In terms of the magnitude of the attenuating effect of the stationary crowd, it is noticeable that the calculations predict that a single stationary person (m2 = 75 kg) is capable of reducing structural vibration levels by more than a factor 2 (for ζ1= 0.5 %). When mentioning this is should be recalled that the modal mass of the empty structure is assumed to be almost 7000 kg, which is a little less than 100 times the weight of the person. The response ratio was also computed for different assumptions for the frequency of the empty structure (systems B1, B2, B3), and results are shown in Fig. 3. Fig. 3 Response ratio e as function of mass of the stationary crowd of people f1= 5 Hz (o), f1= 6 Hz (x), f1= 7 Hz ( ) It appears that the attenuating effect of the stationary crowd of people is also sensitive to the frequency of the empty structure. 6. CONCLUSION AND DISCUSSION The parametric studies of this paper examined the response of different structural systems to the action of jumping, with and without a stationary crowd present atop the structure. It was found that even a very small stationary crowd of people can change the response of the structural system quite significantly. Even a single stationary person is predicted to be capable of reducing structural vibration by more than a factor 2 (for one of the structural systems considered for the studies). It was furthermore found that the vibration attenuating effect of the stationary crowd of people is sensitive to for instance the damping ratio of the empty structure. REFERENCES [1] Sachse, R., Pavic, A., Prichard, S., The influence of a group of humans on modal properties of a structure. In: Proceedings of the fifth international conference on structural dynamics, pp 1241-1246, 2002. [2] Ellis, B.R., Ji. T., Human-structure interaction in vertical vibrations. In: Proceedings of the ICE: Structures and Buildings; Vol. 122, pp. 1-9, 1997 [3] Brownjohn, J.M.W., Energy dissipation from vibrating floor slabs due to human-structure interaction, .Shock and Vibration, Vol. 8 (6), pp. 315-323., 2001 449
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