Vibration of existing slab-on-grade due to large commercial laundry drier

[ajk1]

Given:
An existing installation consists of 3 large commercial laundry driers sitting on a 5" ± thick
housekeeping pad which in turns sits on a 5" (125 mm) thick slab-on-grade reinforced with welded steel wire mesh. The driers are not bolted to the housekeeping pad. The driers are in a nursing home.

Problem:
The issue is that the slab-on-grade can be felt to vibrate when the driers are on.

Questions:
1. Would it reduce the slab vibration if the driers were bolted to the housekeeping pad?
2. How can the vibration of the slab be reduced?
3. Is this something that anyone on this forum has encountered, and how did you address it?

I realize that this may require spring/pad vibration isolators under the driers and this may require retaining a vibration specialist to advise (or perhaps a mechanical engineer who has dealt with this issue).

The drier manufacturer seems unable to help.

Any help that someone on this forum could provide, particularly if you have experience in vibration mitigation, would be appreciated.

I wonder if this question might be better posted on another of the forums? What do you think?

 

[WARose]

1. Would it reduce the slab vibration if the driers were bolted to the housekeeping pad?

Possibly. But you typically have to pretension the bolts for that to be of much use. With the slab thicknesses you describe (and the reinforcing)......you are likely going to crack it badly with any kind of preload in those bolts.

2. How can the vibration of the slab be reduced?

You've typically got two options: add/subtract mass or some sort of pad isolator/damper. You need an analysis to tell you exactly what.

Unless the frequency is pretty high (and that typically isn't the case with a dryer).....you probably can't use any kind of ground isolation system. (I.e. trench, etc.)

Any help that someone on this forum could provide, particularly if you have experience in vibration mitigation, would be appreciated.

I wonder if this question might be better posted on another of the forums? What do you think?

You came to the right place: we've discussed structural vibration problems here quite a few times and I did consulting work in that for years. There are some other knowledgeable people here on that too.

 

[oldestguy]

There are those here that deal with vibrations. Generally the natural frequency of the slab is reduced if you increase its mass. However, dimensions, etc. are needed for this evaluation and for deciding on remediation work. Likely only general help from here, since we don't do calculations for you. Then, practically just adding to the mass may be difficult to get any reasonable improvement. The best I can think of is drilling large diameter (well over one foot diameter)auger holes next to (maybe through) current slab and filling with reinforced concrete tied to the existing slab, MAY accomplice something. In that case progressively adding these masses and observing effects may well do the job. Depth may be limited due to equipment potential, overhead obstructions, etc. but I'd look for at least 6 feet depth and maybe at least four per dryer as a start. You may have to triple these numbers.

 

[hokie66]

I would look at isolators, probably a combination spring and rubber pads. But it not my field, so would seek help.

 

[MIStructE_IRE]

As above, my initial instinct would be to trial some isolators.

The “do nothing” option is also available to the client.. If it vibrates, it vibrates!

 

[Ron]

Agree with isolators. There are many commercially available. Look at isolators for large pump assemblies or for elevator motors. These items are in a similar mass range as a commercial dryer.


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[bootlegend]

Unisorb and Fabreeka are two manufacturers of isolation and damping products that I have used with successful results. Both have technical departments that have been very helpful.

 

[ajk1]

Possibly. But you typically have to pretension the bolts for that to be of much use. With the slab thicknesses you describe (and the reinforcing)......you are likely going to crack it badly with any kind of preload in those bolts.

Why would they need to be pretnsioned much beyond snug tight?

There are those here that deal with vibrations. Generally the natural frequency of the slab is reduced if you increase its mass. However, dimensions, etc. are needed for this evaluation and for deciding on remediation work. Likely only general help from here, since we don't do calculations for you. Then, practically just adding to the mass may be difficult to get any reasonable improvement. The best I can think of is drilling large diameter (well over one foot diameter)auger holes next to (maybe through) current slab and filling with reinforced concrete tied to the existing slab, MAY accomplice something. In that case progressively adding these masses and observing effects may well do the job. Depth may be limited due to equipment potential, overhead obstructions, etc. but I'd look for at least 6 feet depth and maybe at least four per dryer as a start. You may have to triple these numbers.

That has given me an idea: Why not drill some micropiles directly under the area of each drier? Would that do the trick?

I would look at isolators, probably a combination spring and rubber pads. But it not my field, so would seek help.

Thanks hokiee66. Apreciate your contribution, as always.

As above, my initial instinct would be to trial some isolators.

The “do nothing” option is also available to the client.. If it vibrates, it vibrates!

It is a land fill area and the vibration has caused some settlement of the soil under the slab on grade. No god to just let it continue, in my opinion, could eventually be other problems as the area of settlement spreads out.

Agree with isolators. There are many commercially available. Look at isolators for large pump assemblies or for elevator motors. These items are in a similar mass range as a commercial dryer.

