RTU Curb/Catwalk Mounting

[ride5150]

US based, I work for an industrial platform manufacturer. I've been tasked with designing small RTU access platforms on top of a typical joist + metal deck roof. I have designed plenty of RTU frames/joist reinforcing, however it got me thinking - how does an RTU curb physically attach to structure below? I looked through my company's drawings and we have never called out the attachment method. It makes me think that the HVAC contractors typically just put it on top of the metal deck, or literally on top of the roofing membrane and , without any physical connections.

I am in a low seismic zone, but tornados occur fairly often.

In the case of designing the catwalks, i would ideally like to call out new framing elements that are field welded directly to existing joists, and then connect my catwalk columns to the new framing elements. That being said, there appear to be several manufacturers of catwalks that provide "non-penetrating roof mounted catwalks", which seem to either float on top of a thick neoporene pad with no actual connection to the roof deck/joists, or are attached via clamps to standing-seam deck (not my condition). No idea how they can justify a floating structure.

So (2) questions:

1. How does an RTU curb typically connect to structure below?
2. Are there any areas of the IBC that allow for a "floating" structure (or with "deadman" weights, to keep it in place), with no physical attachments?

Thank you in advance.

 

[lexpatrie]

1. Probably self drilling screws. And I'll mention that Floridarwquires AC units be mounted above the roof, as this is a hurricane zone and we've only recently had any design information on RTU wind loads, I've never figured out why.

2. As the Chris Elliot Doritos commercial says "not really", but they exist. From an engineering standpoint most of those are decidedly NOT engineered, and if an engineer is involved they'd be under "alternative design" where they'd need to demonstrate (via calculation) that the design is equivalent to code in life safety. If the amount of ballast on these looks unreasonable, then it was probably engineered and the engineer inspected it personally.

When you say catwalk do you mean on top of the roof? Is it elevated? Like a pipe crossover? Otherwise I'm more familiar with catwalk having to do with lighting and whatnot for theatrical venues and it's hanging below the roof.

 

[ride5150]

1. Probably self drilling screws. And I'll mention that Floridarwquires AC units be mounted above the roof, as this is a hurricane zone and we've only recently had any design information on RTU wind loads, I've never figured out why.

2. As the Chris Elliot Doritos commercial says "not really", but they exist. From an engineering standpoint most of those are decidedly NOT engineered, and if an engineer is involved they'd be under "alternative design" where they'd need to demonstrate (via calculation) that the design is equivalent to code in life safety. If the amount of ballast on these looks unreasonable, then it was probably engineered and the engineer inspected it personally.

When you say catwalk do you mean on top of the roof? Is it elevated? Like a pipe crossover? Otherwise I'm more familiar with catwalk having to do with lighting and whatnot for theatrical venues and it's hanging below the roof.

Thanks for the reply! So the curb sits on the roof deck directly, and secures to the deck via screws? This is in line what ive gathered from watching RTU curb installation videos on youtube. However this would also have potentially large deck "crushing" forces at the locations where the deck crosses over joists. I've never seen any details calling out blocking/something to infill the metal deck flutes.

Yes correct, elevated catwalks on top of the roof. Basically they need a permanent way to access "stuff" on the RTU that is too high to reach from just standing on the roof. Catwalks will have fixed straight ladders to get from t/roof to t/catwalk. T/catwalk will be ~4' from t/roof.

 

[lexpatrie]

I suppose if they aren't attached to the roof they aren't exactly an "appurtenance" and it's more of a stairs on wheels just really bad wheels. They wouldn't be very likely to be occupied during a design windstorm or a hurricane. They'd maybe fall under OSHA more than a building code. Work platform, non fixed ladder, movable equipment?

There are checks for point loads on metal deck, web crippling and whatnot, cantilever, etc. Most RTU aren't all that heavy and the angle supports we tend to put in for new RTUs on new buildings are not all that necessary, but they serve nicely to mark where the roof penetration is supposed to happen have done one walkway that was integral with the large RTU as the RTU was new on an existing roof and we needed to also avoid the snow drift under the unit, hence elevating, which then needed the work platform, guards, stairs, etc. But it was all attached through the roof as it was also the support for the RTU.