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!

 

[marwin23]

evo10: I noticed that operating frequency is 9.66HZ (580rpm), but still it comes through my natural one. Moreover if they ask for not having 7to12hz, I believe that it is wise to assume that equipment may operate in such range. As you noticed they just added plus minus 2.5HZ (or roughly 25%).
GC_Hopi: thank you for the help. I am probably quite well versed with theory, but practical approach suffers. and here I am looking for some help.

Finally, in reality. I have a force of 1KN (or rather 2kN, since it is shown in 2 locations). It causes also some moment (up and down), but that we see in FV1&FV2. If we just concentrate on values of the force (2kN = 450lbs), I really wonder what is the chance to start the resonance. As GC_Hopi noted, knowing what is a damping constant would help, but it can be a dead end. It is not easy to establish theoretically, and the job itself would not allow on any experimental exercises.

Respecting evo10's opinion, is there someone else who has experience in pouring a foundation block on a metal deck, and isolating it with vibration damping?

 

[r13]

Hope this article can shed some light on solving your problem. Link

 

[dik]

ICYC, I often use Fabreeka pads for thermal isolation with bolted end plate connections to reduce thermal transfer into buildings... they make a great product... used to use HDPE, but fabreeka products have a much higher compressive strength.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[evo10]

ive never used robot why does it look like you have some pin supports on the outer corners and mid of your cantilever secction XD

 

[saplanti]

It is difficult to see the details on the model;

Have you added the concrete mass into the model?

Did you try to change the stiffness of the beams and add bracing to increase structural stiffness?

You have these two options to change the frequency of the structure.

If you use sandwich concrete with springs in between you can change the transmissibility of dynamic forces, it perhaps helps to reduce the action on the steelwork. This is widely used in supporting rotating equipments. I am not sure in your case. Additionally this option might be very costly, and you do not know the structure can handle the additional concrete mass locally.

I understand the manufacturer do not employ design engineers anymore, and they are helpless for your case. But you still have a good information in your hand.

I am not familiar with the software you are using, and capabilities of it. You need to start playing with the two options above and plan replacing some members and adding bracing to get outside of the unwanted frequency range. In case there is a seismic and wind forces on the structure the additive stiffness might also be working for you.

 

[evo10]

i would not just add the concrete mass
but account for the stiffness whatever that may be XD

the aisc design guide with vibration, not sure about the acelor mittal, will account for composite beam with decking in the stiffness

but is your deck really composite with the beam

that may have to be taken into consideration in total model

 

[marwin23]

Pins at one end shall not worry you. Do not have good picture, but it is not cantilever, just short span supported (lightly) on precast concrete walls. This side is almost not loaded. Probably if removed from model, there will be no huge impact.

There is no composite action between deck and beams (no studs). Deck itself was modeled with mm precision, and so is concrete (with weigh, etc).

Since there are questions about details (pin supports, soon would be why kN and mm): whole structure with existing equipment was imported from Italy to USA ca. 20 years ago, and is placed inside precast building. On this steel structure there are two separate production lines (mirrored along longer axis), and one is getting replaced. Model shares both IPE (European) and W (US, new to reinforce) beams. Not too many existing drawings, but a lot of assumpions were done. Owner not willing to investigate what are current footings and will release us from responsibility in case of failure. Generally we need to act as quick and precise as possible, to probably minimize impact on the other half of structure, where equipment needs to operate.

 

[marwin23]

saplanti - thank you very much. That is more or less what I am also doing (playing with additional bracing in floor plane and vertical), and really appreciate that Mechanical engineer joined this discussion. I have a huge respect in the subject of dynamic to mechanical (dont confuse with HVAC, what seems to be common in the USA) engineers.

 

[r13]

Finally, in reality. I have a force of 1KN (or rather 2kN, since it is shown in 2 locations).

It seems a typo - the vendor drawing indicates the operating weight is 20400N (20.4kN/4600#).

 

[human909]

It seems a typo - the vendor drawing indicates the operating weight is 20400N (20.4kN/4600#).

I wouldn't expect that to be a typo. 800N is fairly hefty recapacitating load. I've seen entire floors shaking away severely and unpleasantly from undamped reciprocating loads loads of half that value. Same type of machine a vibratory sifter/screener.

 

[r13]

The horizontal loads are 2kN (2x1000N).

 

[dauwerda]

The two most common ways I have seen this type of issue dealt with is:

1. Add stiffness to the supporting floor structure - it is typically the vertical movement of the floor/deck that causes issues.

2. Add mass to the structure to change the natural frequency. This is often times done by hanging weight below the floor, this changes the natural frequency of the floor as well as provides damping to the horizontal motion of the structure. This method of course requires the floor to be strong enough and stiff enough to handle the additional dead load.

 

[marwin23]

@dauwerda - I kind of understand both approaches, but to do them concurrently? Adding stiffness will increase my natural frequency, whereas adding mass will decrease it at the same time.

 

[dauwerda]

Sorry, I was not trying to advocate both at the same time - pick one or the other.

 

[human909]

For the scenario you describe I'd definately be ADDING stiffness and not removing it. That is both based on my experience and on the fact that it says it right there on the spec sheet for the Buhler purifier.

 

[marwin23]

human909: I do not remember providing Manufacturer of it, but you are definitely experienced.

As of right now it seems that if to my deck of 120mm (total), I will add 6-7" of concrete, I am able to reduce first frequency to 6HZ (in fact the 7th mode is the first above 7Hz). Still the structure would have to come through resonance when equipment is starting, but now I have to convey this great approach to owner (adding a lot of weight).

human909: based on your experience, would you find it reasonable to allow the natural frequency be below the equipment frequency (go through resonance), or rather I shall put all effort to increase stiffness. I was pl

aying today with adding more bracing in floor plane (current floor framing attached), but results were very poor.

 

[r13]

For a process plant, I think there are many equipment operate in varies frequencies, which can make structural optimization difficult. I would follow the owner's instruction, do the best to avoid/minimize the interference at a reasonable cost. Can you seat the equipment on a sturdy skid, the skid can be set on the floor using isolation device, or raised above the floor to change its dynamic properties. I still think isolation is the better approach.

 

[marwin23]

r13: fully agree, that it is difficult to find a good frequency which would satisfy each piece of equipment operating requirements. Here I am dealing with some units of 4-4.5HZ, and the previously one mentioned of 7-14HZ (owner's manual).
I really try to play back and forth with that. Just for the sake of exercise, I changed all sections to be W6x8.5 - received around 3.5HZ. I changed all to be W8x31 - 5.1HZ, changed all to be W18x130 - 9.64HZ (I did not check capacity, just eigenfrequency).Seems that it is easier for me to drop it below 7HZ (adding mass) than to raise above 14HZ.

 

[r13]

Understand. And your conclusion seems logically correct. Good/interesting exercise!

 

[evo10]

..moreover if they ask for not having 7to12hz, I believe that it is wise to assume that equipment may operate in such range....

i cant say that equipment operates in the range it may change frequency if say some unbalancing occurs?

here is an illustration of the reason for the range