Rooftop Mechanical Units Wind Loads
Rooftop Mechanical Units Wind Loads
(OP)
ASCE7 6.5.15.1 covers loads on rooftop equipment. However when you go thru the procedure it looks like they apply only a horizontal force. What about uplift on top of the unit?






RE: Rooftop Mechanical Units Wind Loads
ASCE is only addressing the impact on the underlying structure, not the strength of the unit itself.
RE: Rooftop Mechanical Units Wind Loads
F=q(GCr)Ar
q = equals pressure at mean roof height
GCr = 1.5 for an area less than (0.1BL) and it can be reduced linearly from 1.5 to 1.0 for between (0.1BL to BL)
Ar = area of equipment
RE: Rooftop Mechanical Units Wind Loads
RE: Rooftop Mechanical Units Wind Loads
Then in ASCE 7-05 they added the factor and said to design RTU using the 6.5.15 (the other structures section) with the amplification factor.
In my experience the increased lateral loading resulted in an overturning force that more than made up for the lack of an uplift load.
So my advice would simply be to work it both ways. Most RTU's are pretty small and won't have any uplift. But some times its like parking a small trailer up on the roof. Certainly there is uplift. So do it both ways and just see which load case governs.
When in doubt make it stout.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Rooftop Mechanical Units Wind Loads
The formula of ASCE 7-10 that ash060 noted is for the vertical up lift force on roof top structures and equipment.
There also is a lateral force, Fh=qh(GCr)Af(N). where (GCr) = 1.9 for rooftop structures & equipment with Af less than 0.1Bh (and same reduction as vertical equation. Af is the vertical projected area of the equipment.
A few things are not clear to me:
1. I think that the wind loads in ASCE7-10 are strength level loads. Is the (N) in the equation the "Notational Loads" referred to in section 1.4?
2. The velocities used in calculating qh appear to have been increased from a minimum of 85 mph to a minimum of 110 mph. I dont understand why this was done.
RE: Rooftop Mechanical Units Wind Loads
RE: Rooftop Mechanical Units Wind Loads
It only says to apply the load on the projected area of the top of the equipment. The horizontal load is pretty much the same as the ASCE 7-05
RE: Rooftop Mechanical Units Wind Loads
RE: Rooftop Mechanical Units Wind Loads
I worked for a company who was sued over roof top units that were "removed" by a hurricane and deposited in the ocean. Turns out the scope of this at the time (15 years ago probably) was not clear, by the mechanical engineer or the structural engineer? Well, everyone got sued. The large cost was not structural damage but water intrusion through the huge holes in the roof where the RTUs used to be. The place was some type of big box store and they lost merchandise and were closed for several weeks because of this.
"ounce of prevention is worth a pound of cure"
RE: Rooftop Mechanical Units Wind Loads
In the absence of any code to specify loads for the design of the unit itself, it's quite likely that ASCE will be used for that as well, whether intended for that or not.
RE: Rooftop Mechanical Units Wind Loads