Analysis of tower structure for fatigue taking into account wind vortex shedding

Abstract

Vortex shedding is manifested in all tower structures with a solid or lattice cross-section with relatively equal width and depth of the cross-section. The phenomenon is associated with the alternate breakdown of vortices from the edges of the structure, which cause it to oscillate under the influence of inertial masses in a plane perpendicular to the wind. The main condition for the manifestation of vortex shedding is the action of wind with a speed exceeding a certain critical one. As shown by calculations of a real tower structure, oscillations according to the first natural frequency and shape occur already under the influence of a moderate wind. This requires attention, assessment of the number of oscillation cycles and the implementation of criteria for taking into account the combined effect of frontal wind and vortex shedding on the structure and calculation of the structure for endurance. The paradox is that this phenomenon is not widely known in engineering practice, and instructions on the mandatory calculation of structures for vortex excitation appeared only with the implementation of amendment 2 to DBN B.1.2-2:2006 "Loads and influences. Design norms" in 2020. In this analyzed state of research on this issue, an available method for calculating vortex excitation, introduced in EN 1991-1-4:2005, is presented. The calculation and analysis of the McDonald's advertising totem with a height of 25.575 m, implemented in Kyiv in 2016, was performed. Calculations showed that this structure is characterized by oscillations due to vortex shedding. From the available sources of meteorological data, an approximate number of oscillation cycles was determined, criteria for calculating the structure's durability were determined, and generalizations were made that can be applied to all structures. In particular, it is pointed out the need to limit stresses beyond the endurance limit in two ranges - parametric resonance from the frontal wind, and sign-changing cyclic oscillations from vortex excitation. An analysis of the effective cross-sections and effective structural forms of building details when working under the action of a frontal wind with vortex shedding was carried out.