Answer:
The following answer refers to the programs RFEM 5 and RSTAB 8.
If you want to calculate according to second-order theory, you must define load combinations: In a load combination, the loads of the included load cases are combined into "one large load case" considering the respective partial safety factors, which can then be calculated according to second-order theory.
In contrast, a result combination first calculates the included load cases. The results are then superimposed considering the partial safety factors. The calculation theory cannot be set in the result combinations. This is only possible for load cases and load combinations.
For load cases and load combinations, you can specify the calculation theory in the calculation parameters, for example, second-order theory (see Figure 01). If the model contains members of the type "cable," these elements are always calculated according to third-order theory.
In load cases and load combinations, clear internal forces are always output. Result combinations, on the other hand, include an envelope of internal forces (max and min values). Since the results of load cases and load combinations are superimposed in the result combinations, the result combinations can thus contain results according to first or second-order theories.
In your model, you can also combine all LCs in an overall RC with the Or criterion (e.g., RC1 = LC1/permanent or LC2/permanent or LC3/permanent ...). This gives you the maximum internal forces and deformations considering second-order theory (see Figure 02).
The "Permanent-or-superposition" ensures that one of the LCs is always used for the results. When linking the results with a "Potential-or-superposition," it is also possible that none of the LCs are relevant for the results of the RC. The internal forces and deformations could then become zero.
