## JBeam

### Structural Analysis for the *Java*-Platform

## Chapter 7

### Modal Analysis

#### Overview

In addition to the static analysis of displacements and stress
resultants JBeam also supports a modal analysis of the system.
You get eigenfrequencies and eigenmodes as a result.

In modal analysis the lumped mass method is used. This is
no longer an exact calculation but only an approximation
converging to the exact solution using *h-*adaption.

For the practical use this means you have to subdivide longer
beams into several elements. You should perform at least two
calculations at different refinement levels and compare the
results to get an idea of the reached accuracy.

#### Example

The first eigenmode of a frame is calculated using to
refinement levels. The eigenfrequencies of both calculation
do not differ very much (2.24 vs. 2.26 Hz).

To determine higher eingenmodes you will need more refinement.

#### Input and Calculation

The problem description input is analogous to static analysis.
You should not put any loading on the system. The mass of the elements
can be set in the corresponding dialogues in the same way as for sectional values.

To (re)calculate use 'Results - modal analysis'.

#### Result Output

Displacements and stress resultants for the different eigenmodes can
be displayed in arbitraty scalings in the same way as for static analysis.

You can animate the displacement figure using 'Results - animate mode'.

Use 'Results - show next mode' and 'previous mode' two
switch between the desired eigenmodes. Number and frequency of
the current eigenmode is displayed in the status line.