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Parallel Axis Theorem- Composite Areas (STATICS)

  1. Dec 4, 2016 #1
    1. The problem statement, all variables and given/known data
    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:

    2. Relevant equations
    Ix=Ix'+ACy^2

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

    3. 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 thats 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.
     
  2. jcsd
  3. Dec 4, 2016 #2

    PhanthomJay

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    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.
     
  4. Dec 4, 2016 #3
    You're probably 100 percent right. I didn't read the question well enough
     
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