T-shaped canopy unbalanced wind load

[cdowney4]

I know this has been discussed before, but not sure there is a clear answer to me. For a T-shaped canopy, with single column in the middle and equal length cantilever beams in each direction, how do you determine the unbalanced wind load condition per ASCE 7? Using the figure for "Monoslope Free Roofs", since Zones 2 & 3 on either side of the column are equal and equidistant, you would not end up with an unbalanced loads if you are applying wind in only the positive direction or negative direction independently.

What do others do? Apply wind load on only one side of the column? Apply positive on one side of the columns and negative on the other side (seems overly conservative)?

I am not comfortable designing only due to unbalanced snow loads.

Also, for this type of canopy, if trying to do a quick feasibility calc, what would others use for a wind gust effect factor assuming flexible structure? Do you think Gf = 2.0 is too conservative?

 

[JAE]

With ASCE 7-10, could you use Figure 27.4-4 with θ = 0.
You get a Cnw factor on one half of +1.2 / -1.1 and a Cnl factor on the other half of 0.3 / -0.1 for "clear flow" conditions.



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[JLNJ]

I've always wondered about these. When you get a lot of deflection (like you probably will) it seems to me that it changes the aerodynamics of the canopy and it can catch more wind or act like a sail or airfoil. I don't imagine that this is considered in the ASCE 7 coefficients, but it might be.

These are also the first things that seem to fail in high winds (thinking of videos of gas station canopies tearing loose and blowing down the street).

 

[cdowney4]

JAE, thanks. I was looking at the monoslope roof table for C&C rather than MWFRS. Not sure how I flipped right past that one, but your redirection is what I needed.

Any opinion on the gust effect factor? Obviously once I do the design, it can be calculated, but I'm trying to do some quick feasibility on the effects of the supporting structure, not the canopy itself.

 

[JAE]

For G, I would tread carefully and perhaps use the flexible gust coefficient formulation in ASCE 7.
"T" shaped open canopies are generally flexible (based on gut instinct) so using G=0.85 may not be appropriate.

I wrote a spreadsheet to deal with the flexible-based G in ASCE 7 some time ago - and it isn't that bad to do...just takes some time.



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