Wood Question: Which is stronger, a round post or square post?

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(Fred the Red Shirt) said:

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 your foot.
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.
--

FF

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+ + + 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) PvR
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schreef

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.
--

FF

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(Fred the Red Shirt) said:

i'd like to see that mortiser

problem.
etc,
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Bob Gramza wrote:

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?
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[...]

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.
--
Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
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Juergen Hannappel wrote:

Round is the strongest shape in nature. It resists pressure equally in all directions. That's why a submarine's hull is round in cross section.
--
Mortimer Schnerd, RN

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On Wed, 12 Nov 2003 10:25:33 GMT, "Mortimer Schnerd, RN"

Which is important in keeping your fence posts from imploding. ;-)
Greg
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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 directions.
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
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Juergen Hannappel wrote:

Absolutely!
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[...]

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.
--
Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
mailto: snipped-for-privacy@physik.uni-bonn.de Phone: +49 228 73 2447 FAX ... 7869
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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 vulnerable points.
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.
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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 difference...
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So that no advantage is gained by having more wood. This is a simple question.

I don't believe that. Fred answered the same way I did... that a post turned from a tree would have greater strength than a square post.
--
McQualude

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What if the round post is in a square hole?

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Then ask the question of the guy who got it to fit ... his IQ is obviously higher.
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Don't make too many assumptions, this is a simple question.
It is about real world application of fence posts.
--
McQualude

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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 the edges.
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 out ahead.
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.
-Mike
-Mike

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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.
Kelly
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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 off.
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