RTU on Sleepers on Polyiso 1

[XR250]

I have 2800lb chiller that an owner wants to put on a roof (i-beams and metal deck)
As we are going to be installing the unit near a bearing wall, the roof structure has plenty of capacity to carry the weight.
The roof is metal deck w/ 5" polyiso and a membrane. The owner does not want to penetrate the membrane for support legs.
I proposed to lay down double 2x12 flatwise over the beams that carry 6x6 pt beams perpendicular that support the unit.
The 2x12's are intended to distribute the load over polyiso. Based on their surface area, I get a bearing pressure of less than 4 psi. The polyiso is reportedly has a 20 psi rating which, is my understanding, the load that will cause 10% deformation in the foam. Assuming a linear relationship, my load would cause 2% deformation or 1/10"
Does this sound like reasonable approach?

Thanks

 

[DaveAtkins]

Yes. But will there also be snow on top of the unit?

I would also check wind load trying to tip the thing over.

DaveAtkins

 

[KootK]

X2. Reasonable. If you're in a high wind/seismic area, it might be prudent to check that the coefficient pf friction is enough to keep things in place.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Thanks for the advice. Did not think about the snow. It is only about 400 lbs, so it should not change things too much.
Not enough wind or seismic to slide or tip the unit.

 

[manstrom]

I see this a lot. No issues. You may want to put another layer of roofing under the 2x12s.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[XR250]

Good advice

 

[KootK]

I remember reading in ASCE someplace that friction could not be relied on for seismic. Is that just for high seismic zones? I'm sure this is fine. I'm just curious.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

And when the owner re-roofs, the chiller will have to be pulled, and replaced after reroofing with that scenario.

Mike McCann, PE, SE (WA)

 

[wannabeSE]

KootK

ASCE 7-10 section 13.4 prohibits using friction from gravity loads to resist seismic loads on non-structural components.

 

[XR250]

@ Mike - just got a new roof so no worries for about 20 years.

@WannabeSE - Thanks for the info. I have refrigerant lines and electrical conduit anchoring it :> Seriously, we are in a low seismic area and even if it moves a little, it should not be a problem.