Modal testing of a sports stadium

Sports stadia, like many other civil engineering structures, are being pushed to their limits in terms of slenderness and structural efficiency. This normally has benefits such as increased capacities and improved lines of sight for spectators. However, the increased use of more slender stadium structures is causing concern as they may be susceptible to excitation by the increasingly lively spectators that they accommodate. This paper describes the modal testing of one such structure, a grandstand at a football stadium in the UK. The modal testing, performed using a roving APS113 shaker and 6 reference accelerometers, is described in detail. Due to external considerations, the testing had to be performed in very limited timescales and the entire modal test and preliminary estimation of the modal properties of the structure were completed within a single working day. The results from the modal testing are compared with the results of a pre-test finite element analysis that was performed by an experienced stadium designer. Discrepancies between the FE model and the modal test results are highlighted and some explanations are given.