redundancy in plastic moment

[Agandepally]

Hello all,
I have question, as per AISC the nominal flexural capacity of the hollow tube or HSS members is Mn=Mp=Fy.Z (plastic moment).
My question is don't we have to consider any redundancy when we are relating to plastic moment , and am verifying a calculation using a Fy=80 ksi but my understanding is the maximum Fy for plastic analysis is 65 ksi.
Also can any one guide me to a book or publications which ascertained the change of moment capacity from elastic to plastic moment.

I really do appreciate your help.

Thank you
Anvesh

 

[KootK]

My question is don't we have to consider any redundancy when we are relating to plastic moment

I don't think so. It's not redundancy that improves when using elastic design instead of plastic but, rather, overall safety factor for the limit state of section yielding. And most modern codes have offset this effect by adjusting the probabilistic determined material/load factors to maintain a consistent result.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Are you sure that your sections are still Class 1 and 2 sections with such a high Fy?

80 ksi is typically the ultimate strength of most common steel sections, I would like to know where they're getting Fy=80 ksi.

 

[dhengr]

Agandepally:
I wonder if you aren’t kinda mixing/confusing two different concepts as relates to plastic design. When we first started talking about plastic design, back in the late 50's and early 60's, we thought in terms of redundant structures and the fact that the most stressed joint or area in the member could start to yield (go plastic) and the loads and stresses would just be shifted to (sloughed off to) another area in the beam or frame, where there was some reserve strength. When a structure formed a mechanism, it failed. A simple beam formed a mechanism when the center span moment caused any significant yielding. A fixed-fixed beam could start to yield at each fixed end and there would be some reserve (serviceability) in that the center moment region had not yet reached yield and could pick up some additional load before it did yield and finally form a mechanism (failure). The same thing applied to a portal frame, several joints or areas may start to yield and as long as there was still some redundance (reserve) in the structure it had not yet formed a failure mechanism.

Today, this has morphed, through many mathematical machinations, multipliers on loads and divisors on strengths and capacities into what we call LFRD. This has gotten so that you really don’t recognize the loads any longer, they are just numbers in a formula, you don’t have any idea what you think the actual stress is and then you apply a bunch of statistical and probabilistic b.s. to all of this, and you have a final design solution. Now, we are talking about some of these joints or highly stressed areas starting to yield, and absorbing a bunch of energy under static overloading or dynamic cyclic load (wind or EQ lateral loading). And now, we have classes of cross sections which indicate how much plastic deformation can take place before the section just crumples because is not stiff enough to tolerate any, or any further plastification without just folding-up.

They both involve plastification of the section, but they are two very different philosophies of structural thinking. We did account for much of this stuff, in the back of our minds during design, we just didn’t have 700pages of code to tell us how not to do it right. We did think, gee, a 5% overstress here (at this joint or location) should not be a killer, as likely as not the Fy is 10% higher than the min., the rest of the structure is well under stressed, lets take a look at the deflections under this new condition; if it’s o.k. everything is fine.