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Frequently Asked Questions (FAQ)
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AnswerTo consider nonlinearities in Dynamics, you need the addition of a Nonlinear Time History Method in RFEM / RSTAB. This addition is different in RSTAB and RFEM by the type of nonlinearities to be applied.RSTAB nonlinear time history method:Nonlinear member types such as tension and compression members as well as cablesMember nonlinearities such as failure, tearing, and creeping under tension or compressionSupports nonlinearities such as failure, friction, diagram, and partial actionNonlinearities of hinges such as friction, partial action, diagram and fast in case of positive or negative internal forcesRFEM Nonlinear Time-Course Processin addition, you can work with nonlinear material behavior
AnswerPlease note that in the context of the nonlinear calculation, the reinforcement is not increased automatically. A nonlinear calculation is based on an already provided reinforcement. This provided reinforcement can result, for example, from the ultimate limit state designs or a defined ground reinforcement. The reinforcement applied for the SLS designs can be displayed graphically (see Figure 2).Cause of issue:If no reinforcement results from the ULS or the basic reinforcement in some areas, the serviceability limit state design is carried out without the applied reinforcement, and very high results may result, for example in the crack width.Solution to the problem:It should be ensured that there is a reinforcement in each zone of the structure. This can be implemented by applying a basic reinforcement (see Figure 3).
Finite elements with plastic material are divided into 10 layers. First, a normal elastic analysis is performed in the first iteration. Then, the stress in each element is calculated according to the set strength hypothesis in each individual layer. If the limit stress in one of the layers is exceeded, the stiffness of this layer is reduced. Based on the reduced stiffnesses of the 10 layers, an overall stiffness is determined for each element. With this new stiffness, we start a new calculation iteration.
Iterates until the changes are only small.
The total stress is converted into the stresses of the individual layers by means of the laminate theory. This theory is also applied for the conversion between the stiffnesses of the layers and the total stiffness.
4 different strength models can be used as the limit stress:
- Shape Change Energy Hypothesis (von Mises-tension)
- Shear stress hypothesis (Tresca stress)
- Failure hypothesis according to Drucker-Prager
- Failure hypothesis after Mohr-Coloumb
The choice of the failure hypothesis is shown in Figure 1.
The von Mises hypothesis is preset because it is the most frequently used strength theory.
AnswerIn RF-STAGES, the temporary loads in the respective construction stages are calculated only linearly, according to the linear static analysis. The permanent loads that are embossed into the system become nonlinear, according to Theorie III. Order, calculated. In the combinations that can be created in the add-on module, the results of the individual load cases are combined.
AnswerThe 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.
AnswerFor this, the Duo Duct on the bottom side of the foundation body has to be determined and the load z. For example, apply surface loads. Then, you can adjust the properties of the nodal support, line support, or surface support such that it fails if the model is pulled or lifted (see Figure and example model).
AnswerThis should be possible if only one combination is used in the non-linear calculation. Theoretical background is that RF-SOILIN can not consider several situations in parallel with different bedding parameters. It can only be considered a case. If several load situations are considered in RF-BETON NL, theoretically several systems with different stiffness result. In the context of RF-SOILIN this leads to a contradictory situation.
AnswerIn RSTAB, it is also possible to determine plastic internal forces. To do this, you have to select a plastic hinge as nonlinearity by using the member properties (see Figure 1). However, this does not include an interaction of the plastic internal forces. Another possibility is to set releases with nonlinear properties directly. However, you have to use this method to estimate beforehand where these joints form.RFEM provides the option to select a corresponding material model (see Figure 2).
AnswerWhen calculating nonlinear material models, especially for plastic material models, the calculation is always performed in relation to the original constant internal forces.Therefore, you should set the smoothing of the FE elements to "Constant."Furthermore, the stresses are related to the principal stresses $ \ sigma_1 $ or $ \ sigma_2 $.Even if all these factors have been taken into account, there may still be deviations from the defined limit stress. These usually result from the defined hardening modulus.This fact is explained in detail for the technical articles listed below.
No, when using a nonlinear material model, it is not absolutely necessary to determine according to Theorie II. Or III. To calculate order. The material nonlinearity is also considered in the calculation according to the linear static analysis.
The calculation according to Theorie II. Or III. Order means that the equilibrium is set on the deformed system. Thus, it is a geometric nonlinearity.
Difference between II. And III. Order is that at III. Also large rotations may occur.
Thus, if there is no stability problem or the stability problem is analyzed in another way, the calculation according to the linear deformation analysis is sufficient.
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