Further Information

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

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?

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.

• Is it possible to display or export certain results over a period of time from the time history calculation in RF-DYNAM Pro - Forced Vibrations?

With the time history monitor, you can view all results over a period of time. In this case, it is also possible to select several parts of the structure and then export the results directly to Excel.
• In RF-/DYNAM Pro, the option "From Self-Weight of Structure" is available in the mass case. Does this option always have to be activated to consider the self-weight of the structure?

No, this option does not necessarily have to be activated to consider the self-weight. If the masses are imported from a load case that already contains the self-weight, this option must not be activated. Otherwise, the self-weight of structure is doubled.
• Is there a possibility to display the stresses for a solid model according to the dynamic time history analysis?

In the results tables of the RF-DYNAM Pro module, the stresses for solids are not displayed. To be able to display the stresses and internal forces, it is necessary to export the results to a load case or to a result combination. Then, you can look at the results in a load case or a result combination as usual.

• In RF-/DYNAM Pro, it is possible to find the Rayleigh damping. How do I determine these coefficients and what is their application?

For some solution methods, the Rayleigh coefficients are absolutely necessary. Since only the Lehr's damping values are given in the literature, they have to be converted.

The following formula is used for converting Lehr's damping values into Rayleigh coefficients:

${\mathrm D}_{\mathrm r}\:=\:\frac12\;\left(\frac{\mathrm\alpha}{{\mathrm\omega}_{\mathrm r}}\;+\;\mathrm\beta\;{\mathrm\omega}_{\mathrm r}\right)$

Where α and β are the Rayleigh coefficients. It is necessary to set up a system of equations always containing the angular frequencies of the two most dominant mode shapes. In the case of these two mode shapes, the structure will then be damped with the specified damping value. All other mode shapes of the structure will have different damping values. These result from the curve displayed in Figure 01. The curve shows an example of the two angular frequencies of 10 and 20 rad/s and Lehr's damping of 0.015.

It is also possible to use the 'Calculate from Lehr's Damping ...' button to activate corresponding conversion tool.
• How can I display the results of the RF‑/DYNAM Pro add-on module in the printout report?

The results of the RF‑/DYNAM Pro add-on modules Forced Vibrations , Nonlinear Time History and Equivalent Loads are not listed directly in the printout report. This is generally due to the fact that dynamic calculations require a lot of data and results.

In each of the mentioned add-on modules, it is possible to create a result combination with the envelope results. In this generated result combination, you can find the same results as in the main programs and display them in the printout report as usual.

Furthermore, you can print pictures in the printout report as usual. There is also an option to display the time history graphically in the printout report.
• How can I consider prestressed cables in RF‑/DYNAM Pro?

The prestress of cables has a governing influence on the behavior of the structure. Therefore, it must also be considered in the dynamic analysis.

In the natural vibration cases of the add-on module, it is possible to consider stiffness modifications on the basis of a load case. This option allows you to rewrite the geometric stiffness matrix for the natural vibration analysis. In order to avoid the falsification of the results or to consider any unwanted effects, it is important to only include the prestress and no further loading in the load case to be imported.

If you want to perform the time history analysis, make sure that the implicit Newmark analysis and the explicit analysis do not use the natural vibration cases, but that the calculation parameters are defined directly in the dynamic load case. Therefore, it is also necessary to consider the prestress. For this, the "Stationary State" feature is available. This option allows you to consider the stiffness modifications resulting from the prestress.

• After running a time history analysis, I am not able to see all results on surfaces. Is there a possibility to show them?

In the results of the RF‑/DYNAM Pro add-on module, "Basic Internal Forces" and "Basic Stresses" on surfaces are only provided. However, you can use the option to export the results in a result combination to display all results after the time history analysis.

Simply use this option to create a result combination in the add-on module as shown in Figure 01, and show the results in the graphic. All results, such as "Design Internal Forces" or "Stresses," are available then.
• Is it also possible to use nonlinear material behavior in a time history analysis?

To be able to consider nonlinearities in dynamics, the RF-/DYNAM Pro - Nonlinear Time History add-on module is required in RFEM/RSTAB. This module is different in RFEM and RSTAB, depending on the type of nonlinearities to be applied.

RSTAB – Nonlinear Time History Analysis

• Nonlinear member types, such as tension and compression members as well as cables
• Member nonlinearities, such as failure, tearing, and yielding under tension or compression
• Support nonlinearities, such as failure, friction, diagram, and partial activity
• Hinge nonlinearities, such as friction, partial activity, diagram, and fixed if positive or negative internal forces

RFEM – Nonlinear Time History Analysis

In addition to the nonlinearities mentioned above, it is also possible to use nonlinear material behavior.

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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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