The Earthquake Engineering Online Archive

Stochastic inelastic response of offshore towers to strong motion earthquakes

Kaul, Maharaj K.

UCB/EERC-72/04, Earthquake Engineering Research Center, University of California, Berkeley, 1972-08, 88 pages (531/K28/1972)

Complexities of stochastic analysis of offshore towers include nonlinear effects due to coupling of the hydrodynamic drag forces with structural response, nonlinear effects due to hysteretic force deformation relations to the tower, and nonstationary random excitation and response. They are overcome by introducing certain approximations not affecting the numerical results seriously. Semi-closed form solutions are derived for the time-dependent variances of response. Passing shot noise through a second-order linear filter simulates ground acceleration. Internal tower forces are generated using linear and bilinear hysteretic relations, resulting in a full stiffness matrix where the tridiagonal terms are time dependent to account for hysteretic effects. Finally, equivalent linearizations lead to a system of first-order nonlinear differential equations with the time-dependent variances of response as the unknown, which are determined as solutions of the differential equations. Extreme values of response are generated. Numerical results for realistic excitation and typical tower are presented and compared with the corresponding results obtained by Monte Carlo solutions, i.e., by generating an ensemble of artificial accelerograms satisfying the stochastic model and computing by deterministic procedures the separate time histories of nonlinear response. The desired statistical properties of response are then obtained directly from the output process.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-72-04.pdf (4 MB)