Rooftop Generator

[msquared48]

Have a steel skid mounted 500# rooftop 25 KV generator that needs to set on a 3" concrete isolation slab (roughly 3' X 6") on a 1/4 in 12 sloping roof.

I have done a lot of AC units on the roof, but never a generator with a concrete isolation slab there. Floor, yes, roof no.

Other than any extra structure below the roof sheathing, what is the normal method of installation in a high seismic area considering the slab and new roof penetrations?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Have a steel skid mounted 500# rooftop 25 KV generator that needs to set on a 3" concrete isolation slab (roughly 3' X 6") on a 1/4 in 12 sloping roof.

Just to be clear, Mike. Slab size 3' x 6'? Is the slope only 1/4" in 12"? That would be a flat roof.

BA

 

[msquared48]

BA:

Minimum slope for drainage, yes. But still a slope if the generator has to be flat. That can be taken up in the slab pour.

I am concerned with the detailing - sealing and anchorage - of the concrete slab to the waterproof roof membrane structure and what is normal. I do not want to reinvent the wheel here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Den32]

What is the roof structure? Metal deck on steel bar joists? Is concrete c-i-p or a precast pad?

I havne't done this detail, but would think you would want to go down to the roof deck and repair the roofing around the concrete. So, if concrete needs to be 3" above roofing, the concrete pad will be thicker than 3".

The deck corrugation and concrete interface will resist shear and if worried about uplift, you can tack weld a few headed studs to the supports to tie the concrete down. Not sure what your seismic factors are, but lateral load can't be much on a 500 pound unit.

 

[msquared48]

Wood roof, and the slab is intended to be a "floating" isolation slab, whatever floating means.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MikeHalloran]

"Floating" in the context of a generator, means mounted on springs, or rubber isolators, tuned to minimize the transmission of the generator's primary vibration frequency (1/rev of the engine) into the substrate, in this case a building.

Vibration isolators are commonly used under generator skids on yachts and oil rigs.







Mike Halloran
Pembroke Pines, FL, USA