Damping coefficients by experiments and the application to transient FE analyses of cable trays

Abstract

Damping entails significant effects in transient analyses, and neglecting it to obtain a conservative solution in numerical analyses might return no meaningful results. For steel structures, dimensionless damping coefficients around 1% of the critical damping are widely accepted. However, for structures consisting of several materials, damping coefficients may be higher; estimating their values reliably is important. This paper studies the case of damping estimates for steel trays supporting cable bundles. Free vibration signals were experimentally acquired using a steel beam with and without attached cables, by employing a smartphone app set to record acceleration data. The logarithmic decrements calculated from the signals resulted in different dimensionless damping coefficients for correspondingly different numbers of cables attached to the beam; five configurations were tested, up to twenty cables on the beam. The resulting damping coefficients showed an increase from 0.7% (without cables and consistent with the usual 1% value) to 3% (with twenty cables). These results were applied to a Finite Element (FE) model of a ladder-type cable tray, subjected to a shock pulse as excitation. Thus, 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; instead, with the damping coefficients resulting from the experiment, realistic numerical results were found. It is therefore shown that valuable information could be obtained through a simple experimental setup. Multi-material structures can be easily tested, in order to obtain results that can constitute better input for transient FE analyses.