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Wind Loads on Fence

Wind Loads on Fence

Wind Loads on Fence

(OP)
Any suggestions for designing wind load forces on a 6' aluminum fence (2x2 posts with (15) 5/8" square pickets)? I have a client that wants this fence engineered for 110 mph wind load according to the new Florida Building Code. Using ASCE 7-98, I ran numbers using the Trussed Tower coefficient but Cf comes out to 3.02 which gives me a pressure of 30psf on the solid area. This seems excessive, but what other coefficient should I use?

RE: Wind Loads on Fence

Does your client also want you to check it out for a falling palm tree, or a projectile at 110 mph too? Or maybe a runaway motor vehicle! IMO, seems a bit ridiculous or a least impractical for such a condition    See if the he/she is reallllly willing to pay for the "engineered fence" construction first before you get too bogged down into design and details.

RE: Wind Loads on Fence

(OP)
Ingenuity,
I share your sentiments. They (the building departments)have gone crazy with our new building code (and it's only been in effect since March 1). As far as I'm concerned it is ridiculous, but the manufacturer has asked me to check so it can get permitted in that city.

RE: Wind Loads on Fence

Wbruseski,

For a fence I would use the coefficient for open signs (Table 6-12)that varies from 1.6 to  2.0 for flat sided members.

I think the concern of the Florida Building Code is that the fence does not become a projectile during a hurricane.

AEF

RE: Wind Loads on Fence

Wbruseski...I would use the sign coefficients as well.  Since you are checking for 110 mph, that would imply you are not in a wind-borne debris area of Florida (those start at 120mph), so you should not be subject to those provisions of the code.

Unfortunately you are at the mercy of the local building official.  The new code is no different than the old in that respect!

A 2x2 post on a 6-foot fence might give you a problem with passing, unless it is thicker than the usual posts.  Even a 48-inch high handrail gets marginal for some of the coastal loadings.  Surprising how much sail area you can get from a slatted fence or handrail!

I was tasked with designing a rolling fence gate (8-feet high) for "a major Central Florida tourist attraction" a few years back.  The resulting loads were a bit shocking.  

RE: Wind Loads on Fence

(OP)
Thanks to all for your input. The open sign coefficient helps a lot but I did need to increase the thickness from 0.062 to min 0.120 inch thickness for the 2x2 post.

In regards to the concern about the projectile (dlew), I'm checking for allowable bending and shear stresses (with considerable factors of safety according to the code). These fences have posts with 2 feet of embedment surrounded by 6" dia. x 24" deep concrete. As far as I'm concerned these fence panels would never become "projectiles", they may just bend over a little.

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