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Re: Wind Loads on Fences
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- Subject: Re: Wind Loads on Fences
- From: David Topete <d.topete73(--nospam--at)gmail.com>
- Date: Fri, 25 Feb 2011 13:06:40 -0800
Charles,
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
--
David Topete, SE
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