Filter out critical load combinations

Problem/Question #

How to lower the number of loadcombinations taken into account during the analysis?

Number of loadcombinations raises the time needed for analysis, especially for second-order nonlinear analysis significantly.

Explanation/Solution #

Since the critical and envelope results are obtained by locally generated superposition of the results of the analysis of each loadcase, no prior manual definition or auto generation of load combinations is required for linear static analysis.

Second-order nonlinear static analysis on the other hand is already more time-consuming type of analysis than linear static calculation, and the superposition principle used in linear static calculation is inherently inapplicable. Running a second-order analysis requires predefined load combinations. 

Since the number of possible load combinations according to the standard can be huge, while the number of relevant combinations in regards to the final utilizations are low, it is useful to narrow down the number of calculated load combinations before a nonlinear analysis.

The first step is to run a linear static analysis. Once the results are available, you can view the “Results” table of any result component you want to investigate under the “Table browser” menu item, while standing on the “Results” main item in the “Statics” tab. This is a table for the critical values of the selected result component.

The rightmost column of this table contains the combinations in a copyable format. By right-clicking on the column header, you can select “Select entire table column (Critical combination)”, and click on “Send to the load combination table”.

Use the “OK” button to exit and navigate to “Loads” tab, select the “Load combinations” button. On the “Custom load combinations by loadcases” submenu that pops up by default, you can select “Paste critical load combinations from the Clipboard option” to insert combinations.

Conlcusion #

This relatively fast and simple procedure can significantly speed up nonlinear calculations by reducing the number of load combinations to be considered. At the user’s choice, the number of combinations to be run can be further reduced.
It should be noted, however, that the nonlinear calculation may produce results that lead to other load combinations becoming critical instead of the combinations that were critical after the linear calculation. Always consider the changed results and the combinations that are ultimately used!