Directional Wind-Induced Fatigue of Slender Vertical Structures

Abstract

The wind-excited vibrations of structures induce fluctuating stresses that lead to fatigue damage accumulation and may determine structural failure without exceeding ultimate strength. This paper proposes a mathematical model aimed at deriving the histogram of the stress cycles, the accumulated damage, and the fatigue life of slender vertical structures exposed to simultaneous alongwind and crosswind vibrations. Wind climate is modeled by the joint density function of the mean wind velocity and direction. Wind-induced actions consist of aerodynamic forces on the stationary structure and of aeroelastic forces due to the structural motion. Aerodynamic actions on the stationary structure are caused by the turbulence of the oncoming flow, dealt with through quasisteady theory, and by the vortex wake, considered as independent from turbulence. Aeroelastic forces are aimed at reproducing the lock-in phenomenon. The static and the quasistatic parts of the structural response are evaluated by the influence function technique, taking all modes of vibrations into consideration. The resonant part of the response is related to the fundamental alongwind and crosswind modes only. The probabilistic accumulation of damage due to aerodynamic actions on stationary structures is estimated by a counting cycle method inspired by narrow band processes. The damage in lock-in conditions is superimposed by counting stress cycles deterministically. The examples illustrate the application and the effectiveness of the proposed procedure, focusing attention on the role of upper modes and wind directionality.