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

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I found this question in another group (misc.rural)... Which is stronger, a round post or square post? Assume the posts are both made from the same wood and are both equivalent in width.
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McQualude

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the square post. more wood!

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more does not always mean stronger
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McQualude

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wrote:

Yes it does (in this case). It may not be _efficient_ (strength / weight will go down) but it isn't going to make it weaker (for a simple solid post, placed vertically) -- Die Gotterspammerung - Junkmail of the Gods
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Depends on the load, but I'm assuming you mean under deflection. If so, the square post is stronger as it has a larger surface area associated to the top and bottom chords. This assumes all corners have been broken for stress concentration relief. If you mean a load under tension, it's strictly a matter of cross sectional area (assuming a consistent modulus of elasticity across the section) thus the square section again would win...as it would under compression.
Rob
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Have to be a little careful with compression of a post. A lot depends on the support conditions of the column, but you don't have to get a very slender column (i.e. long in relation to cross-section) before buckling becomes the primary failure mode. In this case, moment of intertia, not cross-sectional area, will be the deciding factor. It still favors a square cross-section over a round one, just not as much as the cross-sectional area alone would lead you to believe.
todd (gotta use that materials engineering education for something these days).
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As another poster indicated, assuming we're talking about bending, the moment of intertia of a square cross-section is L^4/12. For a circular cross-section, it's pi*D^4/64. I had to do the calculations three times to convince myself that this is correct, but all things being equal, a square post is 70% stronger than a round one when subjected to bending (this is assuming that the diameter of the round post is equal to the sides of the square cross-section). Keep in mind that the square has 27% more cross-section to start with, though. Perhaps a fairer comparison would be to compare equivalent cross-sectional areas. If equal cross-sectional areas are assumed, the square cross-section is about 5% stronger.
todd
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If it is made of bois de arc, it doesn't matter. It won't break.
On Mon, 10 Nov 2003 21:25:28 -0600, "todd"

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On 10 Nov 2003, todd spake unto rec.woodworking:

    I had a moment of inertia shortly after finishing my lunch, but then I went back to work.
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well if you measure corner to corner and the round post is the same in that measurement it would be stronger. across the square post would be.
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McQualude writes:

Under what conditions?
Compression loading as in a column?
The bending loading on a post at ground level that is buried a couple of feet in the ground?
You have to define the problem first.
HTH
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We're talking about fence posts.
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McQualude

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He's asking valid questions. Fence posts, if braced, can be under tension (rigid brace) or compression (wire brace). A long or high fence needs to be braced. A corner post can be loaded in two directions simultaneously. They can be bent as cantilevered out of the ground or bent at the middle, braced at top and bottom.
We still haven't seen a decent definition of what "equivalent width" means, only a vague statement.
Mike
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Yes, I answered this question - meaning that the width of the wood makes no difference.
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McQualude

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Answering a different question, here but:
If you start with a tree trunk which is typically what you start with for a fence post, it will be stronger if you leave it round than if you square it up.
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FF

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snipped-for-privacy@spamcop.net (Fred the Red Shirt) said:

No, that is the question. I gave the same answer you gave, but not quite as clearly perhaps. I was hoping that someone would be able to support it, because I have no proof and I don't think math will answer this question.
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(Fred the Red Shirt) said: : : >> I found this question in another group (misc.rural)... : >> Which is stronger, a round post or square post? Assume the : >> posts are both made from the same wood and are both equivalent : >> in width. : : > Answering a different question, here but: : : > If you start with a tree trunk which is typically what you start : > with for a fence post, it will be stronger if you leave it round : > than if you square it up. : : No, that is the question. I gave the same answer you gave, but not : quite as clearly perhaps. I was hoping that someone would be able to : support it, because I have no proof and I don't think math will : answer this question. : -- : McQualude
I would think that utilities would use square telephone poles if they were stronger instead of leaving them round. They spend a lot of money and resources on the poles they push into the ground.
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maybe it's just easier to climb a round pole??
dave
Bob Gramza wrote:

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(Fred the Red Shirt) said:

Apparently, you've never tried to drill a square hole in the ground. ;-)
From an engineering mechanics point of view, this is a very simple problem. However, it probably breaks down for utilities on the basis of cost, i.e., it's just cheaper to get a round pole of similar strength than a square pole. There are probably a hundred other reasons that make round poles more workable (easier to climb with spikes, don't have to be oriented any particular way, insulator bases are designed with round poles in mind, etc, etc).
todd
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todd wrote:

I think it boils down to economy and simplicity. You have to start with a much larger tree to get a square pole with the same strength of a round pole. And, you don't need to send it through a sawmill or buy larger trees. Simplicity depends on the type of wood. In the west, lots of poles are lodgepole pine which grows straight with a long length that changes very little in width.
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