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Frequently Asked Questions (FAQ)
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AnswerYour model is unstable because there is the Compression member type assigned to the columns. In RFEM, a compression member means that the element is only subjected to compression, and has hinges at the ends, so it does not transfer any moments. The hinges cause the instability. These elements are used in steel structures, for example. For this reason, it is recommended to use the Beam member type that can transfer all types of loads and is generally used for columns.
Even if the description of the add-on modules is very similar, the calculations performed are different.
This add-on module performs a general stress analysis by calculating the existing stresses and comparing them with the limit stresses. The designs are performed elastically. The designs do not depend on a standard.
This add-on module, on the other hand, performs all typical designs of ultimate limit state, stability, deformation, and fire resistance for steel members according to Eurocode 3 (numerous National Annexes are available). There is a range of module extensions available within this add-on module. These include: warping torsion analysis, plasticity analysis, designs for cold-formed sections.
Comparison of Both Add-on Modules
The results of RF‑/STEEL can be compared with the results of the cross-section design of RF‑/STEEL EC3.
Why the results can differ is explained in FAQ 003489.
The input of intermediate lateral restraints is used to enter lateral supports on a member without having to insert a node in the model at this location. Furthermore, the design of sets of members using the equivalent member method requires them to define the existing support conditions at the intermediate nodes of the set of members.
The entered intermediate lateral restraints are then considered as additional boundary conditions in the eigenvalue solver. However, some boundary conditions are already included in the input tables of the nodal supports or the effective lengths.
To avoid conflicts or duplicate entries for the same location, the error message "Invalid location of the intermediate lateral restraints" appears. The following locations are not allowed as they are already defined in the other entries:
Design of Members:
- Start and end of a member
Design of Sets of Members:
- Start and end of all members if they are entered using nodal supports
- Start and end of a set of members for the design using the equivalent member method
However, it is very easy to find the error, because the first line with the wrong entry is automatically selected after confirming the error message.
To adjust nodal supports of sets of members, go to Window "1.7 Nodal Supports." There you can adjust the nodal supports for the set of members.
Left-click once next to the check box of the support condition that you want to adjust. A selection symbol appears where you can select the "Spring" type. Now, you can enter the spring properties.
According to DIN EN 1993‑1‑1, 188.8.131.52 (2), the reduction factor χLT can be modified by the f factor for χLT,mod. You can activate or deactivate this option under National Annex Settings.
The RF‑CONCRETE Columns add-on module allows you to define a "creep-producing permanent load." You can find the corresponding tab in Window "1.1 General Data."
The reason for the entry is that RF‑CONCRETE Columns can apply this "creep-producing permanent load" for the automatic determination of the effective creep ratio according to EN 1992‑1‑1, 5.8.4.
In contrast, there is no explicit input option for this creep-producing permanent load in RF‑CONCRETE Members. In RF‑CONCRETE Members, the stability analysis of reinforced concrete columns by means of nonlinear design does not automatically reduce the effective creep ratio. You can find the background to the effective creep ratio applied in RF‑CONCRETE Members in Chapter 2.4.6 of the RF-CONCRETE Members manual.
The same applies to the CONCRETE Columns or CONCRETE add-on modules for RSTAB.
RF-CONCRETE Columns determines the equivalent moment M0e from the moment M02 at the column head and M01 at the column base according to EN 1992‑1‑1, 184.108.40.206 (2), and performs the design according to the model column method with this equivalent moment M0e.
Now, it may happen, for example, that a computationally larger required reinforcement area would result from the cross-section design with the moment M01 at the column head.
To ensure this, message 28) is displayed, according to which the user should perform a standard design with the internal forces according to the linear static analysis. To do this, simply open the RF‑CONCRETE Members add-on module and perform pure design of the internal forces according to the linear static analysis for the member designed in RF‑CONCRETE Columns.
AnswerThe Design According to Formula column lists the equations of the standard used to carry out the design.The abbreviations stand for the following designs:CS Cross-section designST Stability analysisSE Serviceability (SLS design)The numbers directly behind it are internal information.The lower table of the intermediate values shows the design formulas with the design conditions that are relevant for the selected design.
AnswerYes, it is possible to select the high-strength steel SAS 670 for nonlinear design in RF‑CONCRETE Members. In this way, you can perform stability analysis for columns, among other things.The steel can be selected in the Materials section of the add-on module (see Figure 01). Since DIN EN 1992‑1‑1 only allows fyk = 500 N/mm² by default, this limit must be adjusted when using SAS 670.In General Data, you can create a user-defined National Annex where the maximum value of yield strength is increased to fyk ≥ 670 N/mm² in Point 3.2 (see Figure 02).
Since concrete has a nonlinear material behavior that can only be simulated with the CONCRETE NL module, it is not possible to analyze it by using the RF‑STABILITY add-on module.
The use of another material model such as isotropic linear elastic or isotropic plastic would not represent the crack formation correctly, and the results are therefore not usable.
The stability analysis on columns can be performed with RF‑CONCRETE Columns or RF‑CONCRETE NL. You can find a small example under Downloads.
This example includes the design of a column by the RF‑CONCRETE Columns add-on module. Make sure that the calculation of the internal forces in RFEM is performed according to the geometrically linear analysis and that no imperfections are required because the method used in the add-on module takes them into account.
The example also includes the design with RF‑CONCRETE NL. Here, it is also necessary to calculate according to the second-order analysis and it requires the imperfections in the form of inclinations. For better comparability, the layout of the longitudinal reinforcement was aligned with the result from RF‑CONCRETE Columns, as shown in Figure 01 and Figure 02. Since the reinforcement is optimized by the module after a new calculation, the desired reinforcement was saved as a template (see the red arrow).
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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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