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  • Answer

    In RSTAB, it is also possible to determine plastic internal forces. To do this, you have to select a plastic hinge as nonlinearity by using the member properties (see Figure 1). However, this does not include an interaction of the plastic internal forces. Another possibility is to set releases with nonlinear properties directly. However, you have to use this method to estimate beforehand where these joints form.

    RFEM provides the option to select a corresponding material model (see Figure 2).
  • Answer

    No, hinges have to be defined separately in the model and cannot be generated automatically by RF-/STEEL EC3.
  • Answer

    In RFEM, it is possible to calculate a pushover curve (also called capacity curve). The pushover curve data can be exported to Excel. Important steps in order to evaluate this nonlinear curve are listed below:

    1. Comprehensive definition of nonlinear hinges
      1. Plastic hinges in accordance to FEMA 356: This is a nonlinear hinge (elastic‑plastic or rigid‑plastic) with default values for yielding points in the hinge diagram and acceptance criteria, both for steel members (Chapter 5 in FEMA 356). The yielding limits of the members are dependent on the cross‑sections and are set automatically. For moment hinges, the diagram parameters and acceptance criteria are interpolated for different type of cross‑sections. You can adjust all the values of a 'FEMA' hinge using your own user‑defined values. Figure 1 shows the Plastic Hinge dialog box.

        Plastic hinges in accordance to EN 1998‑3: This is a nonlinear hinge with a bilinear definition. The bilinear hinges include predefined values for the yielding points in the hinge diagram, acceptance criteria, and yielding limits of the cross‑sections. These values can also be adjusted manually.

        The plastic hinges are displayed with various colors when viewing the results of several load steps. The colors depend on the state of plasticity. This is very useful to identify exceeded acceptance criteria.

      2. Instead of using hinges, you can use the 'Plastic Hinge' type of member nonlinearity in RFEM. In the relevant dialog box, you can define perfectly plastic behavior and set the yielding limits manually. The main advantage of this option is that the location of the plastic hinge is found automatically during the iterative calculation. See Figure 2 for further explanation.

    2. Definition of load pattern for nonlinear analysis

      You can manually define loads in a specific load case, for example a uniform load pattern.

      To achieve load distribution in accordance to a mode shape of your structure you can use RF‑DYNAM Pro - Equivalent Loads. This add‑on module calculates eigenvalues, mode shapes and equivalent loads by using the multi‑modal response spectrum analysis. For each selected eigenvalue, the program automatically exports the equivalent loads into a load case in RFEM.

    3. Load increments in RFEM

      In the calculation parameters of load cases, you can define an incrementally increasing load. The results of each load step can then be analyzed. When using 'Plastic Hinges', you can easily evaluate the plasticity states due to the color marking of the hinges. It is important to scale the exported equivalent loads from RF‑DYNAM Pro to avoid load increments in too large steps. 

      Figure 3 shows a load case exported from RF‑DYNAM Pro and the recommended calculation parameters.

    4. Calculation diagrams for creating pushover curve

      These diagrams are available in the 'Global Calculation Parameters' dialog box. Here you can set the sum of support forces on the vertical axis and the deformation in a roof plane on the horizontal axis and thus obtain the required pushover curve. The data can easily be exported to Excel. The pushover curve is displayed in Figure 4.

      Figure 5 shows the color representation of the plastic hinges. The color scale can display the hinge diagram parameters or the acceptance criteria.

      The continuing pushover analysis (determination of the inelastic spectrum and performance point) can further be carried out in an external program (for example Excel).

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