RTU Support on Metal Deck
RTU Support on Metal Deck
(OP)
I have 2 RTU's to intall on a roof. The roof is framed with 18" bar joists 3'-4" O.C. with what appears to be 1.0E 26 ga deck. I have taken care of the bar joist analysis for the new loads. As far as supporting the curbs on the decking and joists, I have seen details where angle irons are added beneath the decking to support the curb. I have never really paid much attention to this, but how is the curb actually supported on these and how are the angles attached to the joists? The first curb I am tackling is 4'x7' with an 18x34" opening and weighs about 800 lbs. It is oriented with the 7' dimension parallel to the joist span. I only would like to cut the decking at the duct opening to keep my top chords braced.
Thanks
Thanks






RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
You can also use a pre-fabricated galvanized steel curb that can be attached directly to the deck, depending on the roof insulation thickness. You might need a supplementary nailer to provide shimming for the insulation.
26 ga is a flimsy deck; however, your spans are short so that's good.
Another option is to install the angle "leg up" inside the opening and attach a nailer into the angle leg. This puts the fasteners in shear, so be careful that you have adequate capacity.
RE: RTU Support on Metal Deck
Thoughts?
RE: RTU Support on Metal Deck
The deck flutes are usually 6" o.c. so there is a configuration where a 6" channel should work with the deck flutes, if not, we change the channel width to something that will work. I am not sure I have ever seen a unit where the long dimension wasn't perpendicular to the joists and I have seen quite a few. In the odd instance that this happens, we would probably install a few channels under the unit to distribute the weight of the unit to the joists. In the normal case, where the long dimension is perpendicular to the joists, we normally wouldn't support the "ends" of the unit (as they are usually 3-4 feet wide.
This detail can be found in the book "Designing with Steel Joists, Joist Girders, Steel Deck" written by Jim Fisher and is where I stole it from.
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
Do you have the legs of the angle up or coped and facing down?
The curbs I have looked at are usually 16ga material and around 8" deep. My metal stud background says these can span a fair distance. Funny how little information is provided by the curb manufacturers.
RE: RTU Support on Metal Deck
This is true for my client.
I used to assume this to be the case and felt good about the extra measure of safety. For my client, however, the light gauge curb elements come in stock lengths which will often result in mid-length splices that will generally not be flexurally continuous. I've included some photos of my client's stock 8' curb elements below. Interestingly, the 6', non-continuous curbs are sometimes reinforced with sturdy trussed frames to keep the walls of the light gauge curb from buckling.
I should add that my client's RTU's are generally very large and may not be typical.
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.
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
Website is: www.quickframes.us/
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
RE: RTU Support on Metal Deck
If your deck ribs are at 4" o.c use a C4 or a C8 spacing to be determined in the filed to fit under the unit.
If the unit is parallel to the joists, I would probably space the members a 4'-0" o.c. or something similar.
The only downside I have come across to the detail is that it can't be installed once the roofing is on. I also had one contractor on a new building refuse to use the detail as the owner was responsible for the HVAC and it was difficult to locate the positions of the channels during the HVAC install as they were covered by roofing.
RE: RTU Support on Metal Deck