As wind turbine towers grow in height and their blades become longer, actions on the towers are also increased and their safe and cost‐effective design becomes critical for the further development of the wind energy sector. One potential failure mechanism of tubular steel towers is shell local buckling between ring flanges. In the present paper, the buckling behavior of a 120m tall wind turbine tower under realistic wind loads is investigated numerically, focusing on the buckling response near the man‐hole opening. To that effect, nonlinear finite element analyses accounting for geometrical and material nonlinearity and imperfections (GMNIA) is employed. GMNIA is performed taking into account initial geometric imperfections with the most unfavorable shapes resulting from the three primary buckling modes, determining the imperfection size according to EN1993‐1‐6 depending on the assumed fabrication quality class A, B or C. Numerical results are compared to analytical verification according to EN1993‐1‐6. Different wind directions are considered. Moreover, to investigate the influence of the stiffening of the manhole to the tower buckling strength, a comparison between three alternatives of stiffened and an unstiffened manhole is performed.
In recent years, metamaterials are increasingly used in the field of vibration mitigation due to the unique potential these offer. Periodic placement of appropriately designed resonators, the so-called unit cells, has been shown to arrest the propagation of vibration within a specific frequency range, thus resulting in formation of a bandgap. In rendering metastructures efficient, the breadth of the bandgap should be maximized, while for application in structures, it is also necessary to shift the lower limit of the gap to lower frequencies. One potential solution to this direction is the use of nonlinearity at the unit cell scale. In this direction, the current study investigates finite lattice configurations, which consist of impact damper unit cells. The effectiveness of impact dampers in vibration attenuation has been successfully illustrated in limited degree of freedom systems, and is here extended for use in the metastructure sense, i.e., for multiple degree of freedom systems. For this purpose, a one dimensional lattice of a finite number of unit cells is considered. In order to study and better asses the behaviour of the configuration, variable parameters are taken into account, including the number of unit cells, stiffness and mass ratio, while a comparison to a conventional linear oscillator is further offered. For the evaluation of the system performance, several criteria are utilized, including the oscillation amplitude as well as the energy absorption from the primary system as a result of the individual impacts. The obtained results clearly indicate the ability of impact-based meta-structures to successfully contribute to vibration mitigation under appropriate design.
To prevent the severe effects of earthquake on built systems, structural engineering pursues attenuation of vibrations on structures. A recently surfaced means to structural vibration mitigation exploits the concept of metamaterials, i.e., of configurations able to control wave propagation in specific frequency ranges, termed band gaps. The current study harnesses the potency of a geometrically nonlinear unit-cell design, which can develop negative stiffness, and explores the vibration-attenuation capabilities of the resulting metamaterial device. An analytical approach is followed to calculate the expected attenuation zone, as well as for calculating the dependence on the amplitude of the input, a hallmark of the nonlinear behavior. For the purpose of validation of a proof-of-concept system, dynamic tests are performed on a scaled model assembled using LEGO components. Besides showing that such a nonlinear system can be easily constructed, these tests illustrate the potential of this nonlinear design for vibration reduction within the targeted band-gap frequency zones and the protection that it can offer to a primary system. Finally, numerical analyses are used to verify the analytical calculations of the dispersion relation and are additionally compared to the experimental results, evaluating the incorporated modeling assumptions. The possibility to lower the band gap in the typical seismic engineering frequency range and to maintain a broadband attenuation at low frequency show that negative stiffness may enhance the performance of metamaterials for seismic protection.
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