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Frequently Asked Questions (FAQ)
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AnswerEspecially the definition of slippage is a challenge for the solver due to the nonlinear calculation. In the following, hints are given how instabilities can be avoided.
load incrementsWhen considering nonlinearities, it is often difficult to find an equilibrium. Instabilities can be avoided by applying the loading in several steps (see Figure 01). For example, if two load increments are specified, half of the load is applied in the first step. Iterations are carried out until the equilibrium is found. Then, in the second step, the complete load is applied to the already deformed system and iterations are run again until the state of equilibrium is reached. Please keep in mind that load increments have an unfavorable effect on the computing time. A value of 1 (no gradual load increment) is therefore preset in the text box. In addition, you can specify separately for each load case and load combination how many load increments you want to apply (see Figure 02). The global settings are then ignored.
Sliding definitionSlippage (eg in one connection) is defined by means of the "Partial Effect" nonlinearity (see Figure 03). It can be used to define the hinge displacement from which the forces should be transferred. As can be seen in the diagram, the stop, that is, the stiffness that acts according to the corresponding hinge displacement, is considered as rigid (vertical branch, see the red arrows). However, under certain circumstances, this may lead to numerical problems in the calculation. To avoid this, the stiffness that acts after the hinge displacement should be reduced slightly. This is achieved by defining a very stiff spring (see Figure 04).In addition to the very stiff stop, numerical problems may occur within the slippage. In this case, a small stiffness has to be considered for the effect of the slippage in order to increase the horizontal branch a little bit. The stiffness should be selected so small that it has no decisive influence (see Figure 05). This situation is possible by using the "Diagram" nonlinearity.
Arrangement of Member HingesWhen arranging the hinges, care should be taken to ensure that they are not defined in the same direction on both member ends. Thus, there is a state in which the member is not sufficiently supported and the system fails already in the first iterations. In such a case, the slippage on only one side of the member should be defined and the size of the slippage adjusted accordingly (see Figure 06).
The setting in Figure 01 only controls the influence on the design side. After activating this function, the buckling designs in the "Effective Lengths" window are deactivated. Thus, only a lateral-torsional buckling analysis is performed.In order to calculate with the design values of the stiffnesses, they have to be reduced by the partial safety factor according to  . To do this, select the marked option in the calculation parameters (see Figure 02).If the automatic combinatorics was used for the standard EN 1990 + EN 1995, this setting is automatically activated for combinations for the ULS according to the second-order analysis. For combinations for SLS, this option is automatically deactivated as well as for combinations generated according to the 1st order. However, the automatic assignment is only carried out by means of the calculation type in the "Combination Expressions" window (see Figure 03).If you want to calculate the component stiffnesses with the 5% quantile value of the stiffness parameters divided by the partial safety factor, you have to additionally activate the "Modify Stiffness" function in the calculation parameters and modify the stiffnesses manually.
Yes, the CONCRETE for RSTAB 8 module also includes the nonlinear reinforced concrete design. Thus, you can activate the "Nonlinear design (state II)" in the "Ultimate Limit State" tab.
In the detail settings for the nonlinear design, you can select the "General design method for members subjected to compression according to the second-order analysis".
It is important that you define the imperfections in RSTAB and apply load curves (CO) according to the second-order analysis for the design, no result combinations (RC)!
Note on RFEM 5:
In RFEM 5, the same procedure is possible in RF-CONCRETE Members. However, the add-on module RF-CONCRETE NL in RFEM is required for the non-linear reinforced concrete design.
AnswerNo, this is not possible.A member or a set of members is designed as a 'Structural Component' by the model column method.This means that the longitudinal reinforcement is constant over the entire length of the structural component.The design is carried out by means of the governing internal forces at the governing location of the structural component (member or set of members).A stability analysis involving a graded reinforcement situated over the column can be done by means of the nonlinear design for compression elements in RF-CONCRETE Members (RFEM) or CONCRETE (RSTAB).
AnswerThe RF-CONCRETE Columns add-on module uses the model column method with the nominal curvature or the nominal curvature method according to EN 1992-1-1.You can also find more information about the program features of RF-CONCRETE Columns on the product homepage via the following link. See links below.
AnswerPlease check if all sets of members selected for the design are sets of members. The equivalent member method is only applicable for straight sets of members with a uniform cross-section involving, for example, no taper. In this case, use the preset General Method.
AnswerThe reason is that there is no stability analysis for unsymmetrical, open cross-sections according to EN 1999-1-1 when the compressive normal forces and the bending moments are effective.You can neglect the bending moments in Details in the Stability tab by selecting a filter. Then, the flexural buckling design is performed without moments. However, this is under your own responsibility. An alternative is to use a stability check according to the second-order analysis, which would be possible by means of the RF-/FE-LTB add-on module.
If you need to define different types of lateral intermediate supports, you need to divide the specific member. After that, you can create a set of member and with that done, you can easily define different types of intermediate supports along this set of member, or you can use different nodal supports in the nodes of the set of member.
Currently, there is no option to perform a stability analysis in the RF-CONCRETE Surfaces add-on module. However, it may be possible to perform a simplified design by means of a result beam by means of the nominal curvature method in RF-CONCRETE Columns.
AnswerThis function is intended to detect modeling errors in the structure that may lead to instability. Using this method, it is possible to calculate such systems and to determine the cause of the instability graphically.This function is not suitable for the following problems:
If the system is stable and only stability problems occur during the calculation according to the second-order analysis, all results are set to 0 with this function.The problem solving of instabilities is described in detail in FAQ 2257 .
- Calculation aborts due to overloadings (stability problems)
- Determination of buckling curves and buckling modes
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