Some More Equation Confusion
Posted: Sun Feb 14, 2016 10:32 am
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.
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.