ASCE7-05 wind loads on free standing wall

[dcarr82775]

For a free standing wall ASCE7-05 gives 3 load cases: A, B, and C in Figure 6-20. I have a free standing CMU wall. In my way of thinking load cases A and B apply, but I do not believe C applies to a free standing wall on the ground. Primarily given the wind direction indicted in the Figure. C is intended to give a strong torsion for non orthogonal winds which makes since for elevated signs, but not walls IMO. For my wall it is the difference between 18 psf and 30 psf peak wind load.

What have others done for free standing walls?

 

[PSEPK]

Yes. Case C results in much higher wind pressure, compared to cases A and B; although Note 4 provides some relief. IMHO, Case C has to be considered for freestanding walls, keeping view Note 3 under Figure 6-20. I recently designed brick masonry and CMU freestanding walls, both applying Case C as well. It really made the walls very costly.

 

[JBASE]

Nothing in ASCE 7 indicates that it is not for free standing walls. Many cities in Southern California are enforcing this provision.

 

[azcats]

I think Case C is bunk (for typical screen walls) and I'd have a hard time using it for their design. Thankfully I don't design them. OK, maybe not bunk, but a complete over-reach from a practical design perspective.

Some municipalities (Clark County & Las Vegas, NV) have amended Case B & C out of the code for screen walls up to 10' high.

Add a new exception #7 to Subsection 1609.1.1, as follows:
Exceptions:
(...)
7. Solid and freestanding walls up to and including 10’-0” above the highest adjacent
grade and designed using the provisions of ASCE 7 section 29.4.1 need only consider
CASE A of Figure 29.4-1 with a Cf factor equal to 1.40 and the resultant loading applied
at the geometric center of the wall surfaces subject to the design wind loading.

Southern Nevada 2012 IBC Amendments

 

[dcarr82775]

Azcats,

That makes sense to me. I went through the exercise of Case B, and if I equate the eccentric load to a moment and back out a pressure from that I get an average pressure at one end in the ballpark of a Case C. So I guess I will go with B/C for my 7ft tall CMU wall.....oye

 

[azcats]

Did you also note in the 'CROSS-SECTION VIEW' that when s/h = 1, you raise the application of 'F' by an additional 5% the height of wall?

And (just for my curiosity) how are you breaking case B down into a pressure? When I do it for signs, I equate it to 5/3 x 'case a pressure' over 60% of the width of the sign. That yields the same eccentricity.

I've also found that on a sign, Case C yields similar total load eccentricities to Case B. Such as a single column supporting a sign the loads on the column are similar between case b & c.

 

[dcarr82775]

I did an P/A + M/S type of application for Case B. Then I took an average pressure over the last length of wall for my design. It was nice ot see that B and C gave similar results, but that of course makes me wonder why we have both.