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• ### What is the sign definition for the nonlinearity of a line release?

The direction determination is basically determined by the line release axis system and the location of the cut-out object. The line-sharing axis system display can be viewed in the Project Navigator - Show under Model -> Line enables are activated (see Figure 1).
Determining the direction for the nonlinearity determination is important how the released object moves relative to the line release axis system from the original element.
In Fig. 2, the area 2 with the line release u z fixed if v z is defined as a negative object. The load shown at 1 and 2 would move the released surface 2 against the z-axis of the line release axis system. The line release would therefore not be effective at these loads, ie the surfaces 1 and 2 would be firmly connected.
• ### Why do I need the RF-CONCRETE NL add-on module for the stability design in RF-CONCRETE Members? EC 2, Section 5.8.6, only speaks about geometric nonlinearity.

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.

• ### I've got a mechanical system that behaves nonlinearly, and I want to analyze it via direct time step integration (in time range / dynamically). Which method is best used for this?

In RFEM 5 or RF-DYNAM Pro - Nonlinear Time History, there are two different methods (also called "solvers" hereafter) available to you for nonlinear, dynamic analyses: the explicit central difference method and the implicit NEWMARK method of mean acceleration (γ = ½ and β = ¼).

In the case of linear systems, the implicit solver is preferable in most cases, because numerically it is absolutely stable, regardless of which time step length is selected. Of course this statement has to be somewhat relativized, given the fact that if the time steps are selected too crudely, substantial inaccuracies in the solution are to be expected. The explicit solver also has only limited stability in linear systems; it becomes stable, when the selected time step is smaller than a specific, critical time step:

$\triangle t\leq\triangle t_{cr}=\frac{T_n}\pi$

In this equation, Tn represents the smallest natural vibration period of the FE mesh, which leads to the following statement: The finer the FE mesh gets, the smaller the selected time step should become, in order to ensure numerical stability.

The calculation time of a single time step of the explicit solver is very short, but countless, very fine time steps may just be necessary to get a result at all. For that reason, the implicit NEWMARK solver for dynamic loadings that act over a long period of time, is preferable most of the time. The explicit solver is preferred, when you need to select very fine time steps anyway to get a useful (converging) result. This is the case, for example, in short-term and erratically variable loadings such as loads from shock or explosion.

In nonlinear systems, both methods are "only" numerically stable, but the implicit NEWMARK solver is still more stable than the central difference method in most cases. For that reason, the same statements as for linear systems apply to nonlinear systems. When the loads are erratically variable and short-term, the explicit solver is preferable, but in most other cases the NEWMARK solver of mean acceleration is preferred.
• ### Are nonlinear effects considered for the calculation in RF-STABILITY?

FAQ 002285 EN-US Add-on Modules RF-STABILITY

It is possible to consider effects from structural and material nonlinearities in RF-STABILITY. Set the calculation method “Increase load until structural failure” in the module.

These nonlinear effects are not considered for linear calculation.

• ### Do the DYNAM add-on modules perform a nonlinear analysis as well?

No, the calculation is always linear. All nonlinarities (for example failure at tension, nonlinear material, etc.) are ignored. Cables or tension members can be replaced by the "Beam" member type, for example.
• ### My model is stiffened by tension members. The calculation is terminated with the message about instability. How can I finish the calculation?

FAQ 000221 EN-US Calculation RSTAB RFEM

The calculation can be terminated due to an unstable structural system for various reasons. There can be a "real" instability due to overloading the system, but the instability effects may also be caused by failing members.

In the calulcation parameters, you can deactivate the nonlinearity "Members due to member type" (see Figure 01). If the calculation is then possible without the error message, the problem is probably caused by failing members.

The option "Failing members to be removed individually during successive iterations" in the Global Calculation Parameters dialog box (see Figure 02) allows you to prevent the complete failure of tension members. This will help in most cases. For this, the number of possible iterations should be sufficiently large.

An alternative method is to apply a prestress to the tension members in order to prevent the failure of them.