Modal Analysis for RFEM 6 / RSTAB 9

Features

• Automatic consideration of masses from self-weight
• Direct import of masses from load cases or load combinations
• Optional definition of additional masses (nodal, linear, or surface masses, as well as inertia masses) directly in the load cases
• Optional neglect of masses (e.g. mass of foundations)
• Combination of masses in different load cases and load combinations
• Preset combination coefficients for various standards (EC 8, SIA 261, ASCE 7,...)
• Optional import of initial states (e.g. to consider prestress and imperfection)
• Structure modification
• Consideration of failed supports or members/ surfaces/ solids
• Definition of several modal analyses (e.g. to analyze different masses or stiffness modifications)
• Selection of mass matrix type (diagonal matrix, consistent matrix, unit matrix) including user-defined specification of translational and rotational degrees of freedom
• Methods for determining the number of mode shapes (user-defined, automatic - to reach effective modal mass factors, automatic - to reach the maximum natural frequency - only available in RSTAB)
• Determination of mode shapes and masses in nodes or FE mesh points
• Output of eigenvalue, angular frequency, natural frequency, and natural period
• Output of modal masses, effective modal masses, modal mass factors, and participation factors
• Tabular and graphical output of masses in mesh points
• Visualization and animation of mode shapes
• Various scaling options for mode shapes
• Documentation of numerical and graphical results in the printout report

Input

All the necessary data for the determination of the natural frequencies, such as mass shapes and eigenvalue solvers is entered in the modal analysis settings.

The Modal Analysis add-on determines the lowest eigenvalues of the structure. The number of eigenvalues can be adjusted or automatically determined to reach either effective modal mass factors or maximum natural frequencies. Masses are directly imported from load cases or load combinations (with the option to consider the total mass, load components in global Z-direction, or only the load component in the direction of gravity).

Additional masses can be defined manually at nodes, lines, members, or surfaces. In addition, you can influence the stiffness matrix by importing axial forces or stiffness modifications of a load case or load combination.

Calculation

In RFEM, three powerful eigenvalue solvers are available:

• Root of the characteristic polynomial
• Method by Lanczos
• Subspace iteration

In RSTAB, two eigenvalue solvers are available:

• Subspace iteration
• Shifted inverse iteration

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

Results

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 ensures clearly structured documentation. In addition, it is possible to export the tables to MS Excel.