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For a non-circular section, the quantity J appearing in the twist expression is

  • A.The same as the diametral second moment of area
  • B.A torsion constant, not the polar value
  • C.Always the polar second moment of area
  • D.Always zero

Answer: B

Only for a circular section do the two coincide. For any other shape the torsion constant is smaller, sometimes very much smaller, because warping relieves part of the section's ability to resist twist. Substituting the polar value for a non-circular section badly overstates the stiffness.

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