Rooftop Mechanical Units Wind Loads

[ron9876]

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?

 

[ajh1]

The uplift on the unit would generally not be more than the uplift on the equivalent roof area below the unit if the unit wasn't there. Other than some localized effects wherever the unit is physically attached to the building it would be unlikely for the building structure to be affected adversely. The horizontal force creates overturning conditions that would not have been present without the unit being there.
ASCE is only addressing the impact on the underlying structure, not the strength of the unit itself.

 

[ash060]

It has been addressed in ASCE 7-10 with the equation being:

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

 

[ron9876]

Thanks for the input. ash060 does it indicate that the pressure is applied to the sides and top or sides only?

 

[southard2]

In ASCE 7-02 you either used the section on other structures or you did what I did and that was to treat it like small builing on top of the roof. I would bascially apply the MFWRS and components and cladding loads the way you would on an enclosed building thus I was able to calculate uplift.

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

 

[OldPaperMaker]

ron9876,
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.

 

[WillisV]

OldPaperMaker - you sortof answered your own question regarding the wind speeds, the winds are strength level now and are to be used with load factors using 1.0W, hence the increase in velocity. For most locations you end up with the same overall factored force as previous. For instance for your minimum speed example, since pressure is based on velocity squared, 110^2 / 85^2 = 1.67 which is approximately equal to the older 1.6W factor used in ASCE 7-05 (they rounded the wind speeds to it doesn't hit 1.6 exactly, but you get the point).

 

[ash060]

ron9876
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

 

[ron9876]

Thanks for the input.

 

[a2mfk]

Haven't done it recently, but used to always use sign loads for the lateral and C and C uplift from my roof design. Looks like ASCE has filled in some gaps. Even in light of ASCE and their sometimes very confusing and always changing formulas and tables and charts, I would go with whatever produces the highest overturning force and design your uplift straps/cables for these reactions. Think in terms of economics, what is the difference in strap size or a few extra screws? Nothing. The copper pipes going to the RTU cost way more than anything you'll specify to tie it down. You are not economizing for an entire structure, that I understand...

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"

 

[JStephen]

"ASCE is only addressing the impact on the underlying structure, not the strength of the unit itself."

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.

 

[structuresguy]

Funny, we always took this new ASCE rooftop unit pressures as the required pressures for, well, the rooftop unit itself. I mean, that is what it says. So we force the manufacturers to provide units that can withstand the calculated pressures. At first, this was a problem, as very few A/C companies had equipment that could handle these pressures in Florida. But now we don't have any problems.