Wind loads on rooftop equipment
Wind loads on rooftop equipment
(OP)
A small debate in our office. We have two 12' x 24' long (in plan view) condensers sitting above a roof of a large building. The condensers are 16 feet tall. They will be set parallel to one another such that the 24 ft lengths will align.
The two 12 ft. widths will be separated by a 4 foot gap.
The units will sit on a steel framework of beams supported by columns extending through the roof of the building.
We are using ASCE 7-05 for the wind load on roof-top equipment. In ASCE 7 it has a section titled "shielding" that states that for buildings and other structures you cannot use one element to shield another.
The debate is whether, in this case with two large condensers sitting only 4 feet apart, does the "projected area of the units" mean we have to put the wind load on both of these or does the small width of the gap (4 feet) mean that in essence we have "one" unit and receive wind on one face only?
See the attached sketch.
The two 12 ft. widths will be separated by a 4 foot gap.
The units will sit on a steel framework of beams supported by columns extending through the roof of the building.
We are using ASCE 7-05 for the wind load on roof-top equipment. In ASCE 7 it has a section titled "shielding" that states that for buildings and other structures you cannot use one element to shield another.
The debate is whether, in this case with two large condensers sitting only 4 feet apart, does the "projected area of the units" mean we have to put the wind load on both of these or does the small width of the gap (4 feet) mean that in essence we have "one" unit and receive wind on one face only?
See the attached sketch.






RE: Wind loads on rooftop equipment
RE: Wind loads on rooftop equipment
In your case, based on the aspect ratio of the gap, it might act as a single unit.
I am curious to see the responses.
RE: Wind loads on rooftop equipment
However, in this case, if one of the units is taken down, the framing that supports that unit would not see any increase in wind area.
The ASCE 7 commentary states that no research exists that would tell us what actually happens between physical elements like this.
Per WillisV, there could be some factor of gap width to wind pressure - if you take it to ridiculous extremes:
1" gap - I'm certain that the rear unit sees no windward wind.
50 ft gap - I'm certain that the rear unit sees 100% wind force.
In between gap - ??
RE: Wind loads on rooftop equipment
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Wind loads on rooftop equipment
As to what gap would I be comfy with before I started worrying about it personally, in absence of anything from code or testing? About 10ft, gut feeling, FWIW ;)
RE: Wind loads on rooftop equipment
If shared framing...who knows? Engineering judgment prevails so just document what you decide, why you decided it, and move on.
RE: Wind loads on rooftop equipment
We have, for the time being, settled on 100% wind on the windward unit and 50% wind on the shielded, leeward unit.
Still open to comments - just wondering if anyone else deals with or has dealt with this.
(Mike - we can't close off the gap - condensers need minimum space between for free air flow and the client needs free access)
RE: Wind loads on rooftop equipment
I would agree with your proposal, although I think it is likely a bit conservative.
BA
RE: Wind loads on rooftop equipment
I'd use a projected area of 16'x24' and apply + & - pressures to a cube shape which was 28' long in the wind's direction, with appropriate near edge pressure increases. Then for good measure I would take those code pressures and apply them to the faces perpendicular to the wind in the 4' gap, both + & -, but only on a strip around the edge of the gap, say (4' gap width)/2 = 2' wide at the top and the two sides, but maybe the bottom if wind can get under your support frame, thus you have a downward open "U" shaped loaded areas on the gap faces, or maybe a narrow four sided loading area.
If the condition of one of the units being removed is real, I agree that the wind loading won't change by much, but you better check to see that you don't literally have uplift on the unloaded legs under some wind load orientations. I think that is what a2mfk was suggesting.
I don't know that my approach is absolutely correct or not, but these damn codes these days and our general changing approach to engineering inhibits the use of any engineering judgement or common sense in the engineering process. We have more questions than answers if the code doesn't cover the condition explicitly. Does every condition need a wind tunnel test? Or are we allowed to use reasonable engineering judgement and experience at times, or will the codes ultimately cover every detail under every possible condition?
RE: Wind loads on rooftop equipment
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Wind loads on rooftop equipment
This partially addresses other questions ie.,
what happens with wind 10 to 20 deg off-perpendicular to the 24'side?.
Is there any wind tunnel effect with wind paralel to the 24' side?
RE: Wind loads on rooftop equipment
ISBN 0-7844-0262-0
"Wind Loads and Anchor Bolt Design for Petrochemical Facilities"
1997
RE: Wind loads on rooftop equipment
And don't forget the 1.9X multiplier in 6.5.15.1.
RE: Wind loads on rooftop equipment
Attachment would be predicated by treating them as individual smaller units.
Dik
RE: Wind loads on rooftop equipment
The framework below is the issue and the other comments above are very helpful - thanks all.
Other comments are still welcome. SAIL3 - I'll check out that book.
We will check the case with one unit on the near or far location alone.
RE: Wind loads on rooftop equipment
One thing that concerns me abit about the ASCE code and roof top equipment is how we define roof top equipment. Some of these roof top units aproach the sizes of small buildings and in some cases are larger than stair well towers. It would be nice if at some point in the future ASCE addresses some kind of size limitation. At what point do you say OK this is part of the building and apply the traditional leeward and windward wind loads. That said with the 1.9 multiplier noted above the roof top unit loading in many cases might be more conservative than the MRWRS calcs.
And yeah I was totally thinking about the unit and its connections and not the framing below.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Wind loads on rooftop equipment
See attached Values of shelter factor to reduce the net pressure coefficient
RE: Wind loads on rooftop equipment
interesting... do you have the other 26 charts or can you quote a source?
Dik
RE: Wind loads on rooftop equipment
RE: Wind loads on rooftop equipment
I though i did quote the source "BS". slickdeals got it right it's from British standard.
You will also find a simplify version of the reduction factor on EC-1-4 (Euro code) see attached.
RE: Wind loads on rooftop equipment
with h ≤ 60 ft".
This wind force then must have an adequate load path through the MWFRS. There is a provision for reducing the 1.9 factor down to 1.0 depending on the size of the AHU.