Fatigue-induced Risk Assessment of Slender Structures with Circular Cross-Section at Lock-In: Fid-Bau Portal

Fatigue-induced Risk Assessment of Slender Structures with Circular Cross-Section at Lock-In

Pastò, Stefano

The present work deals with the fatigue-risk of slender structures with cir¬cular cross section undergoing vortex induced vibrations. A body performing synchronized oscillations is an elastic body attaining vibrations induced by its unsteady wake consisting of the vortices which alternatively shed by its side regions. When the vortices are shed at the same frequency of oscillation of the body, the latter starts to oscillate reaching high amplitudes of oscilla¬tion. The phenom¬enon extends along a certain velocity range until the lock-in end. The main risk of real structures attaining vortex induced vibrations con¬sists in a worrying fatigue damage accumulation which may cause the struc¬tural failure without exceeding the design wind loads. In fact, the lock-in phenomenon is associated to low wind velocities whose probabilities of occurrence (hazard) are greater than the exceeding probability of the design wind speed fixed to 0.02. Every time the wind velocity falls within the lock-in range, the structure accumulates fatigue damage and its vulnerability increases un¬til the collapse. A circular cylinder has been tested in the CRIACIV -¬DIC boundary layer wind tunnel. The investigation has been performed with different mechanical and flow configurations, discussing the main features of the lock-in phenomenon. Afterwards, a first numerical model has been implemented to study and to discuss the mechanism of synchronization. The results of the above experimental and numerical investigations have been used to approach a numerical model predicting the response of chimneys undergo¬ing vortex induced vibrations. Subsequently, the fatigue-risk has been assessed. Finally, the control of the risky syn¬chronized oscillations has been also contemplated.