FAQ 000425 EN-US

12/01/2017

Robert Vogl Calculation RFEM RSTAB

I would like to calculate my structure according to the second-order analysis and for this, I have created several load cases, including imperfection load cases. Do I have to combine the load cases in load combinations or result combinations?

Answer

If you want to use the second-order analysis, you must define load combinations. In the case of a load combination, the loads of the contained load cases are combined into "one great load case" under consideration of the corresponding partial safety factors and this load case is then calculated according to the second-order analysis.

In the case of a result combination, on the other hand, the contained load cases are calculated first. Then, the results are superimposed under consideration of partial safety factors. It is not possible to set the calculation method for result combinations. This is only possible for load cases and load combinations.

For load cases and load combinations, you can specify the method of analysis in the calculation parameters, for example the second-order analysis (see Figure 01). If the model includes members of the "Cable" type, these elements are calculated according to the large deformation analysis.

For load cases and load combinations, you always obtain unambiguous internal forces. On the other hand, the result combinations include an envelope of the internal forces (max and min values). Since the results of load cases and load combinations are superimposed in the result combinations, the RCs can thus contain the results according to the static linear analysis or the second-order analysis.

In your model, you can combine the defined LCs in a summary RC with the or criterion (for example, RC1 = CO1/permanent or CO2/permanent or CO3/permament ...). Thus, you obtain the maximum internal forces and deformations under consideration of the second-order analysis (see Figure 02).

The "permanent or superposition" has the effect that the one or both of the COs are used for the results. If the results are connected in RCs with a "alternatively or superposition," it may also happen that no COs will be applied. Then, the internal forces and deformations become zero.

Keywords

load combination result combinations second-order analysis superposition combination method of analysis cable

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