round tube vs square tube

[Stomper]

This is a question on square tube vs round tube of the same wall thickness and the same size. If you had a three foot peice each of 1" round tube and 1" square tube of the same material and same wall thickness. they were both supported 4" from each end. Pressure was exerted in the middle of both. Which would fail first. This is just a general question for a small non buggy related project that I want to build.

[fabr]

The sq is stronger so long as the force is exerted parallel to the box tubes sides.When the force is from a corner then it is weaker since it will just collapse. Round tube has equal strength regardless of the direction force is applied from.

[Engineer]

Did you calculate that Fabr?  I am not so sure.  First thing is it's apples and oranges.  there is quite a bit more material in a 1 x 1 x .125wall square tube. (.4375 in^2)  Versus a 1" round x .125 wall tube which is (.343 in^2).

So you could have a heavier wall round to get the same weight per foot as a square.

I agree with Fabr that the angle of the force being applied is more critical on a square than a round.


The number that really defines the strength that you are talking about is the Moment of Inertia for a particular steel shape.  It quantifies the amount of material, and the distance it is from the bending axis.  The further the steel is from the center the more strength that it causes.  There are formulas in the Machinery Handbook for calculating it.  I will run the numbers on 4 sizes.  1-1/2 x 1-1/2 x .125 wall tubing sitting flat on a side, and sitting up on a corner.  Then 1-1/2 round x .125 wall, and 1-1/2" round x .155 wall which isn't a size but would weight the same per foot as the square tube.

Modulus for square on the flat edge = .218

Modulus for square sitting on corner = .218 Sorry Fabr!

Round with .125 wall = .128

Round with .155 wall = .150


Square is stronger.

There are many other factors that affect how a specific design will perform. 

[fabr]

LOL!!  I don't care what the numbers say for the square. I know from practical experience and I'm sure you have also that square takes more effort to bend when flat as compared to across the corners. If all we  are to consider is the force necessary to start deflection that may well be true but once the bend is started the square bent across the corners will become less strong very fast due to the rapid crushing flat of the square profile.

[Stomper]

I guess to clarify,  the project that I an building is a climbing tree stand for deer and moose hunting. There will not be any sharp impact on it. A buddy has a manufactured one built out of 1" square tube. I never measured the wall thickness but it"s thin and it is rated for 300 lbs. I was wanting to make the same design but out of 1" round tube with .083 wall. Because I have 1" tube dies. And I was wondering if it would be as strong as the square tube one. It would be a bit heavier but I don't have to carry it far.

[fabr]

fROM WHAT ENGINEER POSTED IT'S A NO question  THING.sQUARE IS MUCH STIFFER THAN ROUND. gd caps lock!

[Stomper]

All right then. I was wanting to get 1" square dies eventualy any way. I guess that settles it. I can get the dies, make the treestand, and still be $$ ahead.
Thanks guys for the info.

[Engineer]

LOL!!  I don't care what the numbers say for the square. I know from practical experience and I'm sure you have also that square takes more effort to bend when flat as compared to across the corners. If all we  are to consider is the force necessary to start deflection that may well be true but once the bend is started the square bent across the corners will become less strong very fast due to the rapid crushing flat of the square profile.

I agree with yah Fabr!  Especially when you get to a smaller wall thickness that can collapse easier.  There again the length and types of joints will matter.

I guess to clarify,  the project that I an building is a climbing tree stand for deer and moose hunting. There will not be any sharp impact on it. A buddy has a manufactured one built out of 1" square tube. I never measured the wall thickness but it"s thin and it is rated for 300 lbs. I was wanting to make the same design but out of 1" round tube with .083 wall. Because I have 1" tube dies. And I was wondering if it would be as strong as the square tube one. It would be a bit heavier but I don't have to carry it far.

Build it, Sit in it, If it feels spongy, add some braces. 

[trojan]

I see you can remember your maths when you want to :P

[Engineer]

This didn't involve CALCULUS. 

[SPEC]

All right then. I was wanting to get 1" square dies eventualy any way. I guess that settles it. I can get the dies, make the treestand, and still be $$ ahead.
Thanks guys for the info.

Where you at Stomper...I have a 1" sq. tube die

[Stomper]

Where you at Stomper...I have a 1" sq. tube die

Nothern Saskatchewan, Canada. You mean just to borrow ( not worth the hassle considering where I live. But thanks anyways)or do you want to sell them.