FAQ 000744 ES


I have carried out a second-order analysis of a frame in RSTAB. The testing engineer says, that the frame is not stable, because the torsional buckling check has failed.

What do I have to be aware of for the second-order analysis? Do I have to calculate the system as 3D, even if it is a 2D frame? Furthermore, I am not sure if the second-order lateral-torsional check is necessary in the first place.

What parts of the program do I need for a possible lateral-torsional check?


If the internal forces of the frame are calculated by second-order analysis, no lateral-torsional check according to the Equivalent Member Method is necessary for the major axis.

However, in the determination of the internal forces, you have to consider in the load combinations the precambers and initial sways of the members.

EN 1993-1-1 prescribes to generally consider the influences from structural deformations, geometric imperfections, slippage in the connections, and, if necessary, the effective width from local plate-buckling. Therefore, create imperfection load cases, which you then can insert in the load combinations without partial safety factors and combination coefficients.

In praxis, the internal forces are predominantly calculated in-plane with imperfections. Out-of-plane buckling and lateral-torsional buckling are often calculated using the equivalent member method, but with the already existing internal forces according to the second-order theory.

With the EN 1993-1-1, the General Method for the stability checks was introduced. First, the ideal critical load of the system is determined out-of-plane under consideration of imperfections. From this, you can first determine the internal forces in the frame plane according to the second-order analysis. From this, you can determine the system slenderness and the reduction factor. First, the internal forces should be calculated by second-order analysis to check buckling and lateral-torsional buckling according to the General Method.

Thus, a special calculation is not absolutely necessary.

Notice that the frame posts and the beam both can fail due to flexural and lateral-torsional buckling.

EN 1993-1-1 uses only one stability check, which contains all forms of buckling.

The stability checks as well as the cross-section checks can be done using the add-on module STEEL EC3. You have the choice between the Equivalent Member Method and the General Method.

If you want to carry out a precise check according to the second-order torsional-flexural analysis under consideration of warping, use the module FE-LTB.

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