MS24.1 - Wind Induced Vibrations of Slender Structures

The role of atmospheric turbulence in the onset mechanism of bridge aeroelastic instabilities has not been fully understood yet, and it often leads to controversial results. Indeed, concerning torsional and coupled flutter, experimental and numerical results have shown that atmospheric turbulence may have either a stabilising or a destabilising influence. In particular, the parametric excitation induced by the variation of the self-excited forces due to the angle of attack associated with large-scale atmospheric turbulence was found to anticipate the coupled flutter onset of about 20% in the case of the Hardanger Bridge, in Norway (Barni et al., 2022a). In that work, the so-called 2D RFA model was employed to predict the parametric effect of large-scale turbulence. Moreover, after simplifying the full bridge structure to a three-degree-of-freedom 2D model, a first attempt to formally study the stability of the bridge under a periodic parametric excitation according to the Floquet theory was reported in Barni et al. (2022b). Along this research line, the present work numerically investigates the stochastic stability of both a 2D and a full bridge model of the Hardanger Bridge exposed to a realistic turbulent wind field. Even if the governing equations of the system result in a linear time-variant state-space model, a Monte Carlo approach is followed due to the lack of validity of the Markovian process assumption, which does not allow the definition of a diffusion problem. Self-excited forces are again modelled according to the 2D RFA model, and the sample time histories of the slowly-varying angle of attack are derived from the realisations of the turbulent random wind field. The flutter stability of the system is statistically assessed by looking at its free-vibration behaviour given some initial conditions and a large number of realisations of the time histories of the angle of attack. Although the Monte Carlo approach does not foster any physical interpretations of the stability of the system, the numerical simulations highlight the sensitivity of the flutter stability to the standard deviation of the slowly-varying angle of attack, which is directly linked to turbulence intensity, band-superposition cut-off frequencies and integral length scale. For the considered case study, the paper emphasises a significant destabilising effect of the turbulence intensity, as well as the strongly stochastic nature of the Hopf bifurcation. References: Barni, N., Øiseth, O. A., Mannini, C. (2022a). Buffeting response of a suspension bridge based on the 2D rational function approximation model for self-excited forces. Engineering Structures, 261, 114267. Barni, N., Gioffrè, M., Mannini, C. (2022b). Bridge flutter stability in turbulent flow. Proceedings of the XVII Conference of the Italian Association for Wind Engineering, IN-VENTO 2022 (under review)

The monumental work of art by Bernar Venet, called ‘Arc Majeur’ wraps the Belgian motorway E411 at Lavaux-Ste-Anne, offering drivers an unmissable landmark in the landscape. This sculpture is a curved steel structure with a height of 60 meters and a 2.25 meters square section. Due to its slenderness, lightness and low vibration frequencies, the structure is very sensitive to wind excitations and mainly to vortex induced vibrations (VIV). After the presentation of the design and the main properties of the structure, the paper presents two approaches to estimate VIV amplitudes. The first one details the mathematical models of VIV included in the Eurocode and several theoretical approaches. The second approach compares the mathematical models to the results of experiments conducted in the wind tunnel facility of the University of Liège. The influence of several parameters is investigated. The experiments revealed lower VIV amplitudes than the theoretical approaches. As the Arc Majeur cannot withstand the fatigue induced by the VIV, it is mitigated by increasing the damping ratio by adding a tuned mass damper (TMD) at the top. Due to the vital importance of its operation, an online processing is developed to verify that the damper operates as intended.

Tall buildings inherently have low natural frequencies, excitable by wind loading through buffeting and vortex shedding. Such vibrations can cause discomfort or even fear in the occupants which is a design failure from vibration serviceability standpoint. Current wind-induced vibration serviceability guidelines such as ISO10137-2007, have proposed their acceptability criteria based on human perception of vibrations. However, recent studies suggest that such perception thresholds may not be an appropriate measure of vibration acceptability. Rather, more direct factors such as influence on work (both cognitive and physical) performance, health and wellbeing, and the emergence of mild motion sickness (sopite syndrome), should be used to assess ‘acceptability’. This study provides experimental evidence of the effects of wind-induced vibrations on cognitive work performance, comfort, and wellbeing of the occupants. The state-of-the-art motion simulator facility, located at the University of Bath (VSimulators) was used to simulate bidirectional random vibrations, typical of tall building response due to wind loading. Under fully controlled conditions, research participants were exposed to six different motion conditions, as a cross-product of two frequencies and three peak accelerations, five of which were deemed acceptable for office buildings according to ISO-10137. Both objective and subjective physiological and psychological measurements were carried out to evaluate work performance, comfort, and wellbeing of participants subjected to these different motion characteristics. The results showed that both peak acceleration and frequency of motion had adverse effects on work performance, comfort and wellbeing of participants and showed evidence of the onset of sopite syndrome symptoms during even relatively short (~2 hour) exposures. It was concluded that even for motion conditions with peak acceleration magnitudes below the threshold of conscious perception, there were negative consequences, especially as exposure to such motions caused participants to experience sopite syndrome. The data here suggest that buildings constructed to current standards might lead to negative consequences for wellbeing and work performance even when people are not consciously aware of any motion. Current serviceability criteria might be insufficient to address acceptable levels of wind-induced vibrations in tall buildings and future design criteria could be based on how vibrations affect health, wellbeing and performance rather than simply on the perceptibility of vibrations.