Plastic Modulus vs Elastic Modulus for Steel Design

[AggieYank]

For beam design, the green ASD manual used the elastic section modulus S for steel design. The 13th AISC edition uses the plastic section modulus Z for steel design.

So you're getting an extra strength capacity per the new code, which relies on the "outer" fibers yielding until the whole section is utilized effectively.

What are your thoughts on this?

 

[Lion06]

The green book relied on this as well. That was the reason for the allowable stress for bending of compact section that are adequately braced bumping 10% from 0.6Fy to 0.66Fy. Zx/Sx is roughly 1.12-1.18 for most W shapes, so they used 1.10.

 

[271828]

Not to pick on you, AggieYank, but I have to take this opportunity, LOL. This goes back to another thread on the 89 book vs the 2005 book. I'd venture to guess that at least half of people using the 89 book don't realize that Z really is embedded in the 0.66.

Everybody complains about the 2005 equations being longer, but all the items such as this are out in the open so that everybody should know the underlying assumptions.

 

[abusementpark]

I see nothing wrong with using the plastic section modulus. 10% can be significant difference sometimes.

 

[dik]

The plastic section modulus is used for beams that can develop and sustain fy over a large rotation without becoming locally unstable. In Canada, this applies to Class 1 and Class 2 sections.

Dik