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Plastic/non-linear analysis

Plastic/non-linear analysis

Plastic/non-linear analysis

(OP)
Hi Everyone!!!

I have a question and would be grateful if anyone could answer it, please. Designing in steel, is non-linear analysis (material non-linear analysis) the same as plastic analysis ( where hinge is formed). I only ask as I have one software that does plastic analysis (with hinge formation) and another software capable of non-linear analysis.

Thank you in advance!!!

RE: Plastic/non-linear analysis

I'm not sure. Plastic analysis generally is based on designing for a 'collapse mechanism'. Non-linear analysis may include for the design from elastic, up to, and possibly, including plastic analysis.

Dik

RE: Plastic/non-linear analysis

well yes...can be the same....
the word non linear analysis is correct
the word 'plastic' analysis could be a translation from the german word 'plastische Analyse'
which means going beyond the elastic behavior ( of steel )

So yes...I would say it is the same


best regards
Klaus

RE: Plastic/non-linear analysis

(OP)
Thanks !!

Has anyone else got any thoughts on this one?

RE: Plastic/non-linear analysis

There are three major sources of non-linearity:

1) Geometric. This is P-delta type stuff.

2) Material. Plastic design would fall into this category.

3) Changing status. This is closed gaps and tension only braces etc.

So I would say that all plastic analysis is non-linear but not all non-linearities are captured plastic analysis. Non-linearity is a bigger bucket that contains plastic analysis as one of it's elements.

I'd be very surprised to hear of a commercial software package that could do plastic analysis but couldn't handle general non-linearity issues. On the other hand, there are plenty of programs that handle general non-linearity but not plastic analysis.

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.

RE: Plastic/non-linear analysis

I've been using plastic design for nearly 40 years. It has several advantages:

It extends the load carrying capacity beyond the elastic range. For example, a two span continuous beam has an elastic design moment of .125 x ql^2. The plastic design moment for the same beam is 0.0858 X ql^2, relying on the strength beyond the initial yield at the support.

You do not normally check for alternate loading conditions. Alternate loadings generally only have an effect on the moments and shears for the splice connections.

There are fewer pieces to handle and this generally results in some economy.

You are restricted to Class 1 sections (Canada), or maybe compact sections, elsewhere. They have to be 'stocky' to prevent premature buckling.

There's unsubstantiated information that it is more costly; I've not encountered this. It's generally cheaper.

It's stiffer than simple span construction. Defl for SS construction approx. 5/384 * and for fixed end is approx. 1/5 of this and for plastic design is approx. 1/3 of the simple span defl.

I've never used the Gerber system for roof beam framing in my entire carreer.

Dik

RE: Plastic/non-linear analysis

There is a lot of mixed terminology used to describe the different type of analyses. Different codes use different words. And, so do the different software. A quick summary:

1) LA = Linear Analysis: the most basic. Nothing added
2) GNA = Geometrically Non-linear analysis. Considers non-linear effects of deformation. This is sometimes called 2nd order analysis. It includes P-Delta effect in it's most basic form, but can include other effects as well.
3) MNA = Material Non-linear analysis. Considers the non-linear effect of the materials. Like plastic hinging or yielding.
4) GMNA = Geometric and Material Non-linear analysis. Both 2 and 3.

You can also throw an "I" in there in some cases for the consideration of "initial imperfections".

Those are the cleanest way to describe the basics. But, it can get clouded again when you start addressing very specific capabilities (gap elements, hook elements, backbone curves, et cetera).

RE: Plastic/non-linear analysis

non-linear analysis has become an annoying term, because it can mean several different things. Without clarification or significant amount of context, it's hard to determine what someone's describing.

RE: Plastic/non-linear analysis

Josh: Your item 3, I assume includes redistribution of moments after yielding; this is what I call plastic design.
TLHS: agreed...

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