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Round tube versus square tube stiffness

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  • Round tube versus square tube stiffness

    I am going to build a "C" shaped frame for an english wheel. I have 4" round or 4" square 1/4" wall thickness tube available. All things considered which choice would make a stiffer frame given a similar shape?

  • #2
    My guess would be the square.....I might be wrong but I'm still interested in hearing the correct answer too.

    John

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    • #3
      The stronger and stiffer material is the 3x3 square tubing.

      According to the Steel Tube Institute in their document Hollow Structural Sections Dimensions and Section Properties the 3x3 square tubing (pg 21) has a higher Moment of Inertia (corrected) and a higher Torsional Stiffness Constant than the 3x3 round tubing (pg 27).
      Last edited by TriHonu; 05-03-2010, 11:35 AM. Reason: Wrong property stated.

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      • #4
        Tube bending

        We bend a lot a tubing at work. It takes more hyd. pressure to bend squire tube and it has less spring back
        Visit my site for machinist videos free charts & more

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        • #5
          Originally posted by TriHonu
          The stronger and stiffer material is the 3x3 square tubing.

          According to the Steel Tube Institute in their document Hollow Structural Sections Dimensions and Section Properties the 3x3 square tubing (pg 21) has a higher Modulus of Elasticity and a higher Torsional Stiffness Constant than the 3x3 round tubing (pg 27).
          Thanks, and thanks for the reference. I've bookmarked that.

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          • #6
            Originally posted by 914Wilhelm
            Thanks, and thanks for the reference. I've bookmarked that.
            Glad I could help. I looked for a while trying to find the engineering data on square tubing. It was not in any of the books I have. I finally found it on that site along with a couple other documents in their Literature section. Lots of good information there.

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            • #7
              I'd also say square, if for no other reason that for equal diameter / width and equal wall thickness, the square tubing has almost 25% more cross sectional area than the round.

              Ian
              All of the gear, no idea...

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              • #8
                And what happens if you fill it with concrete- will it get even better?
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Originally posted by darryl
                  And what happens if you fill it with concrete- will it get even better?
                  Not so you'd notice. Filling hollow sections with concrete increases the mass and adds some damping but it adds little strength.and no perceptible rigidity no matter how intuitive the concept may seem.

                  The concrete filling will fracture at points of greatest structural flexture endng forever what small benefit it may have added when contiguous.

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                  • #10
                    Comparable span of square is much stronger for bending loads when you control the loading. But it is weaker when loads are not square to the cross section. That's why you hear terms like "force oriented the strong way" or "bending it the hard way".

                    Pound for pound, the same span of round is better when you don't control the loading, and stress can be applied from any direction. It also makes compound bends while retaining much of it's strength. That's part of the reason why it's used in roll cages and such.

                    For your application, for a simple open frame fabrication, square would be the obvious choice. However, a nicely formed/arched perimeter frame of round with a "strong back" of plate forming a shear plane would be FAR stronger, (IMO) far more aesthetically pleasing, easier to build (wheel and band roller) AND just as strong as square in a similar configuration. But the only way to quantify the differences would be to model it and run it through FEA.
                    Russ
                    Master Floor Sweeper

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                    • #11
                      Concrete can make a difference, because it can prevent buckling. But this requires a concrete that doesn't shrink (like normal one does), but one that swells during hardening.
                      Weight-wise, you'd better use a thicker wall. Improved damping doesn't help in your case.

                      Don't forget to close your tubes (weld end caps on). It also makes a difference if the tube is closed at bends (C-frame). The increase in stiffness is considerable!

                      Round tube is the most efficient considering weight, that's why you find it in roll cages.


                      Nick

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                      • #12
                        Design criteria

                        Originally posted by TriHonu
                        The stronger and stiffer material is the 3x3 square tubing.

                        According to the Steel Tube Institute in their document Hollow Structural Sections Dimensions and Section Properties the 3x3 square tubing (pg 21) has a higher Modulus of Elasticity and a higher Torsional Stiffness Constant than the 3x3 round tubing (pg 27).
                        I didn't see any mention of the modulus of elasticity (E) which is a function of the material irrespective of size or section.

                        I suggest that people have a look at Machinery's Handbook 27:
                        page 238 - Section Modulus (Z); Moment of Inertia (I); and Radius of Gyration (k)

                        page 253 - Moments of Inertia (I); Modulus of Elasticity (E) etc.

                        page 260 - Deflection of Beams

                        page 279 - curved beams (as in/for an "English Wheel").

                        etc.

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                        • #13
                          The issue of square tubing orientation came up when we designed this boom pole:


                          We decided to put the tube in the "weaker" orientation, and reinforce it with the round bar. That makes it more resistant to bending from side loads without having to add reinforcing. The tubing is 2" with 1/4" wall.

                          We didn't do any calculations to support our decision, but it seemed like a reasonable approach. Did we get it right?
                          Last edited by winchman; 05-03-2010, 04:57 AM.
                          Any products mentioned in my posts have been endorsed by their manufacturer.

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                          • #14
                            Resistance to bending has to do with the moment of inertia of the shape.

                            Moment of inertia is a measure of the distribution of mass away from the center.

                            It looks to me like the square is far superior in all orientations to the round. Yes, there are easy and hard ways to bend square tube, but all would seem to be stiffer than a round.

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                            • #15
                              New ball game

                              Once the beam/SHS/RHS/tube etc. is taken beyond its elastic limit or yield point all the equations as presented become null and void - especially as regards Modulus of Elasticity (E) and deflection (y) and "stiffness".

                              Even if it were annealed or normalised it may not necessarily have the same properties that it had when it was manufactured.

                              This is pretty well a case of "winging" it - ie try it and see.

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