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• ### Is it possible to consider the second-order theory in a dynamic analysis in RSTAB?

There is a detailed technical article for considering the second-order theory in a dynamic analysis in RFEM (see the link at the end of the FAQ). This article describes primarily the usage of the RF‑DYNAM Pro - Equivalent Loads add-on module. This add-on module does not allow for any consideration in RSTAB.

In order to correctly display the second-order theory in RSTAB, it is necessary to use the DYNAM Pro - Forced Vibrations add-on module. In this add-on module, the results are fully calculated within the add-on module. If you wanted to export the load cases first, the internal forces could not be calculated with regard to the modified stiffness matrix.
• ### 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?

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 perform a dynamic analysis of the initial deflection of a structural component in RFEM or RSTAB?

Yes, it is possible by using the RF‑/DYNAM Pro - Forced Vibrations add-on module. This application also allows you to use time history. It is available for both solution methods (linear Newmark analysis or modal analysis), but the procedures are slightly different.

The procedure is as follows:

1. Define a load case containing 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.

3. Then, select the defined time diagram in the dynamic load cases (it is irrelevant which load case you combine it with as 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 afterwards.

Use this procedure to simulate a vibration after the initial deflection. To illustrate this, you can find an example file under Downloads where this method is shown on a single-mass oscillator.

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

Yes, it is possible to consider the static zero position in a time history analysis. For this, you can use the "Stationary state" function (not available in the linear modal analysis).

This function allows you 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 can be activated in the RF‑/DYNAM Pro add-on module, in the "Calculation Parameters" tab of 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, you can see that the deformations oscillate about the value that is reached by the static zero 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 a time course 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 procedure to perform the design of the individual structural components?

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?

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.
• ### 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.

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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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