Damping coefficients by experiments and the application to transient FE analyses of cable trays
Damping entails significant effects in transient analyses, and it is a mistake to ignore it to reach a conservative solution. It is also possible that the analysis delivers no meaningful results. For steel structures, a dimensionless damping coefficient of 1% of the critical damping is widely accepted; however, for structures consisting of several materials, damping coefficients may be higher and estimating them reliably is very important. Nevesbu has written a paper in which the case of damping in a cable pack supported by a steel tray is studied. On Wednesday, 12 June, our specialists Jack Reijmers and Alessandro Zambon will present this paper at the NAFEMS congress in Staffordshire, UK.
Retrieving realistic damping coefficients
To retrieve realistic damping coefficients, free-vibration signals were acquired using a steel beam without and with cables attached to it. These registrations were made through a basic smartphone’s app, which uses the device’s own acceleration sensor. The logarithmic decrements calculated from the oscillation signals resulted in different dimensionless damping coefficients for different numbers of cables supported by the steel beam.
The calculated damping coefficient for the beam without cables resulted to be consistent with the usual 1% value. Five configurations were tested, ranging from zero to twenty cables attached to the beam. The resulting damping coefficients showed an increase from 1% (without cables) to 3% (with twenty cables). These test results were applied to an FE model of a ladder-type tray carrying cable packs. This configuration was excited with a shock pulse; then, the transient response was investigated for different cases from nearly zero damping up to a 3% level. With negligible damping, the analysis did not converge; with the damping coefficients found in the experiment, realistic numerical results were found instead.
Hence, it is shown that valuable information could be obtained through a simple experimental setup. Although widely accepted values for damping are available, structures with several material components can easily be tested, and the outcome gives a better input for transient FE analyses.