Pluck tests were performed on a major diagonal member of an offshore platform jacket in the fabrication yard to develop data for assessment of structural damping. The initial displacements were varied in order to investigate dependence of damping on amplitude. The response histories were typically composed of six or seven vibrational modes having closely spaced frequencies in the 3.4–4.2-Hz range. System identification was accomplished using an output error approach. Response histories reconstructed from the system parameters identified by this method exhibit excellent fidelity to the corresponding recorded data traces. The analysis results support the assumption of linear system behavior and lead to a suggested value of 0.002 for the critical damping ratio.