Prediction of gust induced cycle counts and fatigue damage of structures

Abstract

Structural vibrations caused by turbulent wind and their contributions to fatigue are still difficult to predict concisely. With regard to fatigue life prognosis, different methods have been discussed in recent years - but general reliable guidance for the determination of stress cycle counts allowing for damage accumulation concepts (besides individual calculations in the time-domain) is still not available. A parameter study in time and frequency domain has been performed to discover the dependencies of wind conditions and structural properties on the resulting cycle count distributions. The findings allowed to formulate of a first simplification approach (level 1), connecting significant parameters to realistic cycle count distributions for arbitrary wind conditions and dynamic properties of the structure. At this stage, the usage of arbitrary Δσ-N resistance curves is preserved to determine the expected structural damage, but a detailed calculation in time- or frequency domain can be waived on this level.In a second (level 2) simplification, a unique Δσ-N resistance curve with a constant slope and no fatigue endurance limit has been assumed. Due to this restriction, the damage prediction derived is functionally dependent on the ratio of present maximum structural stress Δsmax and the detail category Δσc. Consequently, damage equivalence factors KF are introduced on this level, which allows for a straightforward and safe-side estimation of the expected fatigue damage.Recommendations have been compared to detailed time domain solutions, and examples have been provided to apply both introduced concepts.