(Crossposted to alt.home.repair and sci.engr.civil; followups to
alt.home.repair)

Hello all!

Someone I know is proposing to build a privacy fence in his backyard.
He's asked me for advice a couple of times - I'm not sure why, as I am
most definitely not a civil engineer, but I attempt to be helpful. I
have pretty much suggested following the plans in the "do it yourself"
type books - in brief, dig a hole every 6 to 8 feet, set a post (4x4
or 6x6 inch (10x10 or 15x15cm) pressure-treated lumber) with about 1/3
of its length in the ground, pour concrete down in the hole, wait for
the concrete to set, attach horizontal boards between the posts, then
attach vertical boards to the horizontal ones. He keeps coming up
with... "unique"... alternative designs that I sometimes attempt to
dissuade him from. There are two reasons I care at all: one, I might
help him install this fence, and I don't want to be associated with it
if it blows over in the spring and lands on somebody's house, and two,
*I* live in the same neighborhood and don't particularly want pieces of
this fence landing on _my_ house. My intent in posting here is to see
if I'm overly concerned, or if some of these ideas might actually work.

Anyway, the basic specs are a mostly-wooden privacy fence that is 6 to 8
feet (2 to 2.5 m) high, 8' being the highest allowed by local code. It
is enclosing a rectangular backyard. There will be three long sides;
the fourth side is mostly the house but there will be a couple of short
sections of fence on either side of the house. I haven't measured it
but I would guesstimate it to be around 250 linear feet (75 m) total,
plus or minus 50 feet (15 m) or so. The site is a former river flood
plain in Oklahoma; because of flood-control projects upstream it hasn't
been underwater for at least 50 years or so. I don't know any technical
details of the type or composition of the soil, other than I don't think
much fill dirt was brought in when the neighborhood was built about 40
years ago; everything is pretty much flat. The highest wind speed
recorded in the area has been about 55 mph (90 km/h) from the north-
northwest; this is a one-minute average and doesn't include higher gusts
associated with thunderstorms, tornadoes, etc.

Idea #1 was to use ready-made fence panels from the lumberyard, 8' high
and either 6' or 8' long. But instead of attaching them to 4x4 or 6x6
posts set in concrete, his idea was to attach them to T-posts driven into
the ground. T-posts work great for barbed wire fences and temporary crowd-
control fences; I've helped repair and build these kinds of fences before.
But in both cases, the posts don't have solid panels attached to them, but
rather barbed wire, rope, plastic banners, etc. In other words, the wind
load on the post is very small - each post might be supporting four
strands of wire, each eight feet long, with an average width per strand
of maybe 1/8" (3 mm), for a total of something like 0.3 ft^2 (0.03 m^2)
per post exposed to the wind. Compare this with 48 or 64 ft^2 (4.5 or
6 m^2) per post with the fence panels. It seems to me that the wind, if
perpendicular to the fence, would tend to overpower the T-posts, and
either bend them, break them, or push the the fence over, ripping the
posts out of the ground in the process.

Idea #2 is to use the ready-made panels again, 6' high this time and
either 6' or 8' long. For posts, this time, the idea is to use steel
pipe of at least a couple of inches diameter, driven an unspecified
length into the ground. No concrete or gravel around the pipe - just
pound it into the ground. The fence panels would be mounted with their
bottom edges 2' (0.6 m) off the ground, so their top edges would be at
the legal maximum of 8'. There would be a open area from the ground to
2' above the ground. The posts would extend about halfway up the panels,
or to about 5' (1.5 m) from the ground. The steel pipe sounds more
substantial to me that the T-posts, but I think that means that it
probably wouldn't bend or break in high winds - rather, the whole fence
would get pushed over, the pipes would remain straight, and their lower
ends would rip up out of the ground.

I tried to do some math on idea #2 based on the size of the fence
panels, the wind speed, the density of the air, and the diameter and
length of the post (assuming 5' above ground and 2.5' (0.75 m) below
ground). Basically, I am trying to turn the wind into a point load at
the top of the post, and then assuming that all of that load comes out
over a relatively small area somewhere else on the post. Based on what
happens when you horizontally wiggle a small post or stick that's stuck
in the ground, I figured that the bottom end of the post (underground)
wouldn't move, so you'd end up with a lever with the fulcrum at the
bottom end of the post, the wind pushing on the lever at the top of
the post, and the lever pushing on the soil at ground level. When doing
this, I come up with an odd unit of "pounds per second" which causes me
to doubt my "analysis" somewhat, but the numbers I am coming up with for
a horizontal load on the soil are on the order of 200-250 psi, which
seems to me like "too much" for the soil to take. (Assuming this line
of thinking isn't completely off base, I would be happier with an answer
down around 50 psi or even less. Reason: you can drive a pickup truck
or a car (30-50 psi) over an unimproved dirt road without beating the
road up too much, but much over that starts tearing up the road. I know
that this is vertical and not horizontal, but it at least provides some
kind of reference.)

