The further development and improvement of prediction models for crowd-induced vibrations of footbridges requires detailed information on representative operational loading data. This paper uses an inverse method to estimate the parameters that govern human-structure interaction from the resulting structural response. The parameters of interest concern the dynamic characteristics of a single-degree-of-freedom (SDOF) system, applied to describe the mechanical interaction between the pedestrian and the structure. The parameter estimation procedure assumes that the dynamic behaviour of the emtpy structure and the distribution of step frequencies in the crowd are known. The dynamic characteristics of the SDOF model are estimated by minimizing the discrepancy between the observed and the simulated power spectral density of the structural response.

The proposed approach is applied to a real footbridge. First, the operational modal characteristics of the empty structure are identif ed and used to calibrate the f nite element model of the structure. Second, the structural response induced by a pedestrian density of 1 persons/m2 is observed. The corresponding non-resonant response is used to estimate the distribution of step frequencies in the crowd. Finally, the parameter estimation procedure is applied and a reasonable estimate of the dynamic characteristics of the SDOF model is obtained.