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RE: Wind Load on Wood Fence

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And you will need to apply the Gust Effect Factor as you will find that a fence is typically a flexible structure.  And don't forget that as your post to soil structure interface degrades with repetitive motions, the flexibility will increase requiring a bounding solution for the Gust Factor, at best.


Terrain factors will also be significant and you will want to consider the differing topography along the fence.  This is best accomplished as a dynamic solution considering the influence of one segment of fence against other sections.  I believe SAP has a "PushOver" methodology that, although designed for bridges, can also serve in the analysis of fences, particularly wood-framed fences.


Earthquake forces are not likely dominant but according to the code, you will need to at least consider them.  Especially if your fence is near a fault line, which will necessitate a geologic investigation.  I believe they can perform Cone Penetrometer Testing to determine suitable bearing pressures for the fence posts, but deep borings may be required to find fault lines.


Good luck with your fence.



Bob Garner, S.E.


From: Rhkratzse(--nospam--at) [mailto:Rhkratzse(--nospam--at)]
Sent: Wednesday, December 17, 2008 6:30 PM
To: seaint(--nospam--at)
Subject: Re: Wind Load on Wood Fence


Oh, I do, I do!!  My clients definitely hire me to perform fundamental research on such critical, essential, and vital, lifesaving structures as 6' fences.  You betcham. 

And I am sooooo glad that we have Powers That Be who feel it necessary to require us to jump through such hoops. 

I just picked the 25 out of the air, altho I do recall that a long time ago most of the residences I deigned used a wind pressure of 25 psf, without 50 pages of research to arrive at.


In a message dated 12/17/08 6:16:58 PM, jeffsmith7(--nospam--at) writes:

Ha! You don't want to calculate the fundemental frequency of a wood fence post and the gust effect factor?
25 psf for a fence in downtown SF seams pretty high.

From: Rhkratzse(--nospam--at) [mailto:Rhkratzse(--nospam--at)]
Sent: Wednesday, December 17, 2008 6:09 PM
To: jeffsmith7(--nospam--at); seaint(--nospam--at)
Subject: Re: Wind Load on Wood Fence

I'm with you Jeff.  I have a similar little project right now and a glance at 6.5.14 and its interminable references to references, etc., make me think it'll take as week and 50 pages and a Ph.D. just to get the pressure that used to be simply 25 psf.


In a message dated 12/17/08 6:04:37 PM, jeffsmith7(--nospam--at) writes:

I was hoping there might be a simplified procedure. The the 97 UBC allowed 75% of the normal force method for fences less than 12'.

From: Conrad Harrison [mailto:sch.tectonic(--nospam--at)]
Sent: Wednesday, December 17, 2008 5:53 PM
To: seaint(--nospam--at)
Subject: RE: Wind Load on Wood Fence

Seems the right approach to me. We use pressures on free standing walls for fences. If open then use the drag force coefficients for open framing ( AS1170.2). When using ASCE7-05 looks like need to go elsewhere for open framing drag forces.
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust

South Australia

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