Calculation of the natural frequencies of a grandstand

Fig. 19-12: A grandstand full of spectators

It is common for a grandstand, either permanent or temporary, to be full of spectators during a sports event or a pop concert as shown in Fig. 19-12. In this situation, the human mass can be of the same order as the mass of the structure. When calculating the natural frequencies of a grandstand for design purposes, it is necessary to consider how the human mass should be represented. It is recommended in BS6399: Part 1: Loading for Buildings [19.6] and the Interim Guidance [19.7] that empty structures should be used for calculating their natural frequencies. i.e., the human mass should not be included. This is because the worst situation for design consideration is when people move rather than when people are stationary. Therefore engineers will not under-estimate the natural frequencies of grandstands through adding the mass of a crowd to the structural mass.