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Frequently Asked Questions (FAQ)
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For the stability design of arbitrary cross-sections, the add-on modules RF-/STEEL EC3 Warping Torsion (add-on module for RF-/STEEL EC3) and RF-/FE-LTB (stand-alone add-on module) are particularly suitable.By using the calculated critical value, you can determine branch loads and perform a design according to the second-order analysis.
There is no general answer to this problem. In the RF-/STAGES add-on module, however, there is a particularity 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 structural states, etc. However, this possibility means that 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 DuenQ to model any cross-sections and carry out a stress analysis within DuenQ 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 DuenQ can be displayed in RFEM, but there are no possibilities for stress calculation. The approach would therefore be as follows:
1. Create cross-section in DuenQ Creating a cross-section in DuenQ
2. Load cross-section into RFEM and set the reference material correctly
3. Use FE-LTB to calculate the internal forces
4. Perform stress analysis in DuenQ
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 analyses 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).
Im Allgemeinen ist es so, dass RF-/BGDK für eine vollständige Bemessung eindeutige Schnittgrößen über die Stabachse benötigt. Diese Bedingung besteht aufgrund diverser Normanforderungen, die ein zu bemessendes Stabelement als Bauteil ansehen und hierfür zur Bestimmung von Bemessungsparametern Ergebnisgradienten über die Stabachse heranziehen (Normalkraftverläufe, Biegemomentverläufe etc.).
Um dieser Forderung nachzukommen, müssen die zu bemessenden Schnittgrößen eindeutige Bemessungsfälle für den Berechnungskern projizieren. Vertreter solcher eindeutigen Bemessungsfälle sind Lastfälle LF und Lastkombinationen LK.
Da Ergebniskombinationen meist für die umhüllenden Ergebnisse verwendet werden, sind diese generell für die Bemessung in RF-/BGDK gesperrt.
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.
I have the following problem occurring in RF-/FF-LTB: To analyze the effect on the load application point, I have defined the eccentricity e-z one time positively and another time negatively. A positive value, which means a load application on the lower chord, should be a contribution to the stabilization of the structural system. But this is not the case. What do I have to do?
The definition of the point of load application in RF-/FE-LTB depends on the local member axis system. For example, if you define a horizontal member (global Z-axis points downward), the z-axis of the local axis system points downward as well (in the positive Z-direction). Now, if you want the loading to act on the upper side, you have to enter a negative value for e-z. Moreover, for a horizontal member it doesn't matter in which direction the member orientation runs: The local axis z always points downward.
This is different for a vertical member. In this case, the member orientation runs from top to bottom, and the local axis z of the local axis system points to the negative direction X. Now, if a positive value is entered for the eccentricity e-z, this input corresponds to a load application on the upper chord and thus results in a higher utilization.
You can activate the graphical representation of the local axis systems for each member in the Display navigator by selecting the option "Model" > "Members" > "Member Axis Systems x,y,z".
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.
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