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?
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
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
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
RE: steel frame oscillating
RE: steel frame oscillating
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
RE: steel frame oscillating
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
RE: steel frame oscillating
RE: steel frame oscillating
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
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
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
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RE: steel frame oscillating
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
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
RE: steel frame oscillating
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
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
RE: steel frame oscillating
RE: steel frame oscillating
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
3dDAve
JoshPlum
What I need is a good source for Structural dynamics, Good but short. the client will not wait till I read a 300 pages book.
attached is a video of the base plate movement
RE: steel frame oscillating
RE: steel frame oscillating
RE: steel frame oscillating
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
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
RE: steel frame oscillating
Dik
RE: steel frame oscillating
Dik
RE: steel frame oscillating
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
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
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
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
Dik