Abstract
Vibration serviceability problems concerning lightweight, flexible long‐span floors and cantilever structures such as grandstands generally arise from crowd‐induced loading, in particular due to bouncing or jumping activities. Predicting the dynamic responses of these structures induced by bouncing and jumping crowds has therefore become a critical aspect of vibration serviceability design. Although accurate models describing the load induced by a single person are available, essential information on the level of synchronization within the crowd is missing. In answer to this lack of information, this paper experimentally investigates the inter‐ and intraperson variability as well as the global crowd behavior in bouncing crowds. A group size of 48 persons is considered in the experiment whereby the individual body motions are registered synchronously by means of a 3D motion capture system. Preliminary tests verified a new approach to characterize the bouncing motion via markers on the clavicle. Subsequently, the full‐scale experimental study considered various crowd spacing parameters, auditory stimuli, and bouncing frequencies. Moreover, special test cases were performed whereby each participant was wearing an eyepatch to exclude visual effects. Through the analysis of 330 test cases, the interperson variability at the bouncing frequency is identified. In addition, the cross‐correlation and coherence between participants are analyzed. The coherence coefficients between each pair of participants in the same row or column are calculated and can be described by a lognormal distribution function. The influence of the spatial configurations and visual and auditory stimuli is analyzed. For the considered spatial configurations, no relevant impact on the inter‐ and intraperson variability in the bouncing motion nor in the global crowd behavior is observed. Visual stimuli are found to enhance the coordination and synchronization. Without eyesight, the participants are feeling uncertain about their bouncing behavior. The results evaluating the auditory cues indicate that significantly higher levels of synchronization and a lower degree of the intraperson variability are attained when a metronome cue is used in comparison to songs where the tempo often varies.
Highlights
Due to their slenderness, long-span floors and cantilever grandstand structures are very often prone to crowdinduced vibrations [1]
Evaluating the vibration serviceability under crowd-induced rhythmic loading has become a critical aspect of the structural design process. is is relevant for structures such as grandstands used for pop/rock concerts or sporting events as well as the long-span floors accommodating gym and aerobic activities
For the reference configuration 1 m × 1 m with 21 metronome frequencies, the intraperson variability of the achieved bouncing frequencies and corresponding equivalent bandwidth follow a normal distribution
Summary
Long-span floors and cantilever grandstand structures are very often prone to crowdinduced vibrations [1]. When the crowd follows a musical beat or an auditory cue with a specific rhythm, the (near) resonant excitation can induce high structural vibration levels [2]. It has been observed that the stimuli of visual, auditory (metronome and music songs), and tactile cues often have a significant impact upon the mutual interaction of bouncing and jumping persons [11,12,13,14]. Noormohammadi et al [12] studied the effect of an auditory metronome as well as visual and tactile stimuli on the level of synchronization between two jumping or bouncing persons using force plates and a motion capture system.
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