Commissioning: KubanUniversalProjekt GmbH, 350020 Krasnodar, Russia
During the past Olympic and Paralympic Winter Games in Sochi 2014, Russia, the Olympic fire was blazing on a 50m high diagonally projecting torch tower. The building is characterized by its curved, free-form facade construction. However, it was precisely this shape that made the structure susceptible to a particularly problematic form of wind induced vibrations. At the Institute for Steel Construction at RWTH Aachen University, extensive investigations were carried out to ensure the dynamic stability of this structure in the run-up to the two major events.
The evaluation of wind excited structure responses becomes particularly complex when the dynamic load effect depends on the vibration behavior of the structure. In this case one speaks of self-control or self-excited vibrations. Such behavior can occur in slender structures as a result of natural wind flow, and such a mechanism was also feared for the flare. The great danger with self-excited oscillation mechanisms is that the effective excitation load increases steadily as a result of the oscillation movement. Characteristic for this susceptibility is the course of the transverse drive coefficient cy, or its derivation in relation to the angle of incidence α. In general, a negative derivative can be interpreted as a negative damping component for the dynamic structural behavior. If this negative damping contribution exceeds the size of the structural damping, the structure becomes dynamically unstable, depending on the prevailing wind speed. The wind speed at which the negative damping predominates is called the onset speed for the self-excited oscillation mechanism.
Flow visualization on a 1:100 model of the Olympic torch in the boundary layer wind tunnel of the Institute for Steel Construction of the RWTH Aachen University
The identification of the effect and the determination of the corresponding critical insertion speed was carried out in wind tunnel measurements. Also the development.
Various measures were considered to prevent this critical vibration effect: changing the cross-sectional shape, detuning the structure or increasing the structural damping. The first mentioned two possibilities were no longer considered for the concrete structure due to the already advanced execution at the time of the expert opinion. Therefore a design of damping measures was examined and three dynamic vibration dampers with active masses of 500-700 kg were recommended. Due to the potential hazard situation for the structure, the dampers were designed redundantly (two for the galloping vibrations, one perpendicular to them). In addition, a regular effectiveness check was recommended. Finally, fine-tuning of the finished dampers was carried out on site based on natural frequency measurements.