Unsupported Precision Round Rail Calculator

[Jonathan]

Those sheets are great!

I've just found that SolidWorks calculates moments of inertia, including second moment of area. I just drew out the SBR20 rail (20mm, supported) to try and compare it to unsupported rail. Here's the results for just the aluminium support on the rail:

Section properties of the selected face of SBR-20 supported round rail

Area = 400.80 millimeters^2

Centroid relative to assembly origin: ( millimeters )
X = 22.50
Y = 5.85
Z = 0.00

Moments of inertia of the area, at the centroid: ( millimeters ^ 4 )
Lxx = 8228.45 Lxy = 0.00 Lxz = 0.00
Lyx = 0.00 Lyy = 41748.33 Lyz = 0.00
Lzx = 0.00 Lzy = 0.00 Lzz = 49976.78

X is along the width of the rail, Y is the height and Z is the length.
I for the 20mm steel bar is 7840mm^4 ... so how do I combine these two values? Solid edge will calculate them as one but of course doesn't take into account the different moduli(?) for aluminium and steel.

EDIT: I've just found out how to do it - 'Parallel axis theorem'. Why can't it be simple!