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In this paper, a biomechanical crowd-structure interaction model is proposed and calibrated in order to take into account the change of the modal properties of footbridges in lateral direction induced by the pedestrian walking action.The model involves two submodels, namely, (a) the pedestrian-structure interaction and (b) the crowd submodels. In the first submodel, a single degree of freedom system, which simulates the behaviour of each pedestrian, is projected on the vibration modes of the structure. Herein, the parameters of the first submodel are estimated experimentally from the results of several pedestrian tests conducted on a real footbridge. For the second submodel, the crowd behaviour is modelled via a multiagent method. The performance of the overall model has been assessed successfully via the correlation between the change of the first experimental lateral natural frequency of another footbridge during a pedestrian test and the numerical predictions provided by the proposed model. Therefore, this model becomes a valuable tool to analyse numerically the change of the modal parameters of footbridges due to the pedestrianstructure interaction.KEYWORDS change of natural frequencies, crowd dynamics, footbridge, lateral lock-in phenomenon, pedestrianstructure interaction, simplified biomechanical model | INTRODUCTIONAfter the vibratory problems occurred during the opening ceremony of the Millennium footbridge (London, UK), 1 the analysis of the behaviour of footbridges, due to the lateral pedestrian induced vibrations, has become an essential part of their structural design. 2 During the last two decades, many models have been proposed in the literature to simulate the excitation generated by the pedestrian-induced lateral vibrations. These models may be classified into one of the following categories: (a) direct resonance, (b) internal resonance, and (c) dynamic interaction between the pedestrians and the footbridge. 3 Herein, the proposed model can be ascribed to this third category, where the excitation mechanism is originated by the coexistence of an interaction between both pedestrians and the movement of the bridge and among pedestrians themselves.The first attempt to simulate the pedestrian-structure interaction was based on the assumption that a walking pedestrian may be modelled by a pendulum walking model, where a lumped mass situated at the pedestrian's centre of gravity is linked to the structure by a rigid massless element. In 1992, this idea was used by Abe and Fujino 4 to propose a non-linear model for the lateral pedestrian force in terms of the inertial force of the pedestrian and the

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Structure and Infrastructure EngineeringHarmony Search (UKF-HS) algorithm is proposed and its implementation details are discussed. In order to validate such hybrid algorithm and further illustrate its performance, the finite-element-model updating of a benchmark footbridge is performed using two different approaches (single-objective and multi-objective) and three different computational algorithms, namely: (i) genetic algorithms; (ii) harmony search; and (iii) the novel UKF-HS hybrid algorithm. The obtained results reveal that the proposed hybrid algorithm may be considered as an adequate alternative tool to efficiently perform the finite-element-model updating of civil engineering structures in practical engineering applications.