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  • Answer

    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.


  • Answer

    Unfortunately, there are two different interfaces with the same file extension - the .stp interface of the German Steel Construction Association (Deutcher STahlbau-Verband, DSTV) on one hand, and the STEP interface (STandard for the Exchange of Product model data) on the other hand, which is defined in the ISO standard 10303.

    While the DSTV interface refers to the system axes of members, that is to the structural model, the STEP interface refers to the model geometry in the form of boundary lines, surfaces and simple solids. In the latter case, the models can be imported either as RFEM objects (nodes, lines, surfaces, solids) or as background layers so that the nodes can be set manually. The STEP interface does not usually transfers any system lines, but only the outer contours of structural components so it is necessary to manually create system planes afterwards. You can do this in RFEM and RSTAB, for example, by using a CAD/BIM model.

    If you get the error message shown in Figure 01, you have tried to import an .stp file based on STEP using the DSTV interface. As described above, both formats are not compatible with each other.

    In this case, select the STEP format for the import, as shown in Figure 02. If a message shown in Figure 03 appears, it is because this interface requires the RF-LINK add-on module.

  • Answer

    The calculation will be done faster. HOWEVER: Since only one iteration has been calculated, the results are only correct if there is also just one iteration required for the calculation. For example, this can be the case of load cases using linear static analysis. Also, object with nonlinear properties, such as tension and compression members, may not fail.

    On the contrary, load combinations according to second-order analysis require an iterative calculation. Therefore, the results of one iteration only are not correct.

    The number of possible iterations should always be sufficiently high. The number may also be much higher than the actually needed number of iterations, because the program will perform as many iterations as required. 

    Result Table 4.0 shows the number of performed iterations (see Figure).

  • Answer

    The selection of the time step depends on the natural frequency f of the system or the frequency of the exciting forces. To achieve a sufficient precision, the governing period T = 1/f is divided in about 20 steps, that is, the time step Δt is to be selected as follows: Δt <T/20 = 1/(20 f) = π / (10 ω)

    For transient defined excitations, such as accelerograms or time course monitors in tables, the shortest time period should be divided in 7 steps: Δt = Min{ti+1 - ti} / 7

    In RF-/DYNAM Pro, time steps can be automatically determined for the calculation. However, this is done by means of the defined accelerograms or time course monitors. Therefore, the manual entry of this time step should be considered. Regardless of the calculation, time steps are specified to save the results.

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