A note on the nonstationarity of seismic response of structures

Investigations have been conducted to study the characteristics of the seismic response of structures to stationary and nonstationary random excitations. Exact transient response to stationary excitation is evaluated for several SDOF structures subjected to a wide variety of input motions. For structures with low damping ratio and natural frequency very different from the predominant frequency of the input, the response is dominated by free-vibrations up to several response cycles. For narrow-band excitations, such transient response amplitudes even overshoot the final steady state response. Therefore, to get realistic results, it is proposed to carry out the exact solution for the response to stationary excitations. The nonstationarity of input motion is commonly defined by a stationary process multiplied with a deterministic envelope function. The response amplitudes to such an excitation are influenced by both the subjectively chosen envelope function and the PSDF of the associated stationary excitation. Because the actual nonstationarity in design accelerograms cannot be represented by such simplifying idealizations, the conventional analysis for nonstationary excitations does not provide reliable information for practical use in design applications.