I took the Thaddeus course a few years ago but, long story short, never took the SS test: SO I have a lot of study material without the virtue of his instruction.

In the Reference folder (if any of you also have the material) on page R-15, the Section Modulus and Moment of Inertia for a 2x8 (b=1.5 d=7.25) is given.

S=1.5x7.25^2/6 = 21.39in^3 <-----what Thaddeus gets 13.14 in^3 <---------what I get.

We agree for the beam rotated = 2.72in^3

Moment of Inertia:

I= 1.5x7.25^3/12 = 98.93 in^4 <------what Thaddeus gets 47.63 in^4 <------what I get.

Maybe a fluke. Except on the next page he does the same for a simplified wide flange:

overall depth = 10.25"

overall width = 7.25"

chords and web are all 1.5" wide

leaving a 7.25" deep x 2.875" wide negative space between top and bottom chords

I = I whole - I holes (negative space)

I = 7.25x10.25^3/12 - 2(2.875*7.25^3/12) = 635.40in^4 <-----what Thaddeus gets 468.02in^4 <--------what I get.

I/C is used to get the Section Modulus, which will obviously come out the same if you use his MoI.

What am I doing wrong here?

Edit: I just remembered that, for the 2x8, the Moment of Inertia and Section Modulus should be well documented and easily found -- so I Googled it. Turns out I'm right. Thaddeus's solutions are wrong, I reckon. Let me know if you discover something I'm not seeing. I'm going to assume the same for the wide flange above.