Back to Videos
Is this page helpful?
841x
002458
Dipl.-Ing. (FH) Walter Fröhlich
2021-03-17

KB 000982 | Unstable Model

The most common causes of unstable models are failing member nonlinearities such as tension members.
As the simplest example, there is a frame with supports on the column footing and moment hinges on the column head. This unstable system is stabilized by a cross bracing of tension members. In the case of load combinations with horizontal loads, the system remains stable. However, if it is loaded vertically, both tension members fail and the system becomes unstable, which causes a calculation error.
You can avoid such an error by selecting the exceptional handling of failing members under "Calculate" → "Calculation Parameters" → "Global Calculation Parameters".

Links
  • Mia: AI Assistant
  • Latest Videos
Videos
Models to Download
Knowledge Base Articles
Visualization of Cantilever with Seven Mass Nodes
RF-/DYNAM Pro - Equivalent Loads allows you to determine the loads due to equivalent seismic loads according to the multi‑modal response spectrum method. In the example shown here, this was done for a multi‑mass oscillator.
Option "Deactivate" in RSTAB
Both the determination of natural vibrations and the response spectrum analysis are always performed on a linear system. If nonlinearities exist in the system, they are linearized and thus not taken into account. Straight tension members are very often used in practice. This article will show how you can display them approximately correctly in a dynamic analysis.
Approximate Consideration of P-Delta Effects with Amplification Factors in RF-DYNAM Pro - Equivalent Loads
As gravity loads act on a structure, lateral displacement occurs. In turn, a secondary overturning moment is generated as the gravity load continues to act on the elements in the laterally displaced position. This effect is also known as "P-Delta (Δ)". Sec. 12.9.1.6 of the ASCE 7-16 Standard and the NBC 2015 Commentary specify when P-Delta effects should be considered during a modal response spectrum analysis.
Determination of Maximum Horizontal and Vertical Loads to Calculate Stability Coefficient
RFEM offers the option to perform a response spectrum analysis according to ASCE 7-16. This standard describes the determination of seismic loads for the American market. It might happen that the P-Delta effect has to be considered due to the stiffness of the entire structure in order to calculate the internal forces and carry out the design.
Screenshots
Product Features
Defining 2 FE Mesh Layers for One Gas Solid

The stiffness of gas given by the ideal gas law pV = nRT can be considered in the nonlinear dynamic analysis.

The calculation of gas is available for accelerograms and time diagrams for both the explicit analysis and the nonlinear implicit Newmark analysis. To determine the gas behavior correctly, at least two FE layers for gas solids should be defined.

  • Automatic consideration of masses from self-weight
  • Direct import of masses from load cases or load combinations
  • Optional definition of additional masses (nodal, linear, surface masses, as well as inertia masses)
  • Combination of masses in different mass cases and mass combinations
  • Preset combination coefficients according to EC 8
  • Optional import of normal force distributions (in order to consider prestress, for example)
  • Stiffness modification (for example, deactivated members or stiffnesses can be imported from RF-/CONCRETE)
  • Consideration of failed supports or members
  • Definition of several natural vibration cases (for example, to analyze different masses or stiffness modifications)
  • Results of eigenvalue, angular frequency, natural frequency, and period
  • Determination of mode shapes and masses in nodes or FE mesh points
  • Results of modal masses, effective modal masses, and modal mass factors
  • Visualization and animation of mode shapes
  • Various scaling options for mode shapes
  • Documentation of numerical and graphical results in the printout report
Natural Vibration as Graphical Result Display in RFEM

After the calculation, the eigenvalues, natural frequencies, and natural periods are listed. These result windows are integrated in the main program RFEM/RSTAB. The mode shapes of the structure are included in tables and can be displayed graphically or as an animation.

All result tables and graphics are part of the RFEM/RSTAB printout report. This way, clearly arranged documentation is ensured. Furthermore, it is possible to export the tables to MS Excel.

Calculation Parameters

The RF-DYNAM Pro - Natural Vibrations module of RFEM provides four powerful eigenvalue solvers:*Root of characteristic polynomial

  • Method by Lanczos
  • Subspace Iteration
  • ICG iteration method (Incomplete Conjugate Gradient)

The DYNAM Pro - Natural Vibrations module for RSTAB provides two eigenvalue solvers:

  • Subspace Iteration
  • Shifted inverse power method

The selection of the eigenvalue solver depends primarily on the size of the model.

Frequently Asked Questions (FAQ)
In RF‑/DYNAM Pro, the "From self-weight of structure" option is available in a mass case. Is it always necessary to activate this option in order to consider the self-weight of the structure?
Warum bekomme ich bei einer Berechnung mit RF-/DYNAM Pro nur Erdbebenlasten in eine Richtung berechnet?
During the check in RF‑/DYNAM Pro, I get the message "No natural vibration case or dynamic load case has been selected for the calculation". Why?
Is it possible to display natural frequencies separately for each direction?
When performing a dynamic calculation, I get the following message: "No free mass is assigned to the model. The dynamic calculation is therefore not possible" or "Too many eigenvalues required." How can I calculate mode shapes?
What is the approach for seismic design in RFEM or RSTAB 8?
Customer Projects
Recommended Products for You
Display Product Family