Relating ASD 9th to 13th
Relating ASD 9th to 13th
(OP)
I have a fella that I am checking some work for.
It appears as if he is trying to relate the ASD approach in the 13th back to allowable stresses of ASD 9th..
EX: Equation F6-1 for weak axis bending in the 13th says Mn=Mp= FyZy (assume less than 1.6FySy for now and no local buckling).
Say that Mn = 3517 kip-in and theta = 1.67, then Mn/theta = 2106 kip-in
Can you then divide Mn/theta by the section modulus (plastic? Elastic?) to arrive at an "allowable stress". Something seems wrong about this to me.
If you do so in the case I am looking at, the "allowable" weak axis bending stress for a W36x194 of A36 steel is 34 ksi when dividing by Sy & 21.56ksi when dividing by Zy which is the same thing as Fy/1.67. Using the old 9th edition, Fby = 0.75 FY or 27 ksi.
Thoughts on this goofy approach?
It appears as if he is trying to relate the ASD approach in the 13th back to allowable stresses of ASD 9th..
EX: Equation F6-1 for weak axis bending in the 13th says Mn=Mp= FyZy (assume less than 1.6FySy for now and no local buckling).
Say that Mn = 3517 kip-in and theta = 1.67, then Mn/theta = 2106 kip-in
Can you then divide Mn/theta by the section modulus (plastic? Elastic?) to arrive at an "allowable stress". Something seems wrong about this to me.
If you do so in the case I am looking at, the "allowable" weak axis bending stress for a W36x194 of A36 steel is 34 ksi when dividing by Sy & 21.56ksi when dividing by Zy which is the same thing as Fy/1.67. Using the old 9th edition, Fby = 0.75 FY or 27 ksi.
Thoughts on this goofy approach?






RE: Relating ASD 9th to 13th
Ma / Zy = Fy / 1.67 ~= 0.6 Fy
Solve for stress using Zy and compare that to 0.6 Fy. It's just algebra.
RE: Relating ASD 9th to 13th
I guess I left out the whole reason I posted....
This guy is using an allowable stress of "Fy/1.67" and using Sy in determining stresses.
RE: Relating ASD 9th to 13th
RE: Relating ASD 9th to 13th
not Theta.
ToadJones - that is very conservative then right?
Using Sy and 36/1.67 gives you 22 ksi allowable for weak axis bending.
The 13th edition would limit you to 1.6 x 36 / 1.67 = 34.5 ksi using Sy as you mentioned above.
The 13th edition now refers to plastic moment (see F6) using Zy as a limit - which is not a bowtie stress diagram but a plastic state where the Fy is fully reached through the whole member depth - half in tension and half in compression. So trying to use Sy is not consistent NOW with the 13th edition.
RE: Relating ASD 9th to 13th
RE: Relating ASD 9th to 13th
As for following the code, this design would. The applied moment would be less than Fy Zy / 1.67. By using a conservative procedure, he has ensured that. I don't believe the code says you must perform your calculations in a certain manner, just that the design meet these guidelines.
RE: Relating ASD 9th to 13th