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# Re: Wind Loads on Fences

• To: seaint(--nospam--at)seaint.org
• 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,
If your fence is 78 feet, the total wind force applied at the 20% horizontal offset would be resisted by the one or two piers/footings closest to the force application and nothing is resisted by the outer dozen or so footings?
Rhetorical question.
Most municipalities don't require "engineering" for fences up to 6 feet in height for residences.  Regardless, taking a fairly logical approach would be to design the footing and fence segments for the 5 foot tributary width.  Not knowing what you're designing for or what the target clientele for this fence is makes it difficult to really comment.  But, it seems like a boiler plate, catch-all footing design you're doing.  Therefore, I'd design for the highest wind speeds and the crappiest soil conditions.

On Tue, Feb 22, 2011 at 1:59 PM, Charles R. Ashley Jr. wrote:

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?

Respectfully,

Charles R. Ashley Jr., S.E.

Principal Engineer

(805) 545-0010 x111

(323) 744-0010 x111

www.ashleyvance.com

--
David Topete, SE