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Alongwind actions on structures may produce a great number of stress cycles and lead to significant damage accumulation. A critical analysis of the pertinent methods proposed in the literature provides evidence of the necessity of improving the level of approximation, taking into account the bandwidth spectral properties of the stress processes. This paper pursues this aim by establishing an advanced broadband formulation. Applying the cycle counting method based on the narrow-band assumption, two discrete cycles histogram expressions are first suggested, which lead to an tipper and lower bound of the fatigue damage. As the gap between the two bounds may be very large, a more refined counting method is formulated, based on a bimodal representation of the alongwind induced stress power spectral density functions; the low- and high-frequency spectral contents are associated, respectively, with the quasistatic and the resonant parts of the response. Some examples highlight the properties of the proposed procedure and show the good agreement between the bimodal approach and the numerical results. DOI: 10.1061/(ASCE)0733-9445(2006) 132:6(899) CE Database subject headings: Fatigue life: Gaussian process; Stress; Damage.

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. CE Database subject headings: Buffeting; Crosswind; Wind loads; Fatigue life; Vortex shedding; Wind direction.

The present study is the logical successor to a preceding paper, in which a procedure was developed for treating the peak wind velocity and the equivalent pressure on rigid bodies in a unified way. Based upon this formulation the present paper sets out a closed-form solution (CFS2) of the dynamic alongwind response of structures that substantially generalizes a previous solution (CFS1) given by the author. The method introduced makes the gust-buffeting problem formally consistent by defining an equivalent static pressure on flexible structures from which the equivalent pressure on rigid structures can be derived as a limiting solution. Furthermore, it leaves the user free of calculating the dynamic alongwind response associated with any model of the oncoming wind and with any structural modal shape; under this point of view, CFS1 can be regarded as a particular case of CFS2.

Although the peak wind velocity, the equivalent pressure on rigid bodies, and the dynamic alongwind response of flexible structures are component parts of the gust buffeting problem, different procedures have been developed in the technical literature with reference to each of these subjects. This fact gives rise to conflicting solutions and noticeable inconsistencies of results, especially when these procedures are applied to boundary situations. On the basis of this consideration, this paper and a companion paper propose a revised and generalized formulation of the wind buffeting problem and a unified definition of gust factor. The present paper analyzes the themes of the peak wind velocity and of the equivalent pressure solving both problems in closed form. This goal is pursued by introducing a new general expression of the power spectrum of the alongwind turbulence and by applying the equivalent wind spectrum technique. In the context of this formulation, several well-known specific procedures can be conceptually regarded as particular cases of this more general treatment.