NCARB Practice Problem

[k4dm0nky]

I can't figure this out...

I broke down the shape to individual pieces and got .5 + .5 + 42.7 = 43.7 / 5 ... and that does not equal the question in the back of the guide? What am I doing wrong? Testing tommorow and feel like I am up $hits creek without a paddle!!!!

[Mike-SE]

I have no idea what your numbers represent.
This problem is easy BUT you can screw up similar problems if you do not understand.
The shape is symmetrical about the X axis. So you can take the I of the 6" x 8" rectangle and subtract the I of the two 2.5" x 8" rectangles.
IF it were not symmetrical it would be way more difficult, too difficult for the ARE.
All the best tomorrow. Let me know what time you test and I will pray for you at the start and end.

[k4dm0nky]

Thanks Mike! I test at 2pm!

I guess I know how to do it for a rectangles

What I did was separated the flanges from the web for Sx and then summed them all up.

Top Flange
6(1)^3 / 12 = .5

Bottom Flange
6(1)^3 / 12 = .5

Web
1(8)^3 / 12 = 42.7

Total Sum = 43.7

42.7 +.5 +.5 = 43.7 / 5 (centroid) = 8.7

but that is wrong. I don't know why that way. I got the way you did it so if I see it tomorrow I will be doing it that way. Makes sense. Thanks again Mike as always! God bless

[acadia]

Why don't we subtract the two 2.5 rectangles from the overall 10x6?
K4dm0nky good luck!

[jaraga]

I know exactly how you feel since I was making the same "mistake". I did that because based on KAPLAN's study material you have to do it that way (that is, in pieces, so small rec 1st, then 2nd then 3rd, finally add'em up)... I was just as frustrated and asked around in the office and no one even knew what the hell a section modulus was... go figure...
But doing it the way it was proposed here, it makes sense.