Engineering calcs: product moment of area

Discussion in 'Community Discussion' started by sammich, Mar 14, 2009.

1. sammich macrumors 601

Joined:
Sep 26, 2006
Location:
Sarcasmville.
#1
This one should be easy:

I've got to analyse a simple cross-section for an engineering course assignment. I have analysed part sections of the whole and now I need to use the parallel axis theory to 'shift' each section's properties to the centroid of the main structure. Now, I know I need to use the theory to move the 2nd moments Ixx and Iyy (IN = Ic + Ab^2), but how do I achieve this for Ixy? The structure is a 'I' beam cross-section that's symmetrical in x and y.

Sammich

2. silbeej macrumors 6502a

Joined:
Feb 3, 2007
#2
So what are you trying to find out? The "I" beam wouldn't fail along it's XY axis, since the X or Y would be weaker by themselves. If you are moving the moment of inertia it is just I + Ad^2.

3. sammich thread starter macrumors 601

Joined:
Sep 26, 2006
Location:
Sarcasmville.
#3
I need to find out the direct stress at a particular cross section of the beam when under stress from a moment.

To achieve this I need to find out Ixx, Iyy and Ixy for the cross section of the beam. I've got the moments of area for a segment of this 'I'-beam, but I need to be able to shift these moments to the centroid of the entire cross-section to analyse it as a whole.

How do I move the calculated Ixy to the centroid? I = I + Ad^2 makes sense for Ixx/Iyy since the distance is obvious. What is the 'distance' for Ixy?