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# Re: Wind Load on Wood Fence

• To: gehrlich(--nospam--at)nahb.com, seaint(--nospam--at)seaint.org
• Subject: Re: Wind Load on Wood Fence
• From: Rhkratzse(--nospam--at)aol.com
• Date: Thu, 18 Dec 2008 11:59:00 EST

Playing devil's advocate, let's say you design and build a 100' long fence per Case C, making the end 8' stronger, as the code apparently aliows.

Then someone tears down the end 10' or 20', then what?

I think that's why we design even zero-lot-line homes for lateral wind loads, in spite of their being sheltered by their neighbors.

I've heard that trees at the edge of a natural forest are somewhat shorter and stronger than those within the body of the forest.  It was "proven" by some experimenters who shook selected potted trees daily as they grew.

Thought you'd want to know.

Ralph

In a message dated 12/18/08 6:25:15 AM, gehrlich(--nospam--at)nahb.com writes:
Jeff & Ralph,

Interestingly, I recently calculated the wind load on an 8’ freestanding masonry fence, 90mph Exposure B, using ASCE 7-05 Section 6.5.14. The point force I got for the worst-case scenario at the end zone (first 8 feet) of a very long (100 foot+) fence translated to a uniform pressure of 25.5 psf.

For the record, I assumed G=0.85, Kd=0.85, and I=0.87.

Took me 2 or 3 days to generate a matrix duplicating Figure 6-20 Case C. Although a good portion of that was fiddling with Excel and figuring out how to create little drop-down menus I could use to change occupancy category, exposure, and other things.

Gary
Gary J. Ehrlich, PE
Program Manager, Structural Codes & Standards
National Association of Home Builders (NAHB)
1201 15th Street, NW, Washington, DC 20005
ph: 202-266-8545  or 800-368-5242 x8545
fax: 202-266-8369
gehrlich(--nospam--at)nahb.com
Attend the 2009 International Builders' Show
January 20-23, 2009, Las Vegas, NV

From: Jeff Smith [mailto:jeffsmith7(--nospam--at)comcast.net]
Sent: Wednesday, December 17, 2008 9:17 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wind Load on Wood Fence

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.

Jeff

From: Rhkratzse(--nospam--at)aol.com [mailto:Rhkratzse(--nospam--at)aol.com]
Sent: Wednesday, December 17, 2008 6:09 PM
To: jeffsmith7(--nospam--at)comcast.net; seaint(--nospam--at)seaint.org
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.

Ralph

In a message dated 12/17/08 6:04:37 PM, jeffsmith7(--nospam--at)comcast.net 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'.
Jeff

Sent: Wednesday, December 17, 2008 5:53 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wind Load on Wood Fence

Jeff,

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.

Regards