Further Information

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• How is the plastic torsion determined in the plastic hinge definition?

The plastic rotation is determined automatically for each hinge, based on the previously determined cross-section and the member length. For this, the following formula is used:

$\varphi_{y,pl}=\frac{W_{y,pl}\cdot f_y\cdot l_b}{6\cdot E\cdot I_y}$

• Is it possible to set a plastic calculation of the internal forces in RSTAB or are the internal forces exclusively calculated elastically?

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).
• My cross-section is classified in Cross-Section Class 1. Are plastic hinges taken into account automatically?

No, hinges have to be defined separately in the model. They cannot be generated automatically by RF-/STEEL EC3.
• Which tools are available for a pushover analysis?

In RFEM, it is possible to determine a pushover curve or a capacity curve and export it to Excel. The following lists the steps that have to be performed:

1. Definition of nonlinear hinges:
• Plastic hinge according to FEMA 356: A nonlinear hinge (elastic-plastic or rigid-plastic) with preset yield values and acceptance criteria for steel members (Chapter 5 of FEMA 356). The yield strength values depend on the member and are preset automatically. The diagram parameters are interpolated, depending on the cross-section type. The "FEMA" hinges can be customized.
• Plastic hinge according to EN 1998-3: Bilinear definition of the yield curve. The bilinear hinges also have preset yield values, acceptance criteria and yield limits, but can be adjusted manually. The use of the plastic hinges has the advantage of a colored display of the hinges when evaluating the individual load steps. Thus, it is possible to quickly detect the exceeding of an acceptance criterion, for example.
• As an alternative to the definition of a hinge, RFEM provides the "Plastic Hinge" member nonlinearity. Here, a perfectly plastic behavior can be defined; the plastic limit values must be entered manually. The advantage of this option is that the location of the created plastic hinge is found automatically.
2. Definition of load schemes for nonlinear analysis: This can be done manually by entering loads into a load case, for example a uniformly distributed load over the building height.

A load distribution similar to the mode shape distribution can be generated automatically with the RF‑DYNAM Pro - Equivalent Loads add-on module. This module determines eigenvalues and equivalent loads based on the response spectrum analysis. For each selected eigenvalue, equivalent loads are generated and exported to RFEM in load cases.

3. Load increase in RFEM: A load increase can be defined in the calculation parameters of the load cases. The results of all load increments can be analyzed. Especially when using the "Plastic Hinges", the plasticization can be evaluated well by the colored marking of the hinges. It is important to scale the exported equivalent loads (from RF‑DYNAM Pro) so that the loads are not increased in too large increments. Figure 03 shows the load case exported from RF‑DYNAM Pro and the recommended calculation parameters.
4. Calculation diagrams for creating the pushover curve: You can find them in the "Global Calculation Parameters". You can define the total seismic load on the vertical axis and the deformation in the roof plane on the horizontal axis to obtain the desired pushover curve. You can easily export the data to Excel.

The color display of the plastic hinges is shown in Figure 05. You can select the color scale according to the acceptance criteria or according to the defined parameters of the hinge diagram.

A further pushover analysis (determination of the inelastic spectrum, performance point) can then be performed, for example, in Excel.

In the download area, you can find a detailed description of these instructions within a PDF document.

If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.

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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