Why do I get different design results for a load combination (CO) and a result combination (RC) in STEEL EC3 despite the same internal forces?

Answer

The difference between designing by means of RC and CO consists mainly in the moment distribution applied for the calculation of the ideal lateral-torsional buckling moment Mcr. For a load combination (CO), it is possible to clearly apply the moment distribution for the present load position. For a result combination of the '+' type or an RC that does not correspond to the 'permanent/or' type, however, you can only analyze the enveloping moment distribution. Then, the more unfavorable distribution (Max or Min) is applied. As a consequence, unfavorable values result for the critical load factor.
Furthermore, it should be noted when designing according to clause 6.3.3 of EN 1993-1-1 that the moment distribution coefficients are set to 1.0 (constant distribution), provided that the moment distribution cannot be determined unambiguously, which may also lead to more conservative results.

Summary:
The internal forces applied for the design can be the same in LC and RC, but the moment distribution in the result combination is applied more unfavorably to determine the critical factor. Thus, the RC provides a higher utilization for the design with STEEL EC3.

Keywords

Result combination Load combination Design Lateral-torsional buckling

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