It is shown that the pseudorelative spectral velocity (PSV) of an equivalent oscillator, which can be represented by a single degree-of-freedom system for excitation by synchronous horizontal excitation, can be extended to describe the PSV spectra of the same oscillator when excited by simultaneous action of horizontal, vertical, and rocking components of strong ground motion. At short periods, the new spectra are governed by differential ground motion and peak ground velocity, and they depend on the transit time of the waves along the length of the structure and on the oscillator frequency. At long periods, PSV spectral amplitudes tend toward an asymptote with amplitude proportional to the maximum rocking angle of ground motion.

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