

July 7, 2020 at 9:12 amKhizerKhanSubscriber
Hello,
I am having some confusion regarding the physical properties of crosssection calculated by Designer Modeler of Ansys.
While moving the sketch around the 2 axis, the centroid and shear center coordinates are shifted as they should and are given with respect to the origin of sketch plane.
However, the Ixx, Iyy, J values do not change irrespective of where the sketch is placed around the axis (Shown in figures). Second moment of Area/ Torsional constants are calculated with respect of a given axis and if the axis changes, these parameters should change. I dont understand why these parameters are not changing. I intend to place my sketch of cross section about desired axes so that they parameters are calculated with respect to that axis.
Maybe ANSYS uses a fixed axis system for these parameters. It would be helpful even if that fixed axis is known, so that these cross sectional properties can easily be calculated using perpendicular axis theorems.
Any help on this would be appreciated. Regards
Ref:
https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v195/wb_dm/dm3dCrossSectUserDefined.html

July 7, 2020 at 12:38 pmKarthik RAdministrator
Please take a look at the following link and see if this answers your question.
Thanks.
Karthik

July 8, 2020 at 6:50 amKhizerKhanSubscriber
Thank you @Kremella for the suggestion.
I have already gone through the Physical properties section of Designer Modeler Help.
It mentions that the Ixx,Iyy etc are the 'Moment of inertia" about respective axes. In the Note below, it also mentions that moment of inertia is treated as the area moment of inertia.
I have 2 questions now.
Can the parameters (Ixx, Iyy etc) mentioned be treated as "Second moments of Area"? and
What is the reference axes from which these parameters are calculated?
Any help would be much appreciated.
Thanks

July 9, 2020 at 12:09 pmKarthik RAdministrator
Based on the information, yes  you should be able to treat Ixx and Iyy as the second moment of area. Also, these are computed with respect to X and Y axes respectively.
Thanks.
Karthik

July 11, 2020 at 10:19 amKhizerKhanSubscriber
Thank you for the response.
However, second part of the question is still not clear. I understand that these parameters are calculated with respect to X and Y axes respectively. However, these parameters should changed if complete sketch is translated/ rotated about some axis. Moment of inertia/ Areas depend on the distance of geometric points from the axis. Hence if we change the position of the geometry, these parameters should change.
However, as shown in the experiments of my first post, the moments do not seem to change irrespective where the geometry is placed around the coordinate system.
From what I understand, I have come to conclude that these moments are calculated with respect to "CENTROID" of the sketch, irrespective of where the geometry is place around the coordinate system.
Proof for the above conclusion is given by the fact that the Second moment of a rectangle is (b*h^3)/12, where b is the width parallel to axis and 'h' is the height perpendicular to the axis about which the second moment is being calculated. Therefore, from the examples given in my first post, the moments become (6*6^3)/12=108 m^4. Same is calculated by Ansys irrespective where the geometry is placed.
Thank you for all the responses. If anyone thinks otherwise or has any other comment, we can continue to discuss here.
Regards

July 15, 2020 at 10:42 amAniketAnsys Employee
KhizerKhan, Your understanding is right, the reference axes for these properties pass through the CG of the section. And is parallel to the global sketching plane axes.

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