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Frequently Asked Questions (FAQ)
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AnswerThe RF-/JOINTS add-on module is divided into several joint groups. For this, see the following FAQ.Therefore, there is no straightforward answer to the special aspects of the design as in this FAQ.In contrast to the RF‑/TIMBER Pro add-on module described in the mentioned FAQ, however, it is obvious that the RF‑/JOINTS add-on modules cannot design EC2, even if manually changing the LDC, for example in the RF‑/JOINTS Timber - Steel to Timber add-on module (see Figure 01).Furthermore, this also applies to the add‑on modules RF‑GLASS and RF‑/CONCRETE NL.The reason for this is that there are stiffnesses exported in the program in the case of some joint groups of the RF‑/JOINTS add‑on module. For nonlinear calculations, the superposition with result combinations is not allowed. In the case of the second result combination mentioned above, there is the special feature that the superposition is no longer conservative, even in the case of simple structures. The design cannot be also performed correctly by manually changing the LDC.Nevertheless, if a result combination should be superimposed with constant and alternative additive, it is necessary to split EC2 in the attached file into load combinations as follows.
- RC2*=CO1 or CO2
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?
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.
The solution for this case is to either run the load cases/load combinations according to a geometrically linear analysis where the results can be superimposed together in a result combination, or the consideration of the combined load cases must only be considered under the Load Combinations tab and evaluated individually rather than in a result combination.
Keep in mind, this same warning will appear when creating a result combination in a model where geometric nonlinearities are also defined. An example includes a slab foundation where a nonlinear surface support is defined to fail in the case of uplift. Geometric nonlinearities can accurately be considered in load combinations opposed to result combinations which may yield incorrect summary results.
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 solution for this case is to either run the load cases/load combinations according to a geometrically linear analysis where the factor can be applied to the result combination, or the factor can be considered under the Load Combinations tab where it will be applied directly to load cases only according to a geometrically linear analysis without consideration to the second-order effects
When creating a load combination, the calculation method "2nd Order" is preset in the calculation type by default. For linear systems, however, which are statically indeterminate, the results for load cases, which are calculated according to the 1st Order analysis, can be different.
When trying to calculate a result combination, I get Warning 30014:
"Incorrect addition of load cases in result combination! The result combination contains load cases or load combinations with results according to the second-order analysis that have nonlinear relations and therefore can not be summed up. "
What does this warning mean?
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, 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 .
These zero values are caused by the superposition type in the combination criterion: if you analyze several load cases or combinations in one result combination and define the load cases as "Variable," then zero is shown for the maximum or minimum value at the locations with the same signs. When using the variable ("eventual") superposition, it can also happen that no load case or no combination becomes effective. Zero is smaller than the smallest maximum value or greater than the greatest minimum value.
A solution would be to define the load cases or combinations contained in the result combination as "Permanent," for example: RC1 = CO1/Permanent or CO2/Permanent or CO3/Permanent or CO4/Permanent
The load cases or combination are then compared as permanently acting. Thus, the obtained extreme values are unequal zero also for the areas with the same signs.
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.
For the calculation of a super combination, it is not necessary that the results of the other opened models are available. The required load cases or combinations will be calculated automatically.
It is also not necessary to open all other models contained in the super combination for the calculation.
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