The effects of crowd occupation on the dynamic performance of a grandstand

A detailed study of the measured modes of a grandstand shows that crowd occupancy alters its modal properties in a way that varies with the crowd configuration. This demonstrates that a crowd must act as a dynamic system attached to and interacting with the structure. To understand this observation, the influence of a crowd on the structure they occupy has been investigated analytically. Initially, the empty grandstand was simulated using finite element (FE) modelling to determine analytically the modal properties. The FE model was then updated to correlate well with the experimental data from the empty stand. The crowd-structure system was then modelled as a two degree of freedom (2-DOF) system with one DOF representing a mode of the structure and the second DOF representing the crowd, using properties derived from previous researchers and weighted according to the mode shape. A comparison of modal properties of the occupied grandstand obtained from the analytical modelling and experimental data were presented. The results show a reasonable agreement for the natural frequencies but generally the analytical modelling produced lower values for the damping ratios.