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RE: seaint Digest for 23 Feb 2011

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Thanks Gary, I really appreciate your response.

I guess I will have to just get used to telling clients "yes, in fact, a 6ft
tall wood fence does require a 4ft to 6ft footing".  And "no I can't explain
how all fences designed prior to ASCE 7-05 are still standing".  


Charles R. Ashley Jr., S.E.
Principal Engineer
(805) 545-0010 x111
(323) 744-0010 x111

-----Original Message-----

Subject: RE: Wind Loads on Fences
From: "Ehrlich, Gary" <gehrlich(--nospam--at)>
To: <seaint(--nospam--at)>


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

In ASCE 7-10, the directionality factor table was revised to indicate
that the Kd=3D0.85 factor could be applied to solid freestanding walls =
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=3D0.87 (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=3D1.15 for a fence next to a
hospital). However, ASCE 7 doesn't explicitly require you to do so (nor
does the IBC).



National Association of Home Builders
D 202 266 8545

From: Charles R. Ashley Jr. [mailto:charles(--nospam--at)]=20
Sent: Tuesday, February 22, 2011 4:59 PM
To: seaint(--nospam--at)
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.
               Fence is solid
	Height (h=3Ds):      6ft
                Length (L=3DB):     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=3Ds)
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=3D5, 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.


Charles R. Ashley Jr., S.E.
Principal Engineer
(805) 545-0010 x111
(323) 744-0010 x111

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