Bimodal alongwind fatigue of structures

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 upper 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.