Second order calculation acc. EN 1993-1-1
The criterion for executing a second order calculation is found in article 5.2.1(3) of EN1993-1-1.
According to formula 5.1 in this article, the second order calculation should be executed when the alpha critical factor is smaller than a prescribed value.
The alpha critical factor is the ratio between the design forces and the critical buckling load and can be determined in SCIA Engineer.
To obtain the alpha critical factor in SCIA Engineer, a stability analysis needs to be executed.
To do this, activate the stability functionality in the project settings and define stability combinations in your project.
A stability combination should be created for every linear ULS sub-combination, similar to the definition of nonlinear combinations.
If this leads to a too large number of combinations, we advise to filter on the combinations that generate the biggest compression forces in the structure.
Therefore you could use the Results table to check the combinations with the highest compression forces and convert these combinations to stability combinations via the Action button New combination from Combination key:
For a stability calculation it is also necessary to increase the number of elements on 1D members. Try for the number of buckling modes a relatively low amount to start with and search for the first global buckling mode, i.e. where a big part of the structure or multiply elements at once take part in the buckling shape. You could start with 1 buckling mode, but often this will result in a local buckling mode. Raise setting for amount of buckling modes in case there was no global mode present for the initial setting.
Run the stability analysis.
In the Results workstation (or via Main menu > Results) you will now find the ‘critical load coefficients’ command, which will give you an overview of the calculated alpha critical values.
The lowest of these values needs to be checked in formula 5.1 of EN1993-1-1.
Information on how to perform a second order analysis in SCIA can be found in our Steel code check manual.