Abstract Summary
Vibration serviceability (VS) under human-induced loading has become a key design criterion for footbridges, in particular involving crowded conditions. Decisive parameters in the VS assessment are the probability distribution function (PDF) of the step frequencies in the crowd and human-structure interaction (HSI). In the literature, the PDF of the step frequencies is described by a Gaussian distribution. It is generally assumed that for increasing pedestrian densities, the mean step frequency decreases and the standard deviation increases. HSI on the other hand, describes the mechanical interaction between the human body and the supporting structure. In the vertical direction, this phenomenon has shown to have a significant impact on the dynamic behavior of the coupled crowd-structure system. The most significant HSI effect is the effective modal damping ratios of the coupled crowd-structure system that are much higher than the inherent modal damping ratios of the empty footbridge. Despite the extensive research in the last two decades, there is still a lack of experimental research for the validation of models for crowd-induced loading. The aim of this contribution is to verify experimentally the reliability of an equivalent spectral model (ESM) for crowd-induced loading [1,2] in the vibration serviceability assessment of footbridges. To this end, the Eeklo Footbridge benchmark dataset is used [3], involving four data blocks for two pedestrian densities (0.25 pedestrians/m² and 0.50 pedestrians/m²). In this dataset, the footbridge and pedestrian motion are registered simultaneously and the trajectory and time-variant pacing rate of every pedestrian in the crowd has been identified. Analysis shows that the PDF of the step frequencies in the crowd can be well represented by a Gaussian distribution. The ESM, function of the PDF of step frequencies and the average weight of the pedestrians, is then used to predict the structural response. HSI is in turn accounted for by means of an equivalent 2DOF-system representing respectively a structural mode and the crowd [4,5]. The reliability of the ESM is assessed comparing the power spectral density function of the recorded structural response and the one obtained theoretically from the equivalent crowd-structure 2DOF system and modelling the loading through ESM. [1] G. Piccardo and F. Tubino, “Equivalent spectral model and maximum dynamic response for the serviceability analysis of footbridges”, Engineering Structures, 40, pp. 445-456 (2012). [2] A. Ferrarotti and F. Tubino, “Generalized Equivalent Spectral Model for serviceability analysis of footbridges”, Journal of Bridge Engineering ASCE, 21(12): 04016091 (2016). [3] K. Van Nimmen, J. Hauwermeiren and P. Van den Broeck, “Eeklo Footbridge: benchmark dataset on pedestrian-induced vibrations”, Journal of Bridge Engineering ASCE: 05021007 (2021). [4] K. Van Nimmen, P. Van den Broeck. Forthcoming. “Using full-scale observations on footbridges to estimate the parameters governing human-structure interaction” Bridge Engineering. [5] F. Tubino, G. Piccardo, “Tuned Mass Damper optimization for the mitigation of human-induced vibrations of pedestrian bridges”, Meccanica 50, pp. 809-824 (2015).
