Hollow circular tube versus hollow rectangular shaft

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SUMMARY

The discussion compares the bending resistance of a 4ft long hollow circular tube with a 1" diameter to a hollow rectangular shaft with a 1" square cross-section, both made from the same aluminum material and thickness. The circular tube exhibits uniform resistance to bending in all directions, while the rectangular shaft shows greater stiffness in horizontal and vertical orientations but is weaker when bent diagonally. The stiffness ratio calculated from the second moments of area indicates that the square tube is approximately 1.7 times stiffer than the circular tube.

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  • Understanding of material properties, specifically aluminum.
  • Knowledge of structural engineering concepts, particularly bending resistance.
  • Familiarity with the second moment of area calculations.
  • Basic principles of geometry related to shapes and their properties.
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  • Research the second moment of area for various shapes, including circular and rectangular tubes.
  • Explore material selection criteria for structural applications in engineering.
  • Learn about the effects of cross-sectional shape on stiffness and bending resistance.
  • Investigate real-world applications of hollow tubes in construction and manufacturing.
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Structural engineers, materials scientists, and anyone involved in the design and analysis of load-bearing structures will benefit from this discussion.

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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