Components and Cladding Loads on a Canopy?

[structuralengr89]

I've got a job where an engineer is using component and cladding loads for a roof system for a canopy which is open and not attached to my building.

My understanding of C&C loads were that they were introduced into the code to prevent BUILDINGS from going from Enclosed to Partially Enclosed when windows, door, louvers, etc are blown out in a wind event and thus the corresponding increase in wind loading.

Why would you apply C&C loads to a canopy that is always open?

 

[JAE]

C&C wind loads are specific wind pressures associated with components of a structure that resist wind over small areas.

The wind pressure on any structure varies considerably, with peaks and valleys occurring in a highly dynamic manner.

An element that resists wind pressures over a small area is more likely, statistically, to have peak pressures applied over its entire area than an element with a larger tributary area where the pressures average down a bit.

As your tributary area grows, past 500 to 700 s.f., the C&C wind pressure reduces to that of the MWFRS wind pressures.

A smaller canopy, with less than 700 s.f., would be more likely to feel a peak pressure with C&C wind pressures.

 

[structuralengr89]

JAE,

What are your thoughts on reducing Iw to 0.85 (Occ Cat 1) on a free standing canopy outside of a building when your building is designed as Occ Cat II (Iw=1.0) It's not likely that the canopy will be used to protect people in a wind event.

I've got a free standing canopy where the arch specified the purlin spacing for some translucient roof system. Now the engineer for the roof system wants to reduce the purlin spacing because he's coming up with a wind loading of 165 psf.

He's using C&C loading, 140mph, Exp C Iw=1.0
Like you mentioned the wind trib area is very small<10 ft^2 so the C&C loading is extremely high.

I just think we are losing are minds on some of these winds loads. So what if a section or two of an outside canopy is knocked out in a CatIII-IV hurricane from debris. That is expected!

 

[JedClampett]

Well, if part of your canopy fails and blows across a highway, it culd cause an accident. Or it could catch on another structure and cause it to fail. If someone finds out you intentionally ignored the code, you're going to pay and pay dearly. If it fails and you followed the code, oh well, stuff happens.
I would review how he's getting 165 psf. Usually C and C for 140 mph tops out at about 117 psf and that's for Zone 3 only. Is he using obstructed conditions? Is it justified?
But you ignore the code at your own risk.

 

[Ron]

Agree with both JAE and JC....I do a lot of free standing canopy design and I use C&C loading for the very reasons JAE mentioned.

I'm not as concerned about the canopy blowing down during a storm as I am about it becoming windborne debris. I deal mostly with aluminum (these are not light gauge screen type extrusions...these are large special extrusions or standard structural sections), so its dead weight-to-sail area ratio is pretty low! Dead loads are typically less than 10 psf.

Also, check again on that 165psf. Something's not right there.

 

[a2mfk]

JAE- good summary of wind pressures

I did preliminary open canopy design in the Keys before and I think the worse case C and C was around 100psf, if memory serves me correct. I agree with everyone,165 psf seems very high.

Also agree with Ron, these canopies can become THE wind-borne debris.

Early in my career I did a rest area facility about 10 miles from the coast in central FL. The 2004 hurricanes went through there, our building was fine, several of the pre-eng open picnic canopies were on the ground...

 

[larsacious]

I believe that ASCE 7-05 has a whole new wind section devoted to open structures (based on the Australian Code). If you have a free-standing, open structure, then it might be more appropriate to use this section rather than C&C of the adjacent building.

Also, 165 psf seems high.

 

[StructuralHokie08]

I agree with everything that was said for using C&C on the roof panels. 165psf for Zone 3 is not high however. For a 10' x 10' with 10' eave height that is 1:12 pitch for a gable. 140mph Exp 3 I=1.0 classified as Open will be about 126psf for Zone 3 for Clear Wind flow and about 165psf for Obstructed Wind flow.

 

[a2mfk]

165psf is an extremely high wind pressure, though you may be correct that it is indeed the design value for that specific situation. I'd have to double check with ASCE, which is all we were calling attention to... But point taken.

So StructuralHokie08, do you have to pay Hokie66 a name licensing fee of an Aussie nickel every time you post?

 

[rscassar]

I use the c&c factor for open structure design. I had the opportunity to provide a condition assessment following a category 5 hurricane and was actually in a category 3 hurricane and I can assure you that windbourne debris can easily pierce wall framing.