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

[McQualude]

Yes, I answered this question - meaning that the width of the wood makes no difference.

[todd]

[e-mail] (larry) wrote in news: [e-mail]:

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.

todd

[David Binkowski]

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.

[McQualude]

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.

[todd]

McQualude wrote in news:Xns9432E7CBE3C78mcqualude@

24.25.9.43:

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.

todd

[larry]

The integrity of the tree you are tying to maintain in this situation is the grain that runs from end to end in the tree. A tree trunk the size of a post would have all of the grain intact. A tree four times the size of the post split into four pieces would have the same unbroken grain end to end, like a handfull of straws. The cross section of the post is a secondary. So if you have two pieces of wood from the same tree with no grain runout,one round and one square in section and you assume that being the same size refers to cross sectional area, I would think they would be the same strength. From this point someone with a better knowledge of physics would have to take over. I just know wood.

[P van Rijckevorsel]

todd schreef

• + + 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 PvR

[Al Reid]

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?

-- Al Reid

"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

[todd]

"P van Rijckevorsel" wrote in news:3fb4cb92$0$188$ [e-mail]:

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.

todd

[todd]

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").

todd

[Charles Krug]

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 corner posts.

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.

[P van Rijckevorsel]

Al Reid schreef

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. PvR

[P van Rijckevorsel]

todd schreef in

• + + That is a good point, but still assumes posts of sufficient diameter that this outer part is free of juvenile wood. PvR

[Lawrence Wasserman]

Is this post for a fence or a ... MAILBOX?

[A Dog Named Stain]

DUH, everyone knows you use steel, concrete and brick for a mailbox. That way any mail box bashers get their arms broken or better yet, die in a fiery crash.

Please pass the popcorn.

[McQualude]

You still made the same assumption that the square post contains more wood, even after I've told you twice that isn't true.

[todd]

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.

todd

[Michael Daly]

I think we should treat him as a troll. I've already asked him to clarify the width issue and all he does is blather some more. It would be trivial to clarify, but he just wants to be a PITA.

Mike

[Jon Dough]

Some people just can't see the forest for the trees... :) A 4" square post would be stronger than a 4" round post. The square post would be almost the equivilent of nailing a 2"x4" board of the same material to the round post. The only case I know of where a round post was stronger than a square post is the corner post my Father set when I was a teenager. It was about 8" diameter and about 9' long, fresh cut from a hedge tree and set in the spring of the year. Being fresh cut, it took root and grew, now is about a 16" diameter tree over 25 feet tall. It is still round though. It never did grow square.

[McQualude]

The question has already been answered, thanks anyway.