Relating ASD 9th to 13th

[ToadJones]

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?

 

[nutte]

Fy Zy / 1.67 = Ma (Ma is applied service level moment)

Ma / Zy = Fy / 1.67 ~= 0.6 Fy

Solve for stress using Zy and compare that to 0.6 Fy. It's just algebra.

 

[ToadJones]

Nutte-
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.

 

[DWHA]

My response to situations like these are always the same. If the job calls out for a specific code to be followed, follow it. If not, then choose an edition of steel code you perfer to work with and stick with it throughout the analysis. Situations like these can get you into trouble.

 

[JAE]

I thought it was Omega [Ω]
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.

 

[ToadJones]

yes....need to study my Greek.

 

[nutte]

Compared to the 13th edition, that's conservative. Sy is always less than Zy. His "stress" will be about 50% higher than it should be.

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.

 

[ToadJones]

I'm tending to agree with DWHA...if for no other reason than this makes the review work nightmarish.