Shake-Table Response of a Three-Level Cold-Formed Steel Storage Rack Under Harmonic Excitation: Peak Accelerations, Interlevel Displacements, and Amplitude-Dependent Frequency

Broadband acceleration peaks can obscure how a storage rack responds at the excitation frequency. This study examines that distinction through shake-table and free-vibration tests on a full-scale, three-level cold-formed steel rack with restrained added mass at its middle level. Six incremental sine-beat runs at nominal 1 Hz were analysed alongside pre-run and post-run impact records. The synchronous top-to-table amplitude ratio remained nearly constant at 2.89–3.04, while the broadband peak ratio increased from 3.43 in the first run to 5.82 in the last. The latter was sensitive to processing bandwidth. Interlevel displacement demand concentrated in the lower two intervals. Free-vibration frequency decreased with response amplitude, and five pre-run impacts reproduced this dependence. A mechanical discussion distinguishes possible geometric and connection-related contributions without assigning a uniquely identified cause. The post-run frequency difference does not establish permanent stiffness degradation. A systematic discrepancy between the acceleration and displacement acquisition chains limits independent validation of absolute laser displacements. These findings support reporting acceleration demand with its processing definition and frequency with its response amplitude.