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Frequently Asked Questions (FAQ)
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AnswerThe internal forces and deformations are determined according to the second-order analysis for flexural-torsional buckling taking into account 7 degrees of freedom. For a linear calculation of deformations, a vertical/horizontal load results in only one vertical/horizontal deformation. Since the internal forces refer to the deformed system and it is a nonlinear analysis, this is not valid for the second-order analysis for flexural-torsional buckling.
The deformations in the shear center can be checked in RSTAB and RFEM with the RF-/FE-LTB add-on module (see Figure 02). The deformations that additionally result from the displacements or rotations can only be controlled with a surface model in RFEM.
AnswerFor a stability analysis of any cross-section, the add-on modules RF‑/STEEL EC3 Warping Torsion (extension for RF‑/STEEL EC3) and RF‑/FE‑LTB (stand-alone module) are particularly suitable.By using the calculated critical buckling value, you can determine critical loads and perform the design according to the second-order analysis.
AnswerThere is no general answer to this problem. In the RF‑/STAGES add-on module, however, there is a specific feature regarding the structural system. Similar to some other add-on modules, such as RF‑/STEEL Warping Torsion, it is possible to consider the structural system detached from the main program. Thus, there are some advantages regarding the definition of construction stages, and so on. However, this possibility means that the modifications in the main program RFEM or RSTAB are not updated automatically with these add-on modules. Such an update would inevitably lead to incorrect calculations and is therefore blocked.
It is possible to use SHAPE-THIN to model any cross-sections and carry out a stress analysis within SHAPE-THIN or, for example, in RSTAB with STEEL.
The FE-LTB module can be used for the stability analysis of general cross-sections. Modules such as STEEL EC3 depend on the respective standard, which may cause problems for user-defined cross-sections. In this case, it is often the case that they are not covered by the standard and the stability analysis is only valid for certain cross-sections.
For cross-sections consisting of several materials (hybrid cross-sections), you can also work with FE-LTB. The stiffness of such a cross-section created in SHAPE-THIN can be displayed in RFEM, but there is no option for stress calculation. The approach would therefore be as follows:
1. Create cross-section in SHAPE-THIN
2. Import cross-section into RFEM and set the reference material correctly
3. Calculate the internal forces with FE-LTB
4. Perform the stress check in SHAPE-THIN
That is right, both add-on modules calculate with the 7th degree of freedom, the warping.
The difference is that RF‑/FE‑LTB automatically detects only those loads that directly act on a set of members. Loads from other structural components, that indirectly act on the designed set of members, must be added manually as secondary loads. RF‑/FE‑LTB then performs a complete recalculation of the structural system.
On the other hand, RF‑/STEEL Warping Torsion analyzes the internal force distributions from the calculation of the main program and then calculates the loads back. These are then applied again and calculated. Thus, you do not need to enter loads, which saves your time.
In Window 2.2 "Member Loads," you can define the eccentricity of the load by scrolling to the right in the table.
By using a button, you can also graphically select the stress points of a cross-section and thus define the eccentricity on the basis of the cross-section (see figure).
I have calculated a frame according to the second-order analysis. The testing engineer says that the frame is not stable because the torsional buckling check has failed.
What do I have to be aware of in the case of the second-order analysis? Do I have to calculate the system as 3D, even if it is a 2D frame?
What modules do I need for a lateral-torsional buckling analysis when calculating according to the second-order analysis?
If the internal forces were calculated according to the second-order analysis, it is not necessary to perform flexural buckling analysis according to the equivalent member method for the major axis.
However, you have to consider precambers and initial sways of the members in load combinations when determining internal forces.
EN 1993‑1‑1 requires to generally consider the influences due to structural deformations, geometric imperfections, slippage in the connections, and, if necessary, the effective width from local plate-buckling. Therefore, create imperfection load cases, which you then can insert in the load combinations without partial safety factors and combination coefficients.
In praxis, the internal forces are predominantly calculated in-plane with imperfections. Out-of-plane buckling and lateral-torsional buckling are often calculated using the equivalent member method, but with the already existing internal forces according to the second-order analysis.
With the EN 1993‑1‑1, the general method for the stability analysis was introduced. First, the ideal critical load of the system is determined out-of-plane under consideration of imperfections. From this, you can determine the system slenderness and the reduction factor. First, the internal forces should be calculated by second-order analysis to check buckling and lateral-torsional buckling according to the general method. Thus, an in-plane calculation is not absolutely necessary.
Notice that the frame posts and the beam both can fail due to the flexural and lateral-torsional buckling. EN 1993‑1‑1 uses only one stability check, which contains all forms of buckling.
The stability analyses as well as the cross-section designs can be performed using the RF‑/STEEL EC3 add‑on module. You have the choice between the Equivalent Member Method and the General Method.
For a precise check according to the second-order torsional-flexural analysis under consideration of warping, we recommend to use the RF‑/STEEL Warping Torsion extension or the RF‑/FE‑LTB add-on module.
The restraint moment of the frame beam in the columns is not considered, because the sets of members were defined separately for the frame beam and the columns.
FE-LTB imports only the model data and loads (individual and member loads) that belong to the corresponding set of members.
Internal forces from RSTAB are not considered in principle. Therefore, you must try to model the structure as realistically as possible in the FE-LTB module.
The reason for this is simple:
By introducing the 7th degree of freedom (warping), you cannot use the previous internal forces. Therefore, remove a set of members from the system, and thus also the corresponding loads, and specify the corresponding boundary conditions (supports, hinge, springs).
The warping torsion second-order analysis yields different internal forces, which refer to the loading and structural system previously defined in FE-LTB.
Now, if you want to consider further internal forces from adjacent structural components, you must additionally define these internal forces as concentrated loads or also as line loads in the system. Moreover, you can add further loads in FE-LTB, which are important only for the calculation of internal forces in FE-LTB.
For an easier lateral-torsional design of the frame in FE-LTB, it is best to define a single set of members over the frame.
For the stability analysis with the FE-LTB add-on module, it is necessary to specify the application of imperfections. In this case, not just the initial deformation is used for each member, but the eigenvector is normalized to the defined value.
For more information, see the FE-LTB manual.
Only loads acting directly on the continuous members are transferred to FE-LTB.
If loads are passed into the continuous member by connected structural components, they must be added manually.
As an example we can mention hall frames with craneway consoles, 3D halls with purlin roofs, multi-span frames etc.
For example, if a force applies to a cantilevered beam that does not belong to the continuous members, it is necessary to add the loading manually. Then, this load must be shifted into the node and the offset moment must be additionally applied.
According to EN 1993-1-1 and DIN 18800 Part 2, it is necessary to get specified the equivalent geometrical imperfections for the second-order calculation to determine geometric and structural imperfections as well. This pre-deformation should be applied affinely to the lowest buckling or lateral-torsional buckling mode.
Therefore, in the program FE-LTB, the eigenmode belonging to the smallest eigenvalue is calculated (preliminary eigenvalue analysis) and then chosen as initial deformation mode. The imperfection approach is then carried out by the user scaling the eigenmode.
Thus, in RF-/FE-LTB, the pre-deformation is based on the eigenmode. The imperfections defined in RFEM/RSTAB are irrelevant and are therefore not adopted.
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Wind Simulation & Wind Load Generation
With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.
The generated wind loads acting on these objects can be imported to RFEM or RSTAB.
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