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# Polar Moment of Inertia Incorrect for Beam Sections?

## Polar Moment of Inertia Incorrect for Beam Sections?

(OP)
Hello All,

I am sitting here scratching my head wondering if Mechanica's beam section polar moment of intertia (J) is incorrect.

For example, take a 2x2 inch square solid bar.  According to hand calculations J = 2.667 in^4.  Modeling the 2x2 solid bar in Pro/Engineer and analyzing the crossectional properties (Analysis -> Model -> XSection Mass Properties) it also comes up with 2.667 in ^4.

However, in Mechanica when you create a new beam section as square with an "a" dimension of 2 and then click "Review" the polar moment of inertia is displayed as 2.25 in^4.

Am I missing something here?  This looks to be wrong.  I know Mechanica has serious problems in calculating the J for custom sketched sections, but this is bizzare.

Any thoughts?

Steve

### RE: Polar Moment of Inertia Incorrect for Beam Sections?

(OP)
I think I just answered  my own question.  The J shown on the Review screen for the beam properties is only the polar moment of inertia if the cross section is circular.  For all non-circular crossections (including open sections) this value "J" is actually the torsional K factor for beams, which for non-circular sections is typically a fraction of the true polar moment of inertia J for the section.

It would be nice to have PTC make sure that the variable name is "K" not "J" for all sketched and non-circular beam cross sections.

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