RTU Support on Metal Deck

[XR250]

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

 

[jayrod12]

We provide angles (or channels depending on load and span) under the curb as in my experience the curb itself if just screwed down to the metal deck (sometimes they will screw directly into my supporting steel angles). We also specify that they must provide solid blocking in the flutes of the deck between the curb and the supporting steel to prevent localized crushing of the deck.

 

[SteelPE]

We use a channel with the toes point down on top of the deck to support the RTU, typically a C6x8.2. The erector can then weld the flange of the channel through the deck to the support joist below. A nice positive attachment that doesn't rely on shims or "squishing" of the deck.

 

[Stenbrook]

We provide the angles as well. We will go as far as specifying a bolt to attach the curb down to that angle. We also may provide a strap/hold down to prevent any overturning on the unit. The angles themselves we weld to the top chord of the joists. You may have to cope the angle to get it to line up with the deck.I haven't ever had an issue with it though to this point.

 

[jike]

The channel with toes pointed downward does not always work. This will only support the unit where the long dimension is perpendicular to the joists and the unit dimension is equal to or close to the flute spacing. What do you do to support the two other sides parallel with the joists? This is why we always use angles with blocking between flutes. As other have mentioned, we require the curb to be fastened thru to the structural angles to anchor the unit from uplift.

 

[Ron]

Usually an angle frame (L3x3x3/8" is common) is sufficient. Unless the loads are quite high, a simple nailer is placed on top of the deck to receive the curb without much effect on the decking; however, you have a very thin deck so I would agree with jayrod12 that you provide flute filling to prevent deck damage.

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.

 

[XR250]

Thanks for info everyone. I had another idea. My panel points are 24" O.c. What if I laid a 2x12 or 11 7/8" lvl flat over each panel point spanning from joist to joist and support the curb on them? They would spread the load out over the deck so it would not crush and concentrate the load near the panel point so I do not need to add additional webs where angles would be attached. I could also easily screw it down from underneath.

Thoughts?

 

[SteelPE]

JIKE,

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.

 

[XR250]

@SteelPE- This appears to be E type deck so the flutes are 4" O.C. So i guess I can use 4" channels unless you like my wood idea :>

 

[XR250]

@ SteelPE - What is your typical channel spacing? I guess that comes down to how far the curb can span. I recall talking to a Trane engineer and he said the curbs can typically span the entire run of the unit.

 

[structSU10]

when I have done this, the curb guys have always said their curbs need to be 'fully supported'. I had toed down channels in the flutes, and then had angles running over that, set to the size of the curb, to span between channels. Used as many channels as I needed in the deck direction, always run out to terminate at a joists, with the angles spanning between them at the curbs only essentially.

 

[XR250]

Thanks structSU10:
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.

 

[KootK]

I have been doing some work recently with a major RTU supplier as my client. It's been eye opening.

I recall talking to a Trane engineer and he said the curbs can typically span the entire run of the unit.

This is true for my client.

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.

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.

 

[XR250]

Interesting. Thanks for sharing.

 

[hawkaz]

Quickframes makes a pre-fabricated adjustable frame that doesn't require field welding. Very handy.
Website is:

http://www.quickframes.us/

 

[structSU10]

I have the angle legs toed up. it has forced me to run angles on top of the channels parallel to the flutes, so its not very efficient, but its easy to install.

 

[jike]

We usually lap and weld the deck onto the same angle that supports the curb.

 

[SteelPE]

@XR250

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.

 

[XR250]

Thanks