Industrial equipment on a steel structure 3

[marwin23]

Good morning,

I am looking for a piece of advise how to solve the issue when placing industrial equipment on an existing steel structure.

In details: 4sty existing steel frame (columns, beams + metal deck w/concrete) carries currently some flour mill equipment (old, mostly to be replaced). Some units of the new equipment to be placed on the top floor (can not on ground) have an operating frequency between 7-14HZ, whereas it seems that structure natural frequency is close to 8.5HZ. As per manufacturer, I can not provide dampers in between unit and my floor - however here if someone has any experience to argue this statement, then it would be good.

My first thought is:
- Unit operating weight is roughly 5kips. If I pour a block of additional 15kips, then (since mass is in square root (Sqrt(k/m)->(Sqrt(k/(m+3m))) I may be able to reduce the system output frequency twice to 3.5-7HZ. Or maybe it does not work that way, and concrete block would gladly transfer 14HZ from equipment to my deck without damping it (that would be against newton laws probably, but just want to assure myself).

I probably can someone support this weight on the current structure or reinforce as necessary. But at the same time such block would reduce my natural frequency of whole structure (but that would be just a fraction of whole weight). Does this approach make sense?

If someone has any experience - I would appreciate your input!

 

[SwinnyGG]

If the equipment oscillates at 14 Hz, it's going to try to oscillate at 14 Hz regardless of what you attach it to. Putting a big heavy pad will reduce amplitude and forces, but not change the frequency.

My first step would be to review this situation with the equipment manufacturer and determine what damping solution they approve- you are unlikely to be the first to ask them this question.

 

[marwin23]

@SwinnyGG - went that way and got nothing ("You cannot have damper in between the machines base and the floor."). Situation is tough, and neither owner nor manufacturer understands why I am trying to cause problems.

 

[wrxsti]

marwin23 how did you get the natural frequency of the structure?

 

[wrxsti]

I read somewhere that if you can deal with startup resonance
you can do low tunes and keep the natural frequency of different modes (with significant mass participation factors) ratio to machinery frequency
at 0.75 or less

 

[JStephen]

wrxsti's question also came to mind. That could potentially be measured if the building is existing.
Is the vibration in all directions or just lateral or just vertical? And is the building natural frequency in that direction also?
If you just had vibration in a slab, then moving it around on that slab might help.
And it's been ages, but as I recall, if you exactly match the natural frequency of the structure, then the response will depend on damping, and could potentially still be negligible- after all, you never hear of a building collapsing because of a ceiling fan. Effective damping may be kind of a mystery on paper, but if the building is there, could potentially be quantified.
If it turns out that all the solutions can also be done after the equipment is in place, then it might work just as well to set it and see what happens...

 

[GC_Hopi]

I would check the natural frequency of your structure. If you use ASCE 7 paragraph 12.8.2.1 I am getting 1.87 hertz for steel moment frames with a 40 foot height. I have not done a lot with vibration in steel but with concrete ACI recommends being 0.5x lower or 1.5x high on the frequencies.

You also need to check the beam's natural frequency separate from the entire structure.

And just found this in a search. Link

 

[JoshPlumSE]

Interesting that the manufacturer isn't used to this type of question. That may mean that the equipment is well balanced enough that not much dynamic force is produced. Have you inquired about what dynamic forces (magnitudes and direction) are produced by their equipment?

14Hz is actually a pretty low frequency for dynamic equipment. That doesn't mean it's not a problem, it's just not what I'm used to dealing with.

 

[r13]

Instead of the building, I think the important frequency should be checked/considered is the structure, to which the equipment is to be placed on. There are many ways to alter/damp the resulting frequency, though the equipment operation frequency remains constant.

 

[marwin23]

Natural frequency was calculated in Autodesk Robot, and was 8.91Hz. I also did some hand calc and got 8.73Hz. Even if I ommitted something, saying that it is around 8.5Hz sounds justified for me.
Equipment manufacturer cooperates very low with me, replying roughly: we deliver equipment, and you are responsible for structure. The mill itself (owner) does not oarticipate too much in it: we want to get your design asap, just provide stamped drawings showing which 'beams and columns to reinforce'. Many pieces of equipment has frequency of 4Hz, but two pieces specify operating frequency of 7 to 14Hz. I have full force distribution, and for 5k weight unit, horizontal component is probably (now I am at home) 800lbs for each of 4 supports.

r13, could you name few ways, please. Adding mass, isolators & dampers - what else? I can add few beams here and there (stiffness), but kind of moving in the dark.

 

[r13]

Quick come to mind is the equipment vibration damping pad,

https://vibrasystems.com/anti-vibration-pads.html,

you can do a follow up research. A combination of adding mass and vibration damping material should yield satisfactory result.

 

[marwin23]

r13 - thank you. As mentioned earlier: equipment manufacturer requires me to bolt to solid base, and does not allow on any pads in between.

 

[r13]

How about sandwich the pad in between the added concrete pad and the concrete floor? Note that the equipment is fastened to the bolts in the equipment pad, which in turn is attached to the floor through reinforcing/doweling. What you have achieved is separated the equipment vibration from the supporting floor.

 

[marwin23]

r13 - that is a kind of idea I was looking for. Great piece of advice, and surely will check/start thinking of that approach.

If someone has any other experience/idea - would gladly listen to!

 

[evo10]

is that the whole building modeled or a bay for the frequency?
and did you hand calc the frequency of the entire build? XD

 

[evo10]

i also agree with

..You also need to check the beam's natural frequency separate from the entire structure..

you should be doing some kind of localized checks

 

[marwin23]

evo10: in Autodesk Robot it is quite easy to do the whole building. Then I checked also locally whole floor (level). Then I checked single beam (using procedure from ArcelorMittal for composite decks/beams). No matter how I deal, I get values which are dangerously close to what the machine will impose.
BTW, yesterday replying from home I mixed the horizontal force from the said unit with the other - here I am attaching the distribution - it is 1kN so roughly 225lbs.

 

[evo10]

you made it so far in robot, why not try a time history to see the response

 

[GC_Hopi]

You were able to validate your model with some hand calcs. If so then you may have to look at reinforcing or damping.

 

[evo10]

reading that picture it looks like they give some kind of dynamic loading
as well as the frequency is not between 7 and 14 but rather the floor should not be between the range
equipment rpm is given