ASCE 7 Wind on Rooftop Equipment

[spats]

I'm looking at the ASCE 7-05 wind loads on a large rooftop cooling tower. I just noticed for the first time the provision specified in Section 6.5.15.1. It requires that you calculate forces on the unit per Eq. 6-28, AND THEN MULTIPLY THAT FORCE BY 1.9!! There is no explaination in the commentary as to why. I think this is ridiculous! Does anyone know where this provision comes from, and what the logic is? I'm coming up with 43 psf for a paltry 90 mph wind load, which puts an unbelievable wind force of 35,000 lbs. on the cooling tower.

 

[UcfSE]

The 1.9 factor is to modify the gust effect factor on the RTU to be higher.

 

[Archie264]

It's amazing, isn't it? The building codes are getting as convoluted as the tax codes and then after you've gone through the gyrations of applying every conceivable load factor with their Greek alphabet subscripts and superscripts, they then require you to just about double the value you've arrived at.

I wonder if the whole issue is a result of over-reliance on statistical models. For large firms that can devote many man-hours to meeting the constantly-changing code provisions some of the refinements might have some value, but for the small firms and sole-practitioners trying to get projects out the door some of the provisions have become quite onerous.

I guess my point is that if the gust around roof-top units can just about double the value of the applied load, then why the pretense of being able to refine the rest of the applied load so precisely using the various Rube Goldberg equations?

 

[hawkaz]

In ASCE7-10 (29.5.1) we get to start adding uplift to this as well

 

[JStephen]

Doesn't that 1.9 factor get reduced for larger areas of rooftop units?

 

[Ron]

I'll go back to one of my favorite sayings, modified to fit our process....

In design, the codes require that we measure with a micrometer....
In bidding, the contractor then marks it with a crayon.....
In construction, it gets cut with an axe......

I have so many code complaints I can't even keep up with them.

 

[spats]

The gust factor is 1.31. With the 1.9 multiplier, this results in an adjusted "gust factor" of 2.5! Yeah, sounds real reasonable to me.

How this provision could have been added without any explaination in the commentary is beyond me. I'm seriously considering ignoring it. I can just see a bunch of academicians, who have never designed a real structure in their life, sitting around a committee table deciding for the rest of us how we should design. If it wasn't for codes and committees, most of them would have nothing to justify their existance. My favorite (most hated) is the professor at the University of Florida who came up with ACI 318 Appendix D.

 

[ron9876]

Appendix D is the worst I have ever seen. "There is no problem with the procedures in use now but we want to make the code more nearly match the real failure modes".

As logical as the government most of the time.

 

[Archie264]

I'm just surprised practitioners haven't risen up with pitchforks and torches and stormed the towers of academia, or whencever these codes emanate. In addition to making the practice more difficult it also posses a moral hazard in that it's getting ever more difficult to understand the provisions or even their intent.

 

[steellion]

The wind provisions are way too complicated, but a multiplier for screen walls/mechanical units on low-rise roofs does make sense. Damage to screen walls and rooftop units (including blowing completely off of the building) is something that happens, especially in hurricane regions. I'm not sure how they arrived at 1.9, because that seems too conservative, but I'm certainly not going to ignore it.

 

[JedClampett]

Is everyone mad because of the presentation is confusing or that the loads are unreasonable? I'm not going to defend ASCE 7, it's like peeling an onion including the crying and the smell. But the 1.9 factor looks like a pretty close approximation of the zone 5 GCp from Figure 6-11A. I'd rather have a rooftop unit slightly overdesigned, than have it rolling off my roof. And as many have said, if it's a large cooling tower, the factor ramps down based on the area.

 

[JStephen]

35,000 lbs/ 43 psf = 814 square feet, or 28.5' square, so about the size of a house.

 

[Archie264]

Jed, I find them unnecessarily complicated and becoming increasingly worse at an alarming rate. And the most serious aspect of it is that it's gotten to the point that many provisions get lost in the shuffle. Note that the section in question was 6.5.15.1. How many subsections, subscripts, and cross-referenced charts do we have to go into to ferret it all out? And, as noted earlier, ASCE 7-10 has now revamped it all, doubling the length. And to what end? To force us all to buy the latest software packages to help us catch all the provisions? 'Twould seem so...

 

[spats]

The unit is 14' wide x 47'-9" long x 17'+ high, and it's being installed on an EXISTING BUILDING, which is part of the reason for concern. The building is fairly large. If I say B = 300' (it's actually more than that), h = 35', then 0.1Bh = 1050 sq. ft.. Even if I add the depth of the steel frame the unit sits on of 2.5', my area is 47.75' x 19.5' = 926 sq. ft., which is less than 0.1Bh, therefore NO REDUCTION OF THE 1.9 FACTOR! The unit has to be as big as the building to get to a 1.0 multiplier.

Jed, how do you figure (no pun intended)? Zone 4 & 5 GCp from Figure 6-11A is +0.7, -0.8. My GCp is 2.12!

 

[JedClampett]

I was looking at the smaller effective wind areas where Gcp equal 1.1 and 1.4 respectively. Also, a piece of equipment will have a windward and leeward exposure. So it's possible that the .7 positive is been added to the .9 negative.
As far as the other comments, I'll agree with with the criticisms that the wind (and seismic) aections are tough to get through. It does get easier with repetition. Going into something different (like rooftop units) is the challenge.
About 10 years ago, I asked one of the committee members why they were refining the wind from UBC, when there was no evidence of problems with existing designs. No real answer, except they had better information that they couldn't ignore.

 

[Archie264]

Hey, if the committee members have better information that they can't ignore, great, bring it forth. But if it's presented in such a way that only they can decipher then it's hard to see how it serves its intended purpose. What might make for interesting PhD dissertation papers does not necessarily make for useful information for the local practitioners who get stuck having to apply it while also trying to maintain a viable business enterprise. Just my $.02 worth...