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Frequently Asked Questions (FAQ)
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AnswerA pre-deformed equivalent model is not automatically activated for load combinations. You have to set this manually in the calculation parameters of the load combination. There, select the "Extra options" check box and activate the "Initial Deformation from Module RF-IMP" option.To activate the option in several load combinations at the same time, use the option to select several load combinations.The calculation with the pre-deformed FE mesh is performed if the corresponding load combinations will be calculated according to the second-order or the large deformation analysis.
For the design of steel surfaces, please use the add-on module RF-STEEL Surfaces. If you also want to consider local buckling, the add-on modules RF-STABILITY (for buckling shape and buckling loads) and RF-IMP (for determining the imperfection shape, based on the buckling shape) are also recommended.
AnswerThe design of cold-formed, thin-walled structural components and plates is possible as a finite element calculation with the following programs. Basic program RFEM + RF-Stable (determination of bifurcation) + RF-IMP (generation of the pre-deformed FE mesh for the stability analysis) + RF-STEEL Surfaces (design of stresses determined according to the second-order analysis on the pre-deformed model).
The Stability Analysis for plane load-bearing structures can be converted into a pure stress analysis if the calculation is performed according to 2nd Order Theory and the imperfection required by the standard has been applied to the structure.
By means of the RF-STABILITY and RF-IMP modules, you can create imperfections (or a pre-formed FE mesh). The type of imperfection depends strongly on the structural component and the corresponding standard. For members that have been modeled as plane load-bearing structure, the values from DIN EN 1993-1-1: 2005 5.3 can be used. For plane surfaces, it is possible to use e.g. values from DIN EN 1993-1-5: 2006 Annex C. For shells, the problem is much more complex and there are different approaches. I would advise against the generation of imperfections and perform the buckle integrity design by means of the MNA / LBA concept according to DIN EN 1993-1-6, which does not require the application of an imperfection.
For example, if you want to design the surface model of a steel beam, you can proceed as follows:
1. Select a load with relatively high axial forces (compared to other internal forces in the load case), in most cases you can use the self-weight load case or a load combination with the corresponding self-weight. It may be necessary for each load combination to have an individual imperfection.
2. Calculate load combination according to the 1st Order Theory and use it as basis for RF-STABILITY
3. Find the first Eigenmode of a global failure with RF-STABILITY
4. Use the calculated Eigenmode as the basis for an imperfection with RF-IMP. It is possible to apply 1/300 of the beam length as amplitude, for example.
5. Generate a load combination that uses the generated imperfection as a basis and is calculated according to the 2nd Order Theory.
6. Perform a stress analysis on the basis of this load combination which is also a stability analysis of the structure.
An imperfection can be entered manually either using the "New Imperfection Graphically" button or the menu "Insert → Load → Imperfections" (see Figure 01).
This precamber is not to be confused with an imposed precamber, in which, depending on the support condition of the member, bending moments may occur solely by entering the curvature. The load direction in the "Member Load" dialog box is to be understood as a curvature about the corresponding axis (y or z) similarly to a moment (see Figure 02).
Thus, internal forces and deformations are always taken into account in the case of the precamber as a member load (imposed precamber), whereas the precamber as an imperfection only provides internal forces and deformations if an axial force due to another load is present in the imperfect member.
Further option for entering imperfections is to use the RF‑IMP/RS‑IMP module.
If you want to perform a buckling analysis with shell elements, the add-on modules RF-STABILITY, RF-IMP, and RF-STEEL Surfaces are required.
Governing governing loading according to the linear static analysis.
The results of the governing CO are then used in RF-STABILITY to determine the lowest eigenmode (local deflection / buckling of surface).
With the result of RF-STABILITY, you can generate a pre-deformed structure in RF-IMP by scaling the mode shape to a maximum ordinate (maximum value of the initial deformation). The Maximum value of the initial deformation must be defined by the engineer.
Now, you copy the governing LC and dimension it according to the second-order analysis while considering the pre-deformed structure of RF-IMP.
The results are then used to perform a stress analysis with RF-STEEL Surfaces.
The attached video shows the general procedure once more.There are already some very interesting technical articles on this topic on our website.
If a member consists of several partial members, you need to specify in the definition of the imperfection, to which objects the imperfection should refer to. You have the following options to do this:
The imperfection has an influence on one member or independently on several members.
The imperfection acts on the entirety of the members to be specified in a list. The predeformations and inclinations are therefore not applied individually to a member, but as a total imperfection to all members of the member list. By means of a member list, the imperfections can be applied so as to covere several members without having to define continuous members.
Sets of Members
The imperfection acts on a set of members or on several sets of members. Like for a member list, the parameters are applied to all the members contained in a set of members.
The automatic creation of imperfections or imperfect systems is possible with the add-on modules RSIMP und RF-IMP.
The values for the precamber are not shown because the ordinate of the buckling mode is probably too small so that it cannot be considered by using the specify tolerance.
In the "Details" dialog box, you define from which ordinate of the selected buckling mode or deformation the imperfection is to be generated.
Bei der Ermittlung der Schiefstellung muss in RSIMP auch das Nachweisverfahren berücksichtigt werden. In Maske 1.2 können Sie in Spalte B nach einem Klick in die Zelle die Definitionsart festlegen.
Gemäß El. (201) brauchen bei Anwendung des Nachweisverfahrens Elastisch-Elastisch nur 2/3 der Werte angesetzt zu werden.
Stellen Sie dagegen als Definitonsart "DIN 18800 E-P" ein, so wird mit 1/200 gerechnet und Sie erhalten Ihren erwarteten Wert.
In general, you have to pay attention to the following:
- The members must have the same member rotation.
- The direction for the input of imperfections is related to the local axes y and z. A relation to the principal axes u and v is also possible in case of deviating principal axes.
- It is also possible to influence the position by a change of signs for the imperfection value.
Therefore, when defining imperfections, it is often useful to display the local member axis systems: select a member by right-clicking it and activate the display of the local axis systems in the shortcut menu.
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