The combined effect of gravity and vertical excitations on the mass of a structure can introduce parametric excitation terms in the equation of motion. Parametric excitation occurs when the system's parameters, such as mass, stiffness, or damping, vary with time due to external factors. This variation can lead to unstable or chaotic responses in the structure.

The response spectrum, which is commonly used in seismic analysis, provides a graphical representation of the maximum response of a structure at different frequencies. It is typically used to estimate the peak response of a structure under earthquake excitations. However, the conventional response spectrum is primarily applicable to horizontal ground motions.

Is there any way to account for the parametric excitation effects induced by vertical excitations in response spectrum analysis?