Well, I guess you should be surprised. You must be one of those folks who
doesn't let a little thing like physics get in the way of what you think is
right. Maybe you want to enlighten us on how wood resists bending in a
different way conceptually than other materials. Until then, crawl back
into your hole and leave the heavy lifting to someone who has both the
theoretical and practical experience. No material is going to match the
formulas exactly. Wood complicates matters further because it is
anisotropic (that means it has different physical properties in different
directions). However, the formulas (these aren't just theories, by the
way, much less *my* theories) will still give good guidance in answering a
question such as the one posted originally.
I'm listening, explain why a round or square wooden fence post is
stronger if the width of the post is not a factor.
I, and one or two others, postulated that the round post would be
stronger because it maintains the integrity of the tree (assuming the
post started as a whole tree). No engineers have responded to rebut
any of those comments.
Since you're assuming that the round post has the center of the tree as
it's center, I'll assume that the square post is the same. Therefore, the
square post contains all of the material of the round post, plus the
material in the square corners. Unless you can explain how added material
in the corners weakens the post, consider your postulate rebutted.
Note I haven't spent much time talking about whether a square post is an
efficient means of making a post (though I did touch on this point a couple
of posts ago). You could probably make an argument of why the hell are we
talking about this anyway. How much load is a fence post likely to see in
the first place? I answered a general question with a general answer. We
would have to have a *lot* more information to arrive an an in-depth
answer. A notable example would be what would the failure mode of a wooden
post in bending be? It could fail in tension (at the top of the post
(opposite side to the direction of load), in compression at the bottom of
the post, or in shear in the center. You could make a case for being
concerned about the shear strength of a post with the center of the tree
(not an area known for high strength) being located at the area of highest
shear stress. Unless you're prepared to enter into such a discussion, I'd
leave the postulating alone.
+ + +
From a practical point of view this assumption is not realistic.
A round post is likely to be 'whole tree'
A square post is likely to be sawn from a bigger trunk.
This means that a round post consists partly of juvenile wood, a weakening
factor. A square post will consist of mature wood.
Ergo: the strength of the material will not be equal in these two cases. It
will be less in a round post
Ah, but your assumption is that the square post has been cut from the center of
the tree and contains
only heartwood. How do you know it wasn't cut from a section of the tree sear
the growth rings and
thus contains both sapwood and heartwood?
"It ain't what you don't know that gets you into trouble. It's what you know
for sure that just ain't so." --- Mark Twain
center of the tree and contains only heartwood.
+ + +
Just the reverse. If a tree is sawn up into square posts only one of these
will have 'heartwood'. In a perfect world this would be thrown out and not
used as a pole.
I'm telling that from a practical point of view, you don't have the first
clue how the post will fail. (For the record, I've already stated that I
don't know what the compression strength, tensile strength, and shear
strength is for various woods, although I'm sure I could google it if I was
so inclined). Your assumption regarding the strength of juvenile only
matters if shear is the primary failure mode. If the primary failure mode
is tension or compression, then it doesn't really matter what's happening
in the center.
For posts made of a suitable wood in suitable size, the "usual" failure
is that it rots at the ground line.
If your post fails before it rots, it was too small.
Usually, you string the wire on the inside of the line posts, so that an
animal pushing against it doesn't pull the staples, and around the
There are as many ways to brace corners as there are fence builders. My
preferred method was to have the line posts at 16', except at the
corners, where I'd have the first line post 8' away with a horizontal
member in compression, and a wire tensioner making SURE it STAYS in
compression. This was woven wire for sheep, which carries a good bit
more total tension than barbed wire for cows. Dairy farmers got away
with smaller corner posts and less substantial bracing, but they needed
twice as many line posts.
Had a neighbor who had sheep AND a commercial cabinet shop. His fences
were made from "scraps." It didn't hold sheep in (more importantly dogs
out) any better than our fence did, but DAMN did it look nice.
I suppose that sounds good, but I don't think in practice it's really going
to have a very large effect at all, if any. In the worst case, if we
assumed that the shear strength between growth rings was very low, you might
be able to argue that it really wouldn't help, but I don't see how it would
hurt. This isn't like machining a notch into the additional area that would
create a stress concentration, which could actually produce an overall
weaker member (heh, heh....he said "member").
So far, the sum total of your information from what I see looking back in
the thread regarding the dimensions of the post has included the following
"both equivalent in width" and "the width of the wood makes no difference".
Personally, I don't have the faintest idea what the hell you mean by either
of those. I've defined the assumptions I've made pretty clearly. If you'd
care to be specific, I'd tailor my comments to your specific case.
It was a simple question that several people already posted good
answers for. As I stated in the first post, I lifted the question
from another group because I thought a bunch of woodworkers might
have good answers, I gave you all the information that was available
in the original post. The original poster did not answer any
questions that were in the original thread, so I gave you all the
information that I had. I'm not sure how this makes me a troll, but
you can go fuck yourself.
What a lovely sentiment.
Why didn't you just answer "I don't have a clue, I just copied the question"
instead of giving ambiguous and vague answers about width. "Width doesn't
matter" is an irrelevant comment.
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