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?

 

[3DDave]

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"

 

[TLycan]

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

 

[jayrod12]

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).

 

[CELinOttawa]

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.

 

[JoshPlumSE]

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.

 

[3DDave]

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.

 

[oldrunner]

Are you also considering P-Delta?

 

[dik]

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

Dik

 

[dik]

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

 

[charliealphabravo]

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.

 

[CELinOttawa]

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.

 

[TLycan]

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

 

[CELinOttawa]

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).

 

[dik]

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