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Frequently Asked Questions (FAQ)
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This is because the effective lengths or buckling lengths of members and sets of members differ. While the effective length is used for the stability analysis for members, RFEM takes the length of the summarized members for the set of members.
The frame shown in Figure 01 consists of a horizontal beam that is divided into four equally long members. In addition, a set of members is created for the four members. The stability analysis is carried out for both cases according to the equivalent member method.
For the design of members, the program calculates with a length of 1.00 m in each case. In contrast, the set of members has a length of 4.00 m (see Figure 02). This difference in length naturally affects the stability design, which means that the capacities are also different (see Figure 03).
In addition, it is not recommended to calculate all members and sets of members in a single design case because this leads to falsified results.
AnswerFor a vault-free cross-section, a proof of the complete system is recommended using theory II. Order - internal forces and local imperfections. The entry of the effective lengths and nodal bearings (with staff sets) can thus be omitted. However, please activate gamma_M1 for cross-section verification.
AnswerFor the design of steel surfaces please use the module of the same name RF-STEEL Surfaces. If you also want to consider local denting, then the additional modules RF-STABIL (to determine the bulge and branching loads) and RF-IMP (to determine the imperfection figure, based on the bulge pattern) are also recommended.
AnswerUnfortunately not. Intermediate supports and nodal supports have to be set manually. The default configuration for effective lengths is 1,0*member length and for set of members, one nodal support with definition u_y' and phi_x' is defined on each side of the set of member.To speed up the process of defining nodal supports, you can pick more nodes in one step by using the highlighted function in the attached picture.
AnswerIndependent submodels are not interconnected and are considered as separate submodels in the calculation. They are thus independent models without influencing each other (see Figure 2).It is recommended to edit submodels separately as individual files. Then a stability analysis with RSKNICK is possible.Otherwise, the partial models must be connected to each other. In this case, it should be taken into consideration that the static systems of the submodels should be retained when the submodels merge into an overall model (see Figure 3).The feature "Independent Systems" is helpful in detecting partial models. This finds all independent systems and lists them as groups (see Figure 4).One finds this function under Extras -> Model control -> Independent systems.
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).
AnswerIt is quite likely that the high deformations are caused by the consideration of shrinkage and the horizontal storage in the model.The shrinkage is taken into account internally on the load side as elongation, in which connection a failure due to the prevention of shrinkage is also possible. If the shrinkage is prevented by a non-displaceable horizontal bearing, forces are created which can lead to failure of the concrete and thus to a significant increase in deformation or even instability of the model.In this context, it is important that when using the nonlinear deformation calculation, the boundary conditions of the model are mapped as realistically as possible.
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.
AnswerYou can change the cross-section, generally adjust the model and its loads, or activate stabilization measures in STEEL EC3.The rotatable restraint and shear panel settings defined in window Members 1.12 Parameters or 1.13 Threshold Parameters Parameters, which are then considered in the module-internal eigenvalue solver to determine the contraction load, are particularly suitable for this.
The reason for this warning is that sets of members are designed by default according to General Method 6.3.4 of EN 1993-1-1. However, this method is only allowed for loading in the support plane, that is, without transverse bending moment. Therefore, the corresponding warning appears.
You can perform a stability analysis according to the second-order analysis as a cross-section design that is possible with the add-on modules RF- / FE-LTB or RF- / STEEL Warping Torsion.
Alternatively, you can neglect the lateral bending in the filter options of the "Details" dialog tab, tab "Stability" or activate the extended method according to Naumes in the "National Annex" dialog box, which allows for transverse bending.
In this case, however, the best option is to select the equivalent member method for the design of the sets of members in the "Stability" tab of the "Details" dialog box.
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