Is an awning or canopy a roof overhang? 1

[grantstructure]

I'm checking sealed wind load calcs from an awning manufacturer. When I calculated the loads in designing my building, I treated the canopy as a roof overhang under components and cladding. The engineer who did these calcs is treating it as...well, honestly, I can't tell. He seems to be only applying the adjacent wall load to the soffit of the canopy, and neglecting the suction on the top of the canopy. Certainly makes his number lower.

I just want a gut check on the way I'm doing the wind calc before I have to talk to the other engineer. Does anyone think that applying roof overhang provisions to a canopy under components and cladding is incorrect?

 

[ron9876]

I have always looked at this two different ways. One is to apply the positive stagnation pressure to the bottom and roof load to the top. Then I consider overhang loading.

ASCE7 doesn't cover this condition but I don't see how it could be justified to use anything less.

 

[Michel60]

Good morning grantstructure,
I'm with you, unless the awning has sufficient dimension and support to be treated as something of a separate open structure, to me it's an overhang and is subject to uplift accordingly. If it's a common fabric covered sun shade then there possibly could be limits on what kind of forces it could generate before the thing sheds it's skin but I wouldn't want to be making any random guess at what that might be.

regards,
Michel

 

[MiketheEngineer]

Put 20 psf on it and unless you are Florida - should cover it

 

[woodman88]

Under the 2009 IBC Section 3105.3 “Design and construction. Awnings and canopies shall be designed and constructed to withstand wind or other lateral loads and live loads as required by Chapter 16 with due allowance for shape, open construction and similar features that relieve the pressure or loads...”
Which places the design into a large gray area of whether you can and by how much you can reduce the Chapter 16 required loads.
When designing a building to support awnings and canopies, I use the loads I calculate for them. In reviewing sealed shop drawing for them, I made sure their reactions do not exceed my calculated reactions.
The design of the awnings and canopies are the responsibility of the designing/sealing engineer. If you are concerned about the design loads, talking to the client and/or building department usually gets you a “Is it seal by an engineer? If so, whats the problem?”
The only time we were asked to seal one, because the awning supplier did not have an Arizona seal, we had to tell the client that we could not get it to work per our loads and could not justify the suppliers loads. The client went with a supplier that could get an Arizona seal drawings.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Ron]

If it is attached to the building and hangs from the building, it is an awning and becomes an overhang. If it is free standing, it is an open canopy structure that may be influenced by the adjacent structure (snow buildup, etc).

 

[msquared48]

Ron ought to know. He has a lot of snow in Florida.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[grantstructure]

Thanks for the responses.

The calculations for this job are so bad as to make me want to report the engineer to the board. I'd post them if I could. I'm sure we've all seen it before, but it sure hacks me off to see the work of those who drag our profession down.

 

[Ron]

Hey Mike...it doesn't snow in Florida? You mean my designs have been.....conservative?

grantstructure....I do a fair amount of "delegated" engineering work. I only have two clients that I routinely do this for and both are willing to pay for appropriate engineering. Many of these companies will shop around for the cheapest engineering price they can find and will settle on the idiot who's willing to sign and seal the crap he comes up with. It's killing our profession.

 

[steellion]

I generally treat all canopies as open structures. For small canopies, I design the canopy to resist +/- 40 PSF wind load knowing that is plenty.

 

[grantstructure]

I think canopies might qualify as open structures too, but there's not really a good way to justify it, imho; if we're computing loads based on components and cladding, how can we use any other enclosure classification than that of the structure to which the canopy is attached? I'm talking about canopies completely supported by (cantilevering from) the structure.

Just had to have a heart-to-heart with the engineer about his calculations. Trigonometry errors. Errors in his spreadsheet leading to other errors. Wrong moment arm in force calculations. Plus which, he completely ignores compression in his hanger struts in the uplift case. The lackey that answered the phone was clearly afraid that I might be from the city building department.

 

[msquared48]

Ron:

That's one way to look at it...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

grantstructure...if the canopy is attached to the building, particularly a hanging canopy, it is usually not considered an open structure...it is an overhang, or if wide enough, a roof and an overhang. Struts have to be checked for compression for load reversal....it is often the controlling factor in higher wind areas.

 

[RareBugTX]

Grantstructure:

I would also consider it a canopy. If the other engineer's calculations do not make sense to you and are far less conservative than yours, I would just not let them distract me from my approach to the problem. For a canopy I always apply positive and negative pressures in a combination that puts the most stress to the structure. Dead, Snow, Wind, etc. I always remember that a lot of money is allocated to finishes and fancy stuff that are not a matter of safety in a building and they can not take precedence on something that may imply the life of health of somebody.

 

[msquared48]

These things are airfoils. Do not be conservative with your uplift forces.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

Sorry - I meant to say to be conservative with the uplift forces. If there is any doubt as to what it is, use the more restrictive case.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com