rooftop mechanical platform supporting units on isolators

[caseypoi23]

i'm designing an HSS platform on a concrete roof. the platform supports two AC units, and the platform consists of beams which span to short HSS posts. the columns are anchored to the concrete roof slab, over concrete columns (the slab and beams cannot support the AC/platform dead loads). the platform also has grating to support maintenance workers. platforms about 2' high.

i'm designing the anchorage of the units to the platform with Fp determined from asce 7-10 chapter 13, with internally isolated components (Fp = 2.44 * W). also checking the platform beams for these loads.

question:
when anchoring the platform itself to the concrete roof... should i still use the 2.44 * W? seems overkill, although that's what the load path would come down to... also, i'd have to use omega on the anchorage to the roof, which will kill the design. i'm pretty sure this is the case, but would love to hear thoughts on this.

thanks y'all.

 

[wannabeSE]

For heavy platforms, I have used ap = 1 and Rp = 2.5 for the platform self weight and ap = 2.5 and Rp =2 for the equipment weight. It helps a little. I can't think of any good justification for using ap=1 and Rp=2.5 for all of the weight when designing the anchorage.

I have not found a good reference for internally isolated equipment. Often times, the internally isolated elements are a small portion of the overall weight. It seems reasonable to use the higher seismic force for the weight of the internally isolated elements and a lower force for the balance of the weight. For instance, an Air Handling Unit (AHU) may only have isolators on the fans while the coils, filters, frame, enclosure, etcetera are not isolated. But, I have not found anything published that supports this reasoning.

 

[Ron]

Presuming your platform is rather large, 2' of clearance under the platform is insufficient for roofing and maintenance of the roof system. Further, depending on which code you are under, the clearance you note might be a code violation.