Moment of Inertia for built up section (need help)

[jsu0512]

I'm getting different moment of inertia value between the hand calculation and the section property from Shapebuilder (IES) program.

I'm trying to determine the moment of inertia of built up section composed of two cold formed steel channel (2 x 600S162-54).

The section property of 600S162-52 is as below.

Moment of Inertia (weak-axis) (Iy) = 0.18 in^4
Area (A) = 0.557in^2
Shear Center (d) = -1.035 in

To manually calculate the moment of inertia for built up section, I sum the moment of inertia of each channel and the area multiplied by the square of distance between neutral axis of composite section and each channel.

I (composite section) = 2 x Iy + 2 x A x d^2 = 1.553 in^4

However, I'm getting, I = 0.55 in^4, moment of inertia for built up section from Shapebuilder program and can't figure out why..

I'd appreciate for your help on this.

 

[Ingenuity]

You are incorrectly using X from the shear centre (-1.035"), when you should be using X from the geometric centre of the channel section.

 

[jsu0512]

I see what the problem was. Thank you

 

[Standard American]

CAD can calculate section properties for you. Just draw the closed shape, convert it to a region and then use the massprop command. The properties will be based on where your origin is but it will tell you where the centroid is from your origin. I usually use the massprop command to figure out exactly where the origin is then use the UCS command to move the origin to the centroid then run the massprop command again. Just a note if you have a shape with a hole like a HSS you have to subtract out the hole with the subtract command from your region or else you get the section properties of the solid shape.