Parallel Axis Theorem- Composite Areas (STATICS)

[Minescrushessouls]

Homework Statement

Consider the beam shown in (Figure 1) . Suppose that a = 15 in. , b = 8 in. , c = 1 in., and d = 4 in.

Determine the moment of inertia for the beam's cross-sectional area about the x axis.

https://www.google.com/search?q=Det...iw=767&bih=740&dpr=1.25#imgrc=FBSvRrNrtgG_OM:

Homework Equations

Ix=Ix'+ACy^2

Where Cy is the centroid, so ΣACy=Σ(A*Cy) then ΣACy/ΣA=Cy

The Attempt at a Solution

I got Cy to be 1.86 inches

I then started to use the Ix expression:
(1/12)(x)(y)^3+(x)(y)(Cy)^2

I made sure to break it into squares and subtract values that overlap
ex: squares that's are 1x4, 1x13, and 1x8

I've done it two different ways and keep getting 109.8 in^4 as Ix and the homework system says that's not right.

 

[PhanthomJay]

I think you might be using the centroid of the cross section to calculate the distance to the neutral (centroidal) axis, but the problem wants the moment of inertia about the x-axis at the bottom of the cross section.

 

[Minescrushessouls]

You're probably 100 percent right. I didn't read the question well enough