Moment of Inertia, for Load along Z-Z axis?

[subbby]

Assuming that I have a C-Channel

When the load acts along the X-X axis, we take I_yy for the Bending moment calculations. Similarly, when load acts along the Y-Y axis, we take I_xx. But if i have a load that acts on Z-Z axis, how do I calculate the Moment of Inertia?

PLS SEE THE ATTACHMENTS in the following SEQUENCE

FIRST : Attchmnt : TOP VIEW
SECOND : Attchmnt : DETAIL A
THIRD : Attchmnt : ELEVATION

My specific Doubts:

1. Do I have to take I_zz?
2. If yes, is I_zz=I_xx+I_yy (applying Perpendicular Axes Theorem)

Can Anyone help me?

 

[PhanthomJay]

Loading along the z axis would be axial loading (P/A stresses, not bending stresses) except if the axial loading is eccentrically applied (I am not sure if it is) , then it would create weak axis bending stresses as well.

 

[pongo38]

Quote: Similarly, when load acts along the Y-Y axis, we take I_xx end quote

When load is along y-axis, this member will bend about xx but will also twist about zz because it is not being loaded through its shear centre. That involves Izz as well as Ixx/