RTU Support/Framing

[slapjack]

When providing structural framing between bar joists for the support of roof top units, how is the metal roof deck cut to provide chase openings... do they cut the metal deck out for the entire perimeter of the RTU curb or do they only cut the deck where the supply and return chases pass through the roof?

Also, anyone have recommendations for RTU tie-down in high wind zones? Our units are typically supported by W6x9 beams which span between joists under the deck. The RTU curb is then installed over the metal deck.

Thanks!

 

[H57]

I call out the openings in the deck to be at the chase locations, not the entire rtu. I've seen alot for tie downs, but I typically fasten the unit to the curb (with screws, or screws and plates), then fasten the curb to the support structure with screws. Most of the time I don't end up with uplift at the corners from wind although there will be shear, so it typically ends up being a minimal connection and fastening.

 

[lkjh345]

slapjack:

We usually build a frame out of angles below the deck to support the RTU at the curb. We put treated wood blocking in the deck valleys to prevent the deck from crushing under the RTU curb.

For small RTU's, we just allow the contractor to cut out the requied deck opening ( if smaller than 12" x 12") under the RTU as this deck is essentailly carring no load and is supported by the angle frame at the curb.

For larger RTU's, we frame out the deck opening with another frame made up of angles.

Don't forget to specify either RTU load to the joist supplier either through the use of a joist loading diagram or using KCS-series joist.

Generally we leave the tie down detail to the RTU supplier.

 

[mrengineer]

See also threads 507-145660 and 507-117866