Thanks Ron. Appreciate your input as always.

Unisorb and Fabreeka are two manufacturers of isolation and damping products that I have used with successful results. Both have technical departments that have been very helpful.

Thanks. This seems very hopeful source. I will follow up with them on Monday. My preliminary opinion is that isolators may be the way to go, as seemingly relatively simple and hopefully not too expensive, although I suppose the existing driers will have to be raised and re-installed to accommodate the isolators. Hopefully the isolators will not cause the drier itself to vibrate excessively. Is there any chance of that happening?


Thanks everyone. All comments are acknowledged and much appreciated.

 

[Ron]

ajk1....check with Huebsh dryer company (1-800-553-5120). They make probably the most popular commercial dryers and likely have isolation pads for your dryer or similar.


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[hokie66]

I would think the most important thing about the drier itself is to make sure it is level. Ron's contact sounds good.

 

[Ron]

hokie66....lol..you're right. Check to see if level. Also check the mechanical system of the dryer. Worn motor mounts, bushings, and bearings can induce extra vibration.


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[ThomasH]

I would start by trying to figure out if it is a resonance problem or not. If it is resonance perhaps you can change the mass or stiffness to increase the difference between the load frequency and the natural frequenvy of the slab. Another option could be to increase the damping.

Or, as mentioned, isolators. The issue with those can be that if the dryer manufacturer has intended for the dryer to be placed on a foundation, things could get worse.

How strong are the vibrations? What is your concern, what scenario is it that you want to prevent?

Thomas

 

[WARose]

Why would they need to be pretnsioned much beyond snug tight?

Because if you didn't......you really aren't having much impact on the situation unless the skid is completely lifting off the housekeeping pad.

That has given me an idea: Why not drill some micropiles directly under the area of each drier? Would that do the trick?

Possibly.....assuming your slab is reinforced for that.

 

[ajk1]

How much floor to ceiling height is required to install helical piles?

How can the top of the pile be permanently attached to the existing 125 mm ± thick slab-on-grade? Any detail for attaching slab-on-grade to helical pile shaft that has been used on projects would be of interest.

The soil below the existing slab-on-grade is 9 m depth of engineered fill. About how far down into the engineered fill does a helical pile have to penetrate to be sufficient for the pile installation to have adequate strength and rigidity to restrain the existing slab from perceptible vibration that one might feel when standing on the floor while the washing machines are operating? Would something of the order of 2 m penetration into the engineered fill be enough? The machines are washing machines, not driers as I believe I erroneously said earlier.

 

[Canuck65]

Tackling vibration problems can be very difficult. We typically have engaged specialty vibration consultants to provide recommendations that we can then follow up with the design from. If I was to do this as part of the original design, I would have a much larger mass of concrete, independent of the floor, plus vibration isolators. However, you are dealing with an existing condition. The only recommendation that I can offer is to direct you towards Canadian Foundation Engineering Manual. Chapter 14 is a section on machine foundations. There are some interesting comments about the trial sizing of shallow footings in section 14.5.4 (4th edition). They also talk about deep foundations in this chapter.

 

[ajk1]

Tackling vibration problems can be very difficult. We typically have engaged specialty vibration consultants to provide recommendations that we can then follow up with the design from. If I was to do this as part of the original design, I would have a much larger mass of concrete, independent of the floor, plus vibration isolators. However, you are dealing with an existing condition. The only recommendation that I can offer is to direct you towards Canadian Foundation Engineering Manual. Chapter 14 is a section on machine foundations. There are some interesting comments about the trial sizing of shallow footings in section 14.5.4 (4th edition). They also talk about deep foundations in this chapter.

Thanks. I will check that out.
Can I assume that you do not think that helical piles to support the existing floor is worth investigating?

 

[ajk1]

Or maybe I better look at the Canadian Foundation Manual first to see what it says before I conclude anything about helical piles.

 

[WARose]

You may want to try the micropile folks. They do this sort of thing often. (including underpinning existing foundations. They don't need much headroom either.)

I think you will still likely have issues with the reinforcement in that slab (if you go that route).

 

[ajk1]

You may want to try the micropile folks. They do this sort of thing often. (including underpinning existing foundations. They don't need much headroom either.)

I think you will still likely have issues with the reinforcement in that slab (if you go that route).

OK Thanks.

 

[ajk1]

I have now learned that the vibrations are transmitted to the rest of the building, apparently thru the slab-on-grade which is in contact with the building structural columns and the top of the supporting foundation pier caps.
Question: Do you think that installing a grid of micropiles, say about 2 m down into the existing 8 m deep engineered fill under the existing 125 mm thick slab-on-grade, perhaps at 1.7 m centres would reduce this transmission to the rest of the building by damping it, or could it make it worse?