Sample problems 1, 2 & 3 of part 2 AISC 8th ed. 2

[fxp]

I need help. On sample problems #1 of AISC Manual, 8th ed. Part 2, I am clear up to the part where it talks about Fy' - " A check of the Fy' column shows a dash, indicating that Fy' is greater than 65 ksi. Therefore, the shape is compact and the assumed allowable stress of Fb=0.66Fy is correct." I get Fy'= 88ksi which is > 65ksi, but where did 65ksi come from?

On sample problem 2 why did they use the formula Fb=0.60Fy?

On sample problem 3 I am clear up to the part where it says "Since Fy' is less than 50 ksi, the shape is non-compact due to flange criteria. Therefore, the allowable stress is less than 0.66Fy and must be determined from formula(1.5-5a)..." Where did 50ksi come from and why does that make the shape non-compact?

 

[warrenw]

fxp
This is the way I understand it.
AISC ASD limits plastic design to steels with yield strength up to 65ksi.
Example #1(36ksi material) had a f'y greater than 65 which is wht the column had a dash (-) instead of a number. You will notice there is no number greater than 65.
If the f'y had been less than 36, the member would have been non-compact.
Example #3 (50ksi material) had a f'y less then 50ksi so the member is non-compact.

Warren

 

[warrenw]

On example #2 the compression flange is now braced at 9ft which is too far for the member to be consider compact.
Lc = 7.4ft < 9.0ft.

 

[fxp]

Please open spreadsheet and comment: