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FAQ 002108 EN

1 December 2017

Robert Vogl Calculation RFEM RSTAB 5.xx 8.xx

When calculating a result combination, I receive the "Warning No. 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

This warning appears in the case of the following result combination definitions when the load cases/load combinations are analyzed according to a nonlinear analysis such as second-order or large deformation analysis.

Case 1: RC = LC1/CO1 + LC2/CO2

For Case 1, load cases/load combinations cannot be superimposed together in a result combination when each case/combination is ran according to a nonlinear analysis such as a second-order (P-Delta, p-delta) or large deformation analysis. Deformations in both load cases are based on different structural systems and will result in incorrect results as P-Delta effects cannot be simply added together.

Case 2: RC = factor* LC1/CO1 or factor* LC2/CO2

For Case 2, if a load case/load combination is calculated according to a nonlinear analysis, it is not possible to additionally apply a factor to the result combination. The factor would not only be applied to the defined loads on the structure, but also to the nonlinear effects such as P-Delta which is incorrect. 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, the warning must be confirmed 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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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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