An industrial building is a non-classically damped system due to the different damping properties of the primary structure and equipment. The objective of this paper is to quantify the range of applicability of the real model superposition approximation method to the seismic response calculation of industrial buildings. The analysis using lumped mass-and-shear spring models indicates that for the equipment-to-structure frequency ratios γf > 1.1 or γf < 0.9, the non-classical damping effect is limited, and the real mode superposition approximation method provides accurate estimates.