Matt Roberds

wrote:

Hello all!

Hello m,

The highest wind speed recorded in the area has been about 55 mph...

Near the ground, with a 0.00256x55^2 = 7.7 psf wind load?

Idea #1 was to use ready-made fence panels from the lumberyard, 8' high and
either 6' or 8' long... his idea was to attach them to T-posts driven into
the ground... It seems to me that the wind, if perpendicular to the fence,
would tend to overpower the T-posts, and either bend them, break them,
or push the the fence over, ripping the posts out of the ground...

... The fence panels would be mounted with their bottom edges 2' (0.6 m)
off the ground, so their top edges would be at the legal maximum of 8'.

So wind can go under or over the panels. The force on each panel might be
7.7x6x8 = 372 pounds, and the moment might be 5x372 = 1859 ft-lb. This
sounds iffy to me. I can imagine 4 people pulling a T-post out of plumb
with 372 pounds of sideways force 5' above the ground. We might use 2
posts per panel or pound some stone into the ground around the post.

Nick

wrote in message
news:nTBFg.17196$PO.10937@dukeread03...
(Crossposted to alt.home.repair and sci.engr.civil; followups to
alt.home.repair)

Hello all!

novel deleted.

word of advice - not speaking for everyone here, but
many of us who may have wanted to help, are completely
turned off by posters who write war & peace epics and
expect to get a reply.

as the honorable Jack Webb used to say "just the
facts ma'am".

wrote in message
news:nTBFg.17196$PO.10937@dukeread03...
(Crossposted to alt.home.repair and sci.engr.civil; followups to
alt.home.repair)

Hello all!

Someone I know is proposing to build a privacy fence in his backyard.
He's asked me for advice a couple of times - I'm not sure why, as I am
most definitely not a civil engineer, but I attempt to be helpful. I
have pretty much suggested following the plans in the "do it yourself"
type books - in brief, dig a hole every 6 to 8 feet, set a post (4x4
or 6x6 inch (10x10 or 15x15cm) pressure-treated lumber) with about 1/3
of its length in the ground, pour concrete down in the hole, wait for
the concrete to set, attach horizontal boards between the posts, then
attach vertical boards to the horizontal ones. He keeps coming up
with... "unique"... alternative designs that I sometimes attempt to
dissuade him from. There are two reasons I care at all: one, I might
help him install this fence, and I don't want to be associated with it
if it blows over in the spring and lands on somebody's house, and two,
*I* live in the same neighborhood and don't particularly want pieces of
this fence landing on _my_ house. My intent in posting here is to see
if I'm overly concerned, or if some of these ideas might actually work.


I doubt that any of his other ideas would meet code, assuming OK requires a
building permit for fences as do most other areas of the country. I do
know, however that an old school friend of mine moved to OK because he felt
constrained by the rules of living in IL and said that OK left him to do
pretty much what he wanted to do. But that was back in the 70's...OK can't
still be the frontier it was then.

Tom G.

wrote:
(Crossposted to alt.home.repair and sci.engr.civil; followups to
alt.home.repair)

LIE!! NOt cross posted! It's multi-posted! And spam!

Reported

Splash1, you are definitely not speaking for most of us. "mroberd's"
detailed post is much more effective and efficient than the doofus who posts
"...how can I fix my garage". Then those folks desiring to help then need
to drag out additional details. Oh ya, for what it's worth, JACK, he did
include just the facts.

"Splash1" wrote in message
. ..

wrote in message
news:nTBFg.17196$PO.10937@dukeread03...
(Crossposted to alt.home.repair and sci.engr.civil; followups to
alt.home.repair)

Hello all!

novel deleted.

word of advice - not speaking for everyone here, but
many of us who may have wanted to help, are completely
turned off by posters who write war & peace epics and
expect to get a reply.

as the honorable Jack Webb used to say "just the
facts ma'am".