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Frequently Asked Questions (FAQ)
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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.
Nodal supports and boundary conditions are preset, but not imported. As a rule, you have to adjust them.
In the case of the General Method for the set of members design in RF‑/STEEL EC3, nodal supports phi_x ' and u_y' are predefined at the set of members start and end, which represents a support with torsional restraints for all straight sets of members.
In RF-/STEEL Warping Torsion, nodal supports u_x, u_y, u_z, and phi_x are always predefined at the set of members start and end.
For the stability design of compression elements, you need the combination of RF-CONCRETE Members and RF-CONCRETE NL. The reason is the following:
First, the internal forces of the individual load combinations (second-order analysis + imperfection) are subjected to the linear-elastic calculation. For this, you basically only need RFEM.
Then, the cross-section design is performed in RF-CONCRETE Members with these internal forces determined linearly-elastically, and the required bending reinforcement is determined from these internal forces.
This bending reinforcement is then compared with the user-defined entries concerning the existing basic reinforcement or the minimum reinforcement and based on this, the reinforcement concept is generated (dialog box '3.1 Existing Longitudinal Reinforcement' of the module).
This existing longitudinal reinforcement is then used for the nonlinear design.
According to Section 5.8.6 (1), geometric nonlinearities must be taken into account according to the second-order analysis. However, the general rules for nonlinear methods according to 5.7 also apply.
In Sec. 5.7(1), 'an adequate non-linear behaviour for materials is assumed'. According to 5.7(4)P, the use of material characteristics which represent the stiffness in a realistic way but take account of the uncertainties of failure shall be used when using non-linear analysis.
This requires the RF-CONCRETE NL add-on module. Thus, the geometric and material nonlinearities are considered and the requirements of EC 2 regarding the ultimate limit state design are fulfilled.
Similarly, this method is also available in RSTAB in the CONCRETE add-on module.
No, you do not have to worry. If the value alpha*crit (the critical buckling value without torsional stiffness) is displayed in red and thus amounts to '0', there is no reduction of the imperfection factor (European lateral-torsional buckling line). The basic value for the imperfection factor is used.
The 'Mode Shape - Overview' window allows you to check the plausibility of the eigenvalue analysis. You can open this window by clicking the button marked in the figure.
The graphical control of the mode shape graphic is available if the eigenvalue solver was used in the calculation, for example, in the case of the analysis according to General Method in compliance with EN 1993‑1‑1.
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