Performance of a state-of-the-art surface load model: case study on a single span beam and a complex cable stayed structure

The equivalent surface load model (SLM) presented by the AFGC in 2006 takes a lightweight approach for predictions of bridge deck accelerations due to pedestrian traffic. This model aims to envelope any expected vibration caused by random generated traffic streams with a single decisive harmonic surface load. It is also implemented in the current preliminary standard prEN 1991-2:2021. However, as it was developed based on dynamic calculations with single-span beams, it is unknown how accurate the determined design values are if the model is applied to more complex structures. The model neither addresses multimodal vibrations in the critical range of resonance nor the coupling of several spatial directions within a mode. This work analyses dynamic responses of models of different structures with increasing complexity in terms of the quantity of critical eigenmodes and the number of spatial directions considered. Several Monte Carlo simulations with random pedestrian streams are performed according to the specifications of the AFGC report. Subsequently the SLM is applied for each structure and traffic class . The derived maximum accelerations are then compared. Comparing the different dynamic responses suggests that the more complex the structures are, the less the simulation results with the SLM correspond with the results of the MC simulation. It can be concluded that the SLM is not necessarily able to determine the significant eigenmode in the event of multimodal vibrations and/or vibrations of modes with coupled spatial directions. Therefore, it should only be applied if the system is not susceptible to these effects.