Nonstationary response analysis of offshore guyed tower subjected to earthquake loading

Abstract

A method for nonstationary seismic response analysis of an offshore guyed tower is presented. The equation of motion is formulated for the horizontal motion of the guyed tower using a multidegrees-of-freedom model. The nonlinear hydrodamping and the nonlinear restoring force of the guyline system are included in analysis with the aid of the equivalent linearizations. The nonstationarity of the earthquake excitation is modelled by imposing a time varying envelope function onto a stationary random component, The nonstationary responses are obtained in terms of time dependent variance functions. By taking the autocorrelation function of the nonstationary ground acceleration in terms of sinusoidal and exponential functions of time, an analytical procedure is developed for computing time varying variances of the tower responses. Expected maximum responses are evaluated thereafter. Example analysis indicates that the nonstationary method gives consistent estimates for the maximum responses, if the earthquake duration is taken to be longer than the duration of strong excitation, Comparison is made with the results obtained by the conventional spectral method based on the stationary assumption. Copyright (C) 1996 Elsevier Science Ltd.