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Frequently Asked Questions (FAQ)
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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 dimensioning of cold - formed, thin - walled components and sheets is possible as a finite element calculation with the following programs. Basic program RFEM + RF-Stable (determination of the branching figure) + RF-IMP (generation of the pre-deformed FE mesh for the proof of stability) + RF-STAHL surfaces (calculation of the second-order theory determined stresses on the pre-formed model).
The proof of stability for tensile structures can be converted into a pure stress analysis, if the theory is considered to be 2-fold and the imperfection required by the standard has been applied to the system.
With the help of the modules RF-STABIL and RF-IMP imperfection (resp. a preformed FE mesh). The type of imperfection depends heavily on the component and the standard used. For bars, which were modeled as a tensile structure, the values from DIN EN 1993-1-1: 2005 5.3 can be used. For flat surfaces, for example, the values from DIN EN 1993-1-5: 2006 Appendix C can be used. For trays, the problem is much more complex and there are different approaches. From a generation of imperfections I would advise against this and perform the buckle proof by means of MNA / LBA concept according to DIN EN 1993-1-6, which does not require an approach of imperfection.
If, for example, the surface model of a steel girder is to be detected, you can proceed as follows, for example:
First A burden me comparatively (compared to. other internal forces in the load case) select high normal forces, in most cases the self-weight load case or a load case combination with the corresponding own weight is suitable. It may be necessary to provide each load combination with an individual imperfection.
2. Calculate load combination according to 1-order theory and use as the basis for RF-STABIL
3. Using RF-STABILITY to find the first eigenstate of a global failure
4. Using RF-IMP, use the calculated eigenmode as the basis for an imperfection. In this case, for example, 1/300 of the carrier length can be used as the amplitude.
5. Create a load case combination that uses the generated imperfection as a basis and is calculated according to 2-order theory.
6. Perform a proof of tension on the basis of this load case combination, which at the same time is also proof of stability 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.
AnswerIf you want to perform a buckling check with shell elements, the additional modules RF-STABIL, RF-IMP and RF-STEEL surfaces are necessary.
Determine decisive load according to theory of the 1st order.
The results of the authoritative LC are then used in RF-STABIL to determine the lowest eigenform (local evasion / bulging of the surface).
With the result of RF-STABLE, a pre-deformed structure can be generated in RF-IMP by scaling the eigenform to a maximum ordinate (maximum value of pre-deformation). The maximum value of the pre-deformation is to be determined by the engineer.
Now you copy the authoritative LK and measure it according to theory II. Order taking into account the pre-deformed structure of RF-IMP.
The results are then used to perform a stress analysis with RF-STEEL surfaces.
In the attached video the basic procedure is clarified once more.
Beul detection surfaces.wmv (15.03 MB)
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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