# Frequently Asked Questions (FAQ)

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• ### Is it possible to dynamically analyze an initial deflection of a component with RFEM or RSTAB?

#### Answer

Yes, this is possible with the RF-/DYNAM Pro - Forced Vibrations add-on module. This application enables also using time history. It is available for both solution methods (linear Newmark analysis or modal analysis), but the procedures differ slightly.

The procedure is as follows:

1. Define a load case that contains a load that causes the desired initial deflection.
2. Activate the time history method of time diagrams in RF-/DYNAM Pro and define a new time diagram that has a multiplier of 0 over the entire time.

Figure 01 - Time Diagram with Multiplier 0

3. Then, select the defined time diagram in the dynamic load cases (which load case you combine it with is irrelevant because it multiplies the load by 0). You can then import the load case defined in the first step as the initial condition (or only the initial deformation in the modal analysis). Thus, the conditions from this load case are imported at time t = 0 and released immediately afterward.

Figure 02 - Activation of Initial Condition/Deformation in Dynamic Load Cases

Use this procedure to simulate a vibration after an initial deflection. To illustrate this, there is an example file in the download area where this method is displayed on a single-mass oscillator.

• ### Is it possible to consider the static rest position in a time history analysis?

#### Answer

Yes, you can consider the static rest position in a time history analysis. For this you can use the 'Stationary State' function (not available in the linear modal analysis).

With this function, it is possible to read a condition from a load case or a load combination that exists over the entire time history. These include the deformations, the stiffness modifications, and the states of nonlinearities.

This function is activated in the RF-/DYNAM Pro add-on module, in the 'Calculation Parameters' tab of the Dynamic Load Cases.

The time history diagram is well suited for results evaluation. In the attached example, load case 1 with the dead load was defined as a stationary state. In the time diagram, it can be seen that the deformations oscillate about the value that is reached by the static rest position.

• ### What is the approach for seismic design in RFEM/RSTAB?

#### Answer

For seismic design, the add-on modules RF-/DYNAM Pro - Natural Vibrations and RF-/DYNAM Pro - Equivalent Loads are available. They allow you to perform the multimodal response spectrum analysis. After the analysis in the add-on modules, the calculated seismic loads are exported to load cases, which can be evaluated as usual.

Furthermore, you can evaluate the story drift and the horizontal shear of the building. These and other features are described in detail in the webinar "Response Spectrum Analysis in RFEM."
• ### Is it possible to hide background layers?

#### Answer

The background layers can be hidden or displayed in the Display Navigator under Guide Objects → Background Layers. The visibility can be defined individually or for all.

• ### Is there any possibility in RFEM to consider friction for the horizontal reactions of a surface support?

#### Answer

For surface supports, this option is only available if there is also the nonlinearity defined in the local z direction (failure if contact stress in z is negative/positive). In the dialog box where you can edit the nonlinearity, you can find the "Friction in plane xy" option.

This option works as shown in the graphic dialog box: The support in the x and y direction is fully accepted only when reaching the contact stress Tau (contact stress Sigma multiplied by the friction coefficient). It is necessary to reduce this linearly in advance.

In order to use this option, a support must be in the horizontal directions. It can be defined as fixed or with an elastic spring. If the spring is defined with 0, no support is considered even though a friction coefficient has been entered.

• ### In a nonlinear structural system, some load cases do not converge. However, load combinations can be calculated without any problems. What could be the reason for this?

#### Answer

When defining nonlinearities, such as support failure under tension, it may happen that some load cases cannot be calculated. If these are the loads that cannot exist without other stabilizing loads, the problem resolution is simple: You can set the load cases to "Not To Be Calculated." As a result, the load combinations are only considered in the "Calculate All" option of the calculation process. This is possible because some loads can never appear without a dead load, for example.

In the attached example, it is clearly evident that the structural system would buckle in the Wind load case, and thus no convergence is found. In contrast to this, it is possible to calculate the load combination, where the dead load and the wind are combined without any problem because the dead load stabilizes the system.
• ### What is the difference between the materials Isotropic Plastic 1D and Isotropic Nonlinear Elastic 1D?

#### Answer

The difference between both material models is as follows:

In the Isotropic Nonlinear Elastic 1D material model, no plastic deformations are considered. This means that the material returns to its initial state after the load relief.

In the case of the Isotropic Plastic 1D material model, the plastic deformation is considered.

For both material models, the nonlinear properties are defined in an additional dialog box. When entering data by means of a diagram, it is possible in both models to define the distribution after the last step.

The Isotropic Nonlinear Elastic 1D material model allows for the anti-symmetric input of the stress-strain diagram (different for the positive and negative zone), whereas the isotropic Plastic 1D model only allows for symmetric input.

• ### I have used the RF‑/DYNAM Pro add-on module to generate the governing result combinations of seismic loads. What is the next procedure to perform the design of the individual structural components?

#### Answer

With the Equivalent Loads and Forced Vibrations add-on modules, you can create the result combinations that contain the governing combinations of seismic loads. To perform the design using them, they have to be combined further on the basis of the accidental combination. This combination is defined, for example, in EN 1990, Section 6.4.3.4:

${\mathrm E}_{\mathrm d}\;=\;\underset{}{\sum_{}^{}\;{\mathrm G}_{\mathrm k,\mathrm j}\;+\;\mathrm P\;+\;{\mathrm A}_{\mathrm{Ed}}\;+\;}\overset{}{\underset{}{\sum{\mathrm\psi}_{2,\mathrm i}\;{\mathrm Q}_{\mathrm k,\mathrm i}}}$

This accidental combination has to be defined manually in RFEM. Make sure that (for a direction combination with the 100/30% rule), both created result combinations from RF‑/DYNAM Pro have to be added with the "OR" condition. Such a combination is displayed in Figure 02.

This accidental combination can then be used for further design. It is possible to evaluate the governing internal forces as well as to import and calculate this combination in the design modules.
• ### In RF‑/DYNAM Pro, the "From self-weight of structure" option is available in a mass case. Is it always necessary to activate this option in order to consider the dead load of the structure?

#### Answer

No, this option does not necessarily have to be activated to consider the dead load. If the masses are imported from a load case that already contains the dead load, it is not necessary to activate this option. Otherwise, the dead load of the structure will be doubled.
• ### What method is applied in the RF‑/DYNAM Pro - Equivalent Loads add-on module?

#### Answer

Just as in the "Forced Vibrations" add-on module, the "Equivalent Loads" add-on module performs the multimodal response spectrum analysis.

Although the name may suggest otherwise, the simplified response spectrum method is not carried out here, as explained in EN 1998‑1, for example.

The equivalent loads are determined separately for each direction of excitation according to the following formula:

$\begin{Bmatrix}{\mathrm F}_{\mathrm X}\\{\mathrm F}_{\mathrm Y}\\{\mathrm F}_{\mathrm Z}\end{Bmatrix}\;=\;\mathrm\Gamma\;\ast\;\begin{Bmatrix}{\mathrm u}_{\mathrm X}\\{\mathrm u}_{\mathrm Y}\\{\mathrm u}_{\mathrm Z}\end{Bmatrix}\;\ast\;{\mathrm S}_{\mathrm a}(\mathrm T)\;\ast\;\begin{Bmatrix}{\mathrm M}_{\mathrm X}\\{\mathrm M}_{\mathrm Y}\\{\mathrm M}_{\mathrm Z}\end{Bmatrix}\;$

The differences between both add-on modules are described in this FAQ.

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#### First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

#### Wind Simulation & Wind Load Generation

With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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