating

(OP)
CELinottawa,
charlie
dik
and everyone

when the contractor saw this frame swaying he,by himself, made this 20cm RC foot over the original footing in order make base more safe.

later on after I added this question in the forum, he added some other shims plates, I did not see them , under the base and I was told that the vibration was reduce by 50%. after Pouring the R.C. hopefully, praying to, it drops to zero.

CELinottawa,

I can't bring elastomeric bearing it is imported in our country and very expensive. Unfortunately I want the concrete to be monolithic pour around the column to make it a fix base; do you have a solution for the cracking?

Dik

sorry but I don't know what to you mean by "dogleg" , you mean Knee?
and the base is 20mm thick. The maximum axial is 2tons max bending, if fixed, is 1ton.meter ASD which is enough

RE: steel frame oscillating

What will the depth of concrete above the top of the base plate be following the pour?

I would be less concerned if this is a goodly strength (say 30MPa or better) and at least 150mm.

I know of no specific formula, though you could work it out from first principles. perhaps another poster may know of a formula for minimum embedment of an HSS.

Additionally:
- Will the Architect's details all work with this increased slab height?
- Will the shims in any way interfere with the flow of concrete below the plate (I would still recommend grout either way, even if that must now be non-shrink, non-metallic, flowable).

RE: steel frame oscillating

knee might be the same... a 90 degree bend, welded in the haunch so that 3' goes downwards to toward the base and 3' goes towards mid span... this would stiffen the frame significantly.

Dik

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources