Dynamic Behaviour of a steel footbridge under pedestrian loads

Footbridges are generally effective structures concerning the static behavior, since they are subjected to a limited level of live loads. Nevertheless, the frequency range of the pedestrian dynamic actions may fall within the natural frequency interval of the structure, giving high dynamic amplifications. Therefore, dynamic properties of footbridges and effects of pedestrian loads need to be analyzed, comparing experimental and numerical results. This paper is part of a research that aims to characterize the dynamic behavior of a steel footbridge with reference to pedestrian dynamic amplifications. The structure, located in Reggio Emilia (Italy), is about 170 meters long and composed of 5 simple-supported spans, linked at lower-floor level. To investigate the dynamic behavior of the footbridge, an experi-mental campaign has been first performed. Accelerations due to ambient vibrations (wind) and to pedestrian dynamic actions were recorded. In particular, a wide number of pedestrian dynamic loading conditions have been considered, such as excitations induced by people jumping, running and walking with different passing frequencies. Accelerations were acquired by an advanced MEMS-based system. 10 biaxial MEMS sensors were arranged in 3 different setups in order to identify as many natural modes as possible and to investigate the vibration level in several components of the footbridge. The post-processing of experimental data allows to determine both the dynamic properties of the structure (frequencies, mode shapes and damping ratios) and the maximum accelerations caused by pedestrian actions. The dynamic characteristics are identified by means of the classic Enhanced Frequency Domain Decomposition (EFDD) method that is based on the diago-nalization of the spectral density matrix. Then, a finite element model is built and calibrated such that the analytical dynamic predictions agree with the experimental modal properties. Finally, the measured accelerations caused by pedestrian dynamic actions are compared with those given by the numerical model, con-sidering different dynamic load models.