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Frequently Asked Questions (FAQ)
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AnswerThe reason for the warning is that rod sets are detected by default according to General Procedure 6.3.4 EN 1993-1-1. However, this method is only for a load in the supporting plane, so no cross bending moment allowed and therefore the warning appears.You can perform a proof of stability according to theory II. Order as a cross-sectional proof,which is possible with the additional modules FE-BGDK or STAHL arching torsion.Alternatively you can neglect the transverse bending in the filter options of the "Details / Stability" or in the dialog "National Annex" the extended procedure according to Dr. med. Enable Naumes, which allows transverse bending.The best option in this case, however, is to select the equivalent bar method for the design of the bar sets in the "Details / Stability".
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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