why it is necessary to run a linear analysis before non linear one in ETABS???

[keenkeem]

why it is necessary to run a linear analysis before non linear one in ETABS???

THEIR ANSWER:
You need to do a static analysis first and check equilibrium (loads vs reactions, etc) in order to validate your model. You cannot check equilibrium from non-linear analysis.

1. This is clear enough- full sections are used for analysis and the forces and stresses outputs are based on full sections.

2. This means that if the tension/compression limits are exceeded, then the output from the linear analysis is not valid, as the frame has behaved non-linearly (i.e. gone plastic in certain regions)

My questions
working stress design is linear anaylsis right?
ultimate strength design is non-linear analysis ??

when running Etabs analysis I have to take into accound WSD loadings to determine whether its equilibrium or not???
Is that even for real????

 

[JoshPlumSE]

The answers to your questions depend on what you mean by non-linear analysis.

There is geometrically non-linear analysis which takes into account changes in geometry and how that affects the stiffness of the structure. The most straight forward example would be a P-Delta analysis. This is sometimes referred to as a 2nd order analysis.

Materially non-linear analysis takes into account the non-linear behavior of the materials. Yielding, plastic hinges, et cetera. If you do not take this into account, you would refer to your analysis as an ELASTIC analysis.

Now, working stress and ultimate strength design are generally both done with an ELASTIC analysis. Generally speaking you would run one that includes some 2nd order effects (like P-Delta). This is true whether you use WSD or Ultimate strength design.

Now the member capacities that you derive for individual members are certainly based on non-linear material behavior for ultimate strength design. But, that doesn't mean that the analysis itself considers this.

 

[paddingtongreen]

I don't know ETABS, but does "linear" mean a universal load factor of 1.0 for the equilibrium check? Is that what they mean by "linear"? If you are not doing a code check for that run, does the machine care if you overload the members?

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin