# Help: Moment of inertia Ixx, Iyy, Izz, Ixy, Ixz, Iyz, etc?

1. May 14, 2010

### Diquan

Help: Moment of inertia Ixx, Iyy, Izz, Ixy, Ixz, Iyz, etc??

Hi all,

Can someone help with a few equations?, i need to know the moment of inertia of a section.

The section is a column which in the Z direction have 3m, on the X direction has 0.3m and on the Y direction has 0.6m.

I know that Ixx on one direction is b*h^3/12 and for the other side Iyy is b^3*h/12... but what the equation for Izz and rest of the equations? (like Ixy, Ixz, Iyz, etc)

Thank you

-
Diquan

2. May 14, 2010

### tiny-tim

Hi Diquan! Welcome to PF!

(try using the X2 tag just above the Reply box )

The moment of inertia of a body with constant cross-section depends only on the shape of the cross-section, so Izz wil be the same as for a rectangular plate 0.3m by 0.6m

And mixed terms such as Ixy are zero if x y and z are principal axes of the body (and every axis of symmetry is a principal axis ).

3. May 15, 2010

### pongo38

Re: Help: Moment of inertia Ixx, Iyy, Izz, Ixy, Ixz, Iyz, etc??

I think it would help you if you returned to the integral definitions: Ixx = integral y^2 da etc. Draw a diagram of the cross section, and you will be able to see how Ixx and Iyy relate to Izz