# Frequently Asked Questions (FAQ)

#### Customer Support 24/7

In addition to our technical support (e.g. via chat), you’ll find resources on our website that may help you with your design using Dlubal Software.

Receive information including news, useful tips, scheduled events, special offers, and vouchers on a regular basis.

• ### Is it also possible to use nonlinear material behavior in the time history analysis?

To consider nonlinearities in Dynamics, you need the addition of a Nonlinear Time History Method in RFEM / RSTAB. This addition is different in RSTAB and RFEM by the type of nonlinearities to be applied.

RSTAB nonlinear time history method:

Nonlinear member types such as tension and compression members as well as cables

Member nonlinearities such as failure, tearing, and creeping under tension or compression

Supports nonlinearities such as failure, friction, diagram, and partial action

Nonlinearities of hinges such as friction, partial action, diagram and fast in case of positive or negative internal forces

RFEM Nonlinear Time-Course Process

in addition, you can work with nonlinear material behavior

• ### Why can not I activate the "Nonlinear time history method" option in the RF-DYNAM Pro add-on module?

The RF-DYNAM Pro - Nonlinear Time History Analysis add-on module is only available for 3D models. So you have to change over to "3D" in the model's general data.
• ### I create a picture for the time in DYNAM PRO, and paste it via the clipboard in the printout report, this picture appears very blurred. What can I do about it?

In this case, you have the option to print the image in the timeline diagram directly into the printout report. Here you proceed as described in the picture.
• ### With the additional module "RF- / DYNAM Pro" there are the parameters modal mass, participation factor and substitute mass. The manual contains the formulas, the meaning and the explanation, as well as usage would be just as helpful.

Modal mass

Each multi-mass system can usually be represented by a single-mass system. When you do this transformation, you need the modal mass of the system. This mass is needed to generate the frequency of the equivalent single-frequency oscillator.

Beiteilungsfaktor

This factor can also be negative because it is composed of the substitute mass at a node and the associated displacement due to the eigenform. If the deflection is in the negative direction, the participation factor becomes negative. The replacement mass factor is then still positive, since the participation factor is squared. (see formula)

equivalent mass

The equivalent mass of a system is a part of the total mass which is excited due to the vibration of the multi-mass oscillator. The equivalent mass of a system can be between zero and the total mass. The replacement mass factor is only the quotient of the total mass to the substitute mass. As a rule, this makes it possible to check more quickly what proportion of the excited mass of the respective eigenform is. Should it happen that the substitute mass factor is greater than 1, one should check the discretization of the system and, if necessary, refine the division of the nodes or the FE mesh.

For an earth analysis, the substitute mass factor and the substitute mass are usually decisive, since these values are used to calculate the dynamic equivalent loads on the building.
• ### Which time step should I choose for calculating the time history method in the module RF- / DYNAM Pro?

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?

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 analyse it via direct time step integration (in time range / dynamically). Which method is best used for this?

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 relativised, 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.