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Wind Loads on Fences

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I am seeking clarification/advice on ASCE 7-05 Figure 6-20.

 

Scenario:  provide foundation calculations for a fence company.

Parameters:

               Fence is solid

Height (h=s):      6ft

                Length (L=B):     78 linear feet  (fence posts are spaced at 5ft o.c.)

                Exposure:            B

                Wind Speed:      90mph

               

My question has two parts:

 

First, Figure 6-20 specifies the loads be applied with an eccentricity of 0.05h and 0.2B.  Again, if I have a 78 linear feet (B) of 6ft (h=s) tall wall that gives me a point of application of 3.3 ft above grade at 54.6 ft (78/2+0.2*78).  This doesn’t It becomes impractical to design a fence with uniformly spaced posts for this load.  (I would even argue that it is impractical for the design of a cantilevered masonry fence wall where the “post” spacing isn’t the issue.) 

 

Secondly, if I have a 78 linear feet (B) of 6ft (s) tall wall that gives me a B/s aspect ratio of 13.  However, like most fences, I have posts embedded in foundations at 5ft o.c.  If I was to use B=5, then my B/s aspect ratio is less than 1.  (This results in an almost 50% reduction in the forces.) 

 

It is obvious Cases B & C specify an eccentric load to account for torsion or rotation implications on the sign and/or wall.  That makes sense in the case of billboard with one or two legs of support.  But for a continuous fence wall I don’t see applicability especially one with posts at 5 ft o.c.

 

I have not been able to uncover any commentary or clarifying language when it comes to ASCE 7-05 Figure 6-20.  Some building departments accept the logic for just Cases A & B, while a few others are enforcing Case C as well.  Anyone know of documents clarifying this situation?

 

Your feedback is greatly appreciated.

 

Respectfully,

 

Charles R. Ashley Jr., S.E.

Principal Engineer

(805) 545-0010 x111

(323) 744-0010 x111

www.ashleyvance.com