nonlinear analysis and load combinations

[AJA14]

I was wondering why automatic load combinations are not available when nonlinear analysis is being used. This seems to be the case for ACI and LRFD. Are there certain codes that one may use to generate automatic load combinations when nonlinear analysis is being implemented. Or one has to generate all the combinations manually.

[Pawel.Pulak]

Automatic load combinations are using superposition principle and they can be used only in case of linear analysis.

If model contains any objects with non-linear behavior (like tension-only bars, uplift supports, cables, etc.) all load cases and combinations become non-linear. It means that results for any combination cannot be calculated directly from results of simple load cases composing this combination but it is necessary to perform iterative analysis for each combination.

In such case if you need to automatically generate combinations you can use the feature shown in the attached screen capture - it will result in automatic generation of manual combinations.

Please remember that in such case each combination will be iterated and it can take a lot of time - so it may be useful to generate not all combinations possible but only those which may result in extreme results.

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Best regards,