Section Modulus values

[dstein_TM]

I am conducting a
cross section analysis of my model to compare section moduli of a couple parts
through a hole. I click X-Section Mass Properties by means of
Analysis>Model and selected the datum plane that is through the center of
the hole. When I scroll to the bottom of the dialog box I noticed there
are 4 section moduli for the axis I am concerned about. One pair of these
is for the coord. system at the center of gravity of the model and the other is
at the center of the part on the bottom surface. I am using the pair of
values at the CoG, but I don

 

[Luis Aguirre]

Hi DS,


The first set of 4 inertia numbers you see are with respect the coordinate system of the model (ProE part CS). The second set of Inertia are with respect the area centroid of the part (Section CG). The difference between this two sets is nothing more that a coordinate translation of the inertia tensorfrom theProE part CS to the Section CoG CS.You canprove it to yourself by using the parallel axis theorem from one none rotated CS to another.As to which one to use it all depends what you are trying todo. I hope this helps!


Luis

 

[dstein_TM]

I tried recalculating the values using Section Modulus (S) = I/c where I is the Moment of Inertia and c is the surface distance to the center of gravity. The results from this calculation showed that the pair of values from the CoG section modulus related to how the force was applied to the model. One value was for when the part was loaded with a bending force downwards and the other was from when the bending load was applied in the opposite direction, or upwards. The reason that the first number wasn't the same direction was that the parts were modeled by different employees that used the original coordinate system datums differently. One extruded the part in one direction from the original datum, and the other employee extruded the part in the opposite direction at the beginning.

The part wasn't symmetric in shape perpendicular to the bend line, so the different section modulus numbers confused me. I think using S = I/c and recalculating the values helped me understand the direction of the forces better.