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RE: Wind Loads on Fences

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Charles,

 

There is some language regarding Figure 6-20 on page 300 of ASCE 7-05, in the Chapter 6 commentary. It’s not particularly helpful, except to note that Case C was apparently based on some British wind-tunnel and full-scale studies of freestanding walls.

 

There are a few things you might be able to take advantage of to reduce your loads a bit. For starters, you’ll note from Figure 6-20 that if you add a perpendicular segment at each end of your wall you get a reduction over the first several feet; the return wall stiffens the corner. So, for example, a 6 foot return would give you a 25% reduction in the Cf values for Case C. NCMA’s TEK Note on masonry fences makes use of this, as it was the only way they could generate reasonable prescriptive designs.

 

In ASCE 7-10, the directionality factor table was revised to indicate that the Kd=0.85 factor could be applied to solid freestanding walls as well as solid (freestanding) signs. That would give you a 15% reduction in the loads.

 

Also, you could theoretically treat the fence as a Category I structure and use I=0.87 (or 0.77 in a hurricane-prone region). As a practical matter, many engineers would look at what buildings are adjacent to the fence and use the same category (e.g. use I=1.15 for a fence next to a hospital). However, ASCE 7 doesn’t explicitly require you to do so (nor does the IBC).

 

Regards,

Gary

 

GARY J. EHRLICH, P.E.

National Association of Home Builders

D 202 266 8545

gehrlich(--nospam--at)nahb.org

 

From: Charles R. Ashley Jr. [mailto:charles(--nospam--at)ashleyvance.com]
Sent: Tuesday, February 22, 2011 4:59 PM
To: seaint(--nospam--at)seaint.org
Subject: Wind Loads on Fences

 

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

www.ashleyvance.com