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

[McQualude]

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.

[tecwhiz]

[Rob Stokes]

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

[todd]

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

[Lawrence A. Ramsey]

[todd]

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).

[Steve Knight]

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.

[Lew Hodgett]

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

[Fred the Red Shirt]

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.

[Juergen Hannappel]

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...

[Scott Cramer]

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.

[Swingman]

Then ask the question of the guy who got it to fit ... his IQ is obviously higher.

[Milo]

What if the round post is in a square hole?

[McQualude]

more does not always mean stronger

[McQualude]

Don't make too many assumptions, this is a simple question.

It is about real world application of fence posts.

[McQualude]

We're talking about fence posts.

[McQualude]

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.

[Bob Gramza]

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.

[McQualude]

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.

[Bay Area Dave]

maybe it's just easier to climb a round pole??

dave

Bob Gramza wrote:

stronger instead of

push into the ground.