Hollow circular tube versus hollow rectangular shaft

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Discussion Overview

The discussion centers around the comparative bending resistance of a hollow circular tube versus a hollow rectangular shaft, both made from the same aluminum material and with the same thickness. The focus is on the theoretical aspects of stiffness and resistance to bending in different orientations.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • Sportnut questions which shape, a hollow circular tube or a hollow rectangular shaft, would be more resistant to bending under identical conditions.
  • One participant suggests that the circular pipe resists bending equally in all directions, while the square pipe is stronger in horizontal and vertical directions but weaker when bent diagonally.
  • Another participant argues that the square tube is stiffer because more material is positioned further from the centerline, providing a stiffness ratio that favors the square tube.
  • This same participant challenges the previous claim, stating that the square tube is also equally stiff for bending in any direction, although this may not be as apparent as it is for the circular tube.

Areas of Agreement / Disagreement

Participants express differing views on the bending resistance of the two shapes, with no consensus reached on which is definitively more resistant to bending.

Contextual Notes

Participants reference the second moments of area to discuss stiffness, indicating that mathematical considerations play a role in their arguments. The discussion does not resolve the implications of these calculations fully.

daniel wood
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Sportnut:
If I am using exact same aluminum material, with exact same thickness of gauge and I construct a 4ft long hollow circular tube that is 1" diameter, and I also construct a 4ft long hollow rectangular shaft that is 1" square, which would be more resistant to bending?
 
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Hard to say, but the circular pipe would resist bending equally in all directions. Whereas the square pipe would be great in horizontal and vertical directions but would quickly fail if you tried to bend it along a diagonal (making it diamond-shaped)
 
Thanks Bill. I appreciate your response.
 
The square tube would be stiffer because more of the material is further from the centerline.

The stiffness ratio (from the second moments of area) is (1/12) / (pi/64) = about 1.7

Post #2 is wrong. The square tube is also equally stiff for bending in any direction, though this is not so "obvious" as it is for a circular tube.
 

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