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• ### In RF-/DYNAM PRO - Forced Vibrations, why does my time diagram function formula output incorrect values which appear to be off by only a factor?

New

FAQ 004342 EN

In the Dlubal programs, all values are stored internally based on SI units. When a user changes the units in the program to metric or imperial, SI units are still used internally and only the value displayed in the interface is modified. Therefore, all values set in the time diagram function also default to SI units unless the user clarifies an alternative unit.

Let's look at a simple example shown in Figure 1 where the parameter x = 1 ft has been set in RFEM.

In RF-DYNAM PRO - Forced Vibrations, the time diagram function is defined as k(t) = 1*x where 1 is the multiplier (1/ft) to convert x to a dimensionless value. You can see in Figure 2, because all values default to SI units, the Multiplier column produces values of 0.305 instead of the correct value of 1.000.

In order to correct the issue, the user only needs to specify the units of the multiplier as (1/ft) in the function equation. This can be done with the formula modification k(t) = 1/1[ft]*x as shown in Figure 3. Notice the Multiplier column now shows the correct values of 1.000.

In summary, when using units in the program other than SI units, coefficients or multipliers in the time diagram function should be accompanied with alternative units defined in brackets.

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

With the Equivalent Loads and Forced Vibrations add-on modules, you can create result combinations that contain the governing combinations of seismic loads. To perform a design with them, they have to be combined further on the basis of the extraordinary combination. This combination is defined, for example, in EN 1990 Art. 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. A combination like this 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 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 any option to display the stresses for a solid model after a dynamic time history analysis?

The result tables of the RF‑DYNAM Pro add-on module do not display the stresses for solids. In order 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 view the results in a load case or a result combination as usual.

• ### There are two solving methods for a linear time history analysis available in the RF‑/DYNAM Pro add-on module. What is the difference between them?

Both solving methods, the "linear modal analysis" and the "linear implicit Newmark analysis," are available.

###### Linear Modal Analysis

This solving method uses a decoupled structure that is based on eigenvalues and mode shapes of the structure. It is necessary to assign a defined natural vibration case.

This method should only be used if there is a sufficient number of eigenvalues of the structure calculated in the natural vibration case. This means that you should pay attention to achieve an effective modal mass factor of the total structure of approximately 1 in all governing directions. If this is not possible, this method will lead to inaccurate results.

###### Linear Implicit Newmark Analysis

This is a direct time step method that does not require a natural vibration case, and requires enough small time steps to achieve the exact results.

This method is recommended for complex structures, which would require a very large number of mode shapes in order to achieve an effective modal mass factor of about 1.

If a sufficient number of eigenvalues can be guaranteed by means of the linear modal analysis, both solving methods lead to approximately same results. For more information about both methods, read the manual of RF‑DYNAM Pro.

• ### In RF-/DYNAM Pro, the Rayleigh damping is available. How do I determine and use these factors?

For some solving methods, the Rayleigh coefficients are absolutely necessary. Since the Lehr's damping values are only given in the literature, it is necessary to convert them.

The following formula is used for converting the Lehr's damping values into the 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 that always contains the angular frequencies of the two most dominant mode shapes. For both of these mode shapes, the structure will then be damped with the specified Lehr's 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 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 the 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.

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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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