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• ### There are two different modules for the response spectrum analysis in RF-/DYNAM Pro. What are the reasons if the results of both modules differ?

New FAQ 003429 EN-US

The differences between the two modules are explained in this FAQ .

In general, you should also calculate the same results for both add-on modules if the settings are identical. However, this does not apply to existing nonlinearities. This is because no nonlinearities are considered in the RF-/DYNAM Pro add-on module. If the results are output via the Forced Vibrations add-on module, all nonlinearities are ignored. In contrast to this, the equivalent loads are calculated on a linear system, but the exported load cases are then calculated on the real system, that is, with all nonlinearities in RFEM or RSTAB . This may lead to inconsistent results.

If you deactivate the nonlinearities for the exported load cases, they should have identical results.

The way of considering nonlinearities in the response spectrum analysis is described using the tension members in this FAQ.

• ### Which time step should I select for the calculation of the time history analysis in the RF- / DYNAM Pro add-on module?

FAQ 002655 EN-US

There are two options to choose from: an automatic time step selection and a manual one. Especially for a structure with nonlinearities, it is always recommended to manually select the time step and perform a time step convergence study that compares the computation time and accuracy.

The time step to choose depends on many factors, including the excitation frequency, the frequency and size of the structure, and the degree of non-linearity. So no general statement about the size of the time step can be made.

For detailed information on this and many other topics, see the <a href="https://www.dlubal.com/-media/3BEF143083FA4DC3A9463B0E4166CCF0.ashx" target="_blank"> manual </a> of the additional module RF- / DYNAM Pro.

• ### Which explicit method is used in the RF-DYNAM Pro - Nonlinear Time History add-on module?

FAQ 002376 EN-US

The RF-DYNAM Pro - Nonlinear Time History offers, in addition to the implicit NEWMARK method of mean acceleration, also an explicit method. In the manual of this add-on module, it is mentioned that this is a solver which uses the central difference method.

It should be noted that not the "original" version of the central difference method is used here, but a modified form. The modified form is characterized by the fact that it is simply not a central difference when applying the speed difference. The following two equations show the applied speed and acceleration differences.

Speed: (no central difference)
${\dot{\mathrm x}}_{\mathrm n+\frac12}=\frac{{\mathrm x}_{\mathrm n+1}-{\mathrm x}_\mathrm n}{{\mathrm{Δt}}_{\mathrm n+{\displaystyle\frac12}}}$

Acceleration: (central difference)
${\ddot{\mathrm x}}_\mathrm n=\frac{{\dot{\mathrm x}}_{\mathrm n+{\displaystyle\frac12}}-{\dot{\mathrm x}}_{\mathrm n-\frac12}}{{\mathrm{Δt}}_\mathrm n}$

This approach leads to a faster convergence since it responds "faster" to changes in loading or structure (nonlinearities).
• ### I've got a mechanical system that behaves nonlinearly, and I want to analyze it via direct time step integration (in time range / dynamically). Which method is best used for this?

FAQ 002356 EN-US

In RFEM 5 or RF-DYNAM Pro - Nonlinear Time History, there are two different methods (also called "solvers" hereafter) available to you for nonlinear, dynamic analyses: the explicit central difference method and the implicit NEWMARK method of mean acceleration (γ = ½ and β = ¼).

In the case of linear systems, the implicit solver is preferable in most cases, because numerically it is absolutely stable, regardless of which time step length is selected. Of course this statement has to be somewhat relativized, given the fact that if the time steps are selected too crudely, substantial inaccuracies in the solution are to be expected. The explicit solver also has only limited stability in linear systems; it becomes stable, when the selected time step is smaller than a specific, critical time step:

$\triangle t\leq\triangle t_{cr}=\frac{T_n}\pi$

In this equation, Tn represents the smallest natural vibration period of the FE mesh, which leads to the following statement: The finer the FE mesh gets, the smaller the selected time step should become, in order to ensure numerical stability.

The calculation time of a single time step of the explicit solver is very short, but countless, very fine time steps may just be necessary to get a result at all. For that reason, the implicit NEWMARK solver for dynamic loadings that act over a long period of time, is preferable most of the time. The explicit solver is preferred, when you need to select very fine time steps anyway to get a useful (converging) result. This is the case, for example, in short-term and erratically variable loadings such as loads from shock or explosion.

In nonlinear systems, both methods are "only" numerically stable, but the implicit NEWMARK solver is still more stable than the central difference method in most cases. For that reason, the same statements as for linear systems apply to nonlinear systems. When the loads are erratically variable and short-term, the explicit solver is preferable, but in most other cases the NEWMARK solver of mean acceleration is preferred.
• ### Both RF-/DYNAM Pro - Forced Vibrations and RF-/DYNAM Pro - Equivalent Loads preform the multi-modal response spectrum analysis. What is the difference?

FAQ 002062 EN-US

The RF-/DYNAM Pro - Equivalent Loads add-on module analyzes structural equivalent loads and export them into load cases. Further analysis is performed in RFEM or RSTAB then. Result combinations are created.

The RF-/DYNAM Pro - Forced Vibrations add-on module provides the multi-point response spectrum analysis. It means that buildings on different supports can excite different situations. Only the final result combination is created here, so you receive no equivalent load and the traceability is more difficult.

• ### How many mode shapes do I need to consider in the dynamic analysis?

New FAQ 000242 EN-US

In principle, you need to consider so many mode shapes that the sum of the effective modal masses (that is, for a normalization of Mi = 1, the sum of the quadrats of the involved factors) is at least 90 % of the effective total mass.

For this, it is also useful to also calculate the equivalent masses in Window 1.1 General Data.
• ### I am interested in one of the modules for dynamic analysis. Can I buy each module separately or only in a package?

FAQ 000206 EN-US

You always need RF-/DYNAM Pro - Natural Vibrations. The additional modules can be purchased depending on your needs. The following additional modules are available: RF-/DYNAM Pro - Forced Vibrations and RF-/DYNAM Pro - Equivalent Loads.