Roof Framing for Future Roof Top Units

[kodstruct]

Hello,

I am designing a retail center with leasing spaces. Future tenants will install their own RTUs and ductwork.
Does anyone know how to approach the roof framing design? I don't want to be overly conservative and design specify add'l point load on each joist.
Any idea?

Thanks.

"Be nice to people on your way up because you might meet them on your way down"

 

[msquared48]

The RTU's will probably not be too heavy if they are small retail spaces in a strip[ mall. If you allow 2 to 4 psf for Mechanical, and the units are 400# or less, they should be able to be placed closer to the exterior walls and span 3 or 4 joists with sleepers to work.

I would not worry about designing each joist to carry the units via a point load at the middle.

Mike McCann
MMC Engineering

 

[kodstruct]

Thanks msquared48 for your input.

I was thinking of using 3' to 4' joist spacing and show typical details of RTU support when the weight exceeds 600#.

Thanks.

 

[Archie264]

kodstruct,

Regarding "...and show typical details of RTU support when the weight exceeds 600#." If you present it that way make sure to give the upper limit lest someone in the future goes crazy with it and then tries to hold you accountable for it.

 

[hawkaz]

What kind of joists are you using? For steel joists, I normally see 6'-8' spacings at malls. Units have been getting heavier lately. It is not uncommon to see 600-1200 lb units being put on exisitng roofs. A good way to do this is to define zones where units can be placed- and beef up the joists accordingly at these areas.

 

[kodstruct]

@ hawkaz,

I often go with 6'-8' also with typical Vulcraft Joists when locations of RTU's are defined and I transfer loads to joists/beams accordingly.
On this particular project, we are designing the building shells (up to 8 buildings with approx 12,00 sq.ft each), with slab leave-outs for future tenants to finish their own plumbing, HVAC and stuff. A tenant may want to lease 8,000 sq.ft in one building in which I'd anticipate up to 4 RTU's minimum based on past projects. So the RTU locations are unpredictable.

have anyone heard of constant shear design for this type of application? I know one of my old boss (12years ago) used that expression a lot.

Thanks.

 

[dcarr82775]

I have sometimes specified the KCS joists where I know I will have some mechanical loads, but I don't know where. You still spec a uniform load that covers things, but the KCS joist allows some flexibility in the location of the RTU. I am not sure of the cost increase.

 

[RareBugTX]

Another 2 cents. Have in mind that the RTUs will likely be placed at midspans (hvac contractors place the units so that RTUs efficiency is good and to minimize ductwork.

 

[UnthoughtKnown]

add 10 psf live load in the design.

 

[structSU10]

For normal joist you have to be careful about where the shear and moment occur - if the max moment & min shear shift too far from the center it becomes a problem because the webs are designed only for uniform load and any reversal of a web could lead to failure. Really if you design for an extra uniform load to account for the RTU's they would want to be placed near the midspan of the joists to ensure webs dont become overstressed, or you can spec the KCS joists, which has a constant shear capacity and hence more flexibility.

 

[kodstruct]

Thanks all,

I think using KCS joists is a reasonable approach.
Adding 10psf live with closely spaced joists to allow the RTU to span over 3-4 joists also seem reasonable to me.
If I were to chose between KCS joists @ 6'-8' on center VS 30psf live load with joists closely spaces @ 3'-0" on center, I think I will go with the KCS.

 

[OHIOMatt]

One additional way to do this, which I have done in the past is to specify that each joist be designed for a pair of point loads at certain distance apart. So, for a small space use two point loads of 400 pounds at 4' centers. Then have the joist manufacturer design the joist to account for these point loads at any location of the span. Give them realistic loads for total load and live load. Designate the joists as SP. They will produce the most economical joist with this method.

The joist designation would look like 20K 175/100 SP. The 175 would represent you dead + live, the 100 would represent your live / snow. Then direct them on how you want to handle the point loads.

 

[kodstruct]

Thanks OHIOMatt...I like this idea.

 

[jayrod12]

One thing I came across on a design once where I was putting new RTUs on an existing mall roof was the roof decking and joists being designed for 100 PSF which basically meant that snow build-up was not an issue so we only needed to reinforce for the unit itself and not the build up due to the unit.

And in a lot of cases the unit weighs less than the 100 psf and so only minimal reinforcing was required (Some angles or channels to support the unit curb.)