Of course math will answer the question although I suspect there is
some leg-pulling going on here. You want the fence post to be strong,
how, like so it doesn't fall over when a cow leans on the fence, right?
But you don't need math, you just need two brain cells to rub together.
If you start with a tree trunk that is round and square it then you
get a square post that is, at most, as wide accross the diagonals as
the diameter of the original tree. Now, suppose you want to reinforce
it. How would you do that? One way would be to nail extra boards to
all four sides of the post. OK, so take the four slabs you sawed off
to make it square and nail them back on.
The square beam might have a better strength to weight ratio but
who gives a damn? It's sitting in a hole in the ground, not on
If you want to maximize the strength to weight ration then saw the
tree into lumber and construct box beams for your posts. Try sug-
gesting that over on misc.rural. Heck you can drill a little hole
in the side and they can double for birdhouses.
What, you never heard of a designated dirt mortiser?
Yes, if you start with a larger tree why not just stick it in
the ground instead of spending money to square it and make it weaker?
OK, they want the poles to be reasonably uniform in cross section,
but seriously, if you start with an almost round pole (tree trunk)
then the strongest symetrical pole you can get which will also
involve removing the least wood, will be a pole with a circular
cross section tangent to the inside of the trunk.
+ + +
Given the typical size of posts they had better be small birds.
Also pretty industrious to fill it all up to just below the hole!
Wouldn't a cross be stronger than a box?
(The birdhouse could be on top)
That would take a lot of crap indeed. OTOH we have been getting
a lot in this ng lately.
If the post was same weight as a standard post but constructed
as a hollow box beam then it'd at least be big enough for blubirds.
No. A cruciform would not be stronger, again assuming the same weight.
Box beams concentrate material at the extreme fibers, the surfaces
at which the compressive and tensile forces will be maximum when
the beam bends. An I-beam would be better for a fence post than
a box, because the cow can only lean on it one way so you don't have
to worry much about bending parallel to the rails. THen you can
put the bird house on the top.
You are kidding? Making a square pole would require taking
a large round pole and removing wood. Why would they spend
money to shave wood off and make the pole smaller and less
strong? You do know that the poles start out round, right?
For a fence the choice is obvious: If the tree is thin: Make round
poles. If its to thick for that make pie shaped poles by splitting the
tree. If it's so thick that this still is no good idea use the
precious wood for something else.
On Wed, 12 Nov 2003 09:41:13 +0100, Juergen Hannappel
One thing that hasn't been mentioned is "boxing the heart"
If you're going to cut a square post or beam from a tree, and you want
the strength to be comparable to a round post, then you must include
the pith roughly in the centre of the sawn post. Ask any timber
framer. (And also avoid species that fail from the pith). If it's
a beam with a large bending moment in one direction alone, then moving
the pith around may be justifiable (even moving it out of the beam
altogether), but this loses strength considerably in the other
If the round post is simply turned from a square post of equal face
width, then clearly the square post is stronger. But this is by a
negligible amount in most directions, and the weight of the post is
greater (by nearly a third).
If the round post is turned into a square, it also loses strength, but
this is likely to be more than simple geometry suggests, as the rings
are now no longer continuous.
Die Gotterspammerung - Junkmail of the Gods
The unchanged tree trunk is stronger because the parts that would get
cut away for squaring add most to its moment of inertia (as shown in
another posting here) and also leaving the wood intact with the
outside of the last growth ring as the exposed face makes it less
prone to rot than the open grain of a sawn surface.
If you need to have it thinner (or wand to make more than one post
from a lenth of tree) try not to saw byt to split.
Too bad you didn't just say the same circumference. You would
have avoided all the "what do you mean by same size" questions.
If THAT were the question I'd say round has a slight advantage,
not in terms of taking a blow that breaks it in two, but the lack
of square corners begging to be dented and splintered makes
the entire structure more "sound". If you took the same amount
of material and shaped it with alot of delicate edges and thin
areas, the structure is weaker due to its shape, and the many
But if you're just wanting to know which shape would snap
under a destructive load test, you'll have to ask an engineer.
The software said it ran under Windows 98/NT/2000, or better.
So I installed it on Linux...
What dou you mean by "equivalent" in width? Same diameter as length
of side of the square? Then the square is stronger, because the
additional material in the corners adds considerably to the
geometrical moment of inertia.
If "equivalent" is meant that the same strength is reached there is no
The simple answer is round.
The reason is that for a given cross-sectional surface area, a square
has narrow spans and wide spans through the center. A circle only has
one span through the center (which is the diameter). If you don't know
what direction a load will be applied, then a circle gives you even
coverage in all directions, where a square is stronger along the
diagonals, but weaker than a circle of the same area, perpendicular to
On the other hand, if you know where your load forces are likely to
come from, you could orient square posts to fight this, and may come
Another consideration is that trees conveniently grow with a
near-circular cross-section. So for any given tree, making a square
out of its trunk is going to weaken it by removing significant amounts
of material. Ideally a fence-builder will have a supply of
appopriately sized tree trunks, that have been debarked, depending on
the type of fence being built.
As intuitive as this may seem to you, it is, quite simply, very wrong.
A square, with sides of 1 in. and cross-sectional area of 1 in^2, has a
sectional modulus of 0.333 in^3.
A round, with diameter of 1.128 in, has a cross-sectional area of 1
in^2, and a sectional modulus of 0.141 in^2.
That means that in bending, a square is over twice as strong as a
round of the same cross-sectional area.
Have you noticed that, for example when building frames out of steel,
square tubing is much preferred over round tubing, despite the fact
that round tubing is much cheaper for a given size?
Intuition often misleads, engineering calculations rarely do.
It doesn't matter much wheather it is square or round. What matters is
the grain of the post. When a piece of lumber is sawn to a square
shape, the saw cuts across grain lines. This causes a weakness along
those lines. Traditionally round posts would have been split from the
log. This split would have followed the grain producing a perfectly
strong post. Since the splitting process would not produce square
timber and people were not interested in doing extra work, the posts
were either roughly rounded or left as they were split. If you are
wondering about buying round or square posts from your local lumber
yard I would say it is a matter of what you think looks good. Both
were sawn from the tree, the round one just had the corners knocked
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