I have calculated a frame according to the second-order analysis. 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 in the case of the second-order analysis? Do I have to calculate the system as 3D, even if it is a 2D frame?

What modules do I need for a lateral-torsional buckling analysis when calculating according to the second-order analysis?

Answer

If the internal forces were calculated according to the second-order analysis, it is not necessary to perform flexural buckling analysis according to the equivalent member method for the major axis.

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

EN 1993‑1‑1 requires to generally consider the influences due to 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 analysis.

With the EN 1993‑1‑1, the general method for the stability analysis was introduced. First, the ideal critical load of the system is determined out-of-plane under consideration of imperfections. 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, an in-plane calculation is not absolutely necessary.

Notice that the frame posts and the beam both can fail due to the flexural and lateral-torsional buckling. EN 1993‑1‑1 uses only one stability check, which contains all forms of buckling.

The stability analyses as well as the cross-section designs can be performed using the RF‑/STEEL EC3 add‑on module. You have the choice between the Equivalent Member Method and the General Method.

For a precise check according to the second-order torsional-flexural analysis under consideration of warping, we recommend to use the RF‑/STEEL Warping Torsion extension or the RF‑/FE‑LTB add⁢-on module.

Keywords

second-order analysis flexural buckling design flexural buckling lateral-torsional buckling stability

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