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FAQ 002108 EN

7 December 2019

Robert Vogl Calculation RFEM RSTAB 5.xx 8.xx

When trying to calculate a result combination, I get the warning 30014:

"Incorrect addition of load cases in the result combination!
The result combination contains load cases or load combinations with results containing second-order effects, which points towards nonlinear relationships and therefore cannot be added together."

What does this warning mean?

Answer

The warning appears if load cases or load combinations are nonlinear according to theory II. Or III. And then superimposed in a result combination according to the following criteria:

Case 1: RC = CO1 + CO2

Since each load combination is analyzed nonlinearly according to the second-order analysis, the superposition of the respective results is problematic: Deformations and internal forces are based on different structural systems. It would not be correct to simply add up the individual CO results.

Case 2: RC = 'Factor' * CO1 or 'Factor' * CO2

RFEM overlays both load combinations with factors. Since each CO is calculated according to the second-order analysis, the results show a nonlinear correlation. The factors cause each internal force to be multiplied by the corresponding factor. A doubling of the axial force is, for example, not necessarily accompanied by a doubling of the moment. It is assumed that the results are linearly correlated. This contradiction (nonlinear calculation of the results with linear multiplication) results in the warning described above.

The solution for both cases is to calculate the load cases or load combinations linearly according to the linear static analysis and then superimpose them in an RC. If there are only load cases, you can analyze them in a load combination.

Nevertheless, the results of nonlinear analyzes can be superimposed in an RC if the criterion "permanent" is used: RC = CO1 / s or CO2 / s . This determines the result envelope that covers the unfavorable constellation from both COs. In this case, confirm the warning with [Yes]. The RC internal forces then represent the extreme values from the two LCs and can be used for further design.

When a result combination with geometric nonlinearities is selected in the model, a similar message appears. It is described in FAQ 003065 .

Keywords

load case load combination result combination warning incorrect addition second-order analysis superposition

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