mass prop moments of inertia 1

[tmalinski]

When doing a simple deflection of a beam calculation using the machinery's handbook it's using "I" moments of inertia in the formula. so rather than calculating this longhand I construct the model in Solidworks and do a mass properties on it. It reports moments of inertia values, but I'm not sure which values to use. Does it matter where the model sketch is relative to the origin? here is the mass prop I got from t shaped beam

Center of mass: ( inches )
X = 0.00000000
Y = 0.00000000
Z = -5.00000000

Principal axes of inertia and principal moments of inertia: ( pounds * square inches )
Taken at the center of mass.
Ix = (0.00000000, 0.00000000, 1.00000000) Px = 27.74575243
Iy = (1.00000000, 0.00000000, 0.00000000) Py = 83.23725729
Iz = (0.00000000, 1.00000000, 0.00000000) Pz = 101.73442558

Moments of inertia: ( pounds * square inches )
Taken at the center of mass and aligned with the output coordinate system.
Lxx = 83.23725729 Lxy = 0.00000000 Lxz = 0.00000000
Lyx = 0.00000000 Lyy = 101.73442558 Lyz = 0.00000000
Lzx = 0.00000000 Lzy = 0.00000000 Lzz = 27.74575243

Moments of inertia: ( pounds * square inches )
Taken at the output coordinate system.
Ixx = 430.05916266 Ixy = 0.00000000 Ixz = 0.00000000
Iyx = 0.00000000 Iyy = 448.55633095 Iyz = 0.00000000
Izx = 0.00000000 Izy = 0.00000000 Izz = 27.74575243

also, when I construct the same shape as a region in autoCad, it's massprop moments values are totally different. I don't know which to use.

any guidence is appreciated

Tom

Tom Malinski

http://www.okayind.com/

Dell Prec 670, Xeon 3.8,2GB Ram, Nvidia Quadra FX 3450/4000 SDI
SWorks Premium 2008 SP 3.1 & PDMWorks

 

[normm]

I will like to add the following to the point raised by rb1957 as to the difference between centroid axis and neutral axis.

Centroid axis of a cross-section is a line such that the moment of the upper part of the area about the line is same as the moment of the lower part of the section area about the same line. It is a property of the cross-section and independent of the stresses in the section.

Neutral axis is a line such that that the forces on the cross-section (external and internal) above the line is exactly same as those below the line.

Where the cross-section is subjected to external applied force which is applied at an eccentricity to the centroid axis, the neutral axis is different to the centroid axis. In fact if the force is applied through the centroid, it will be under pure compression or tension -- there will be no neutral axis. But of course it will have a centroid axis as calculated from the cross-section geometry. Such situations are common in pre-stressed concrete, high-strength friction grip bolts, ground anchors etc.

In tmalinski's case the T-cross-section is asymmetric but as far as I make out has not got any axial compression. So the 2nd moment of inertia shoud be about the centroid axis, as mentioned by some others before me. This axis will also be the neutral axis.