# Identification of a Human-Structure Interaction Model on an Ultra-Lightweight FRP Footbridge
The previous models often lead to high unreal predictions of the response of lightweight pedestrian structures since human-structure interaction (HSI) is neglected [22]. Due to the lightness and low stiffness of composite materials, interaction phenomenon may be a key aspect on the dynamic behaviour of FRP footbridges since significant vibration amplitudes may be reached [23]. Moreover, accounting for HSI on composite pedestrian structures could be beneficial in the assessment of the structural performance at VLS and the sizing of vibration control devices [24]. Therefore, an accurate estimation of human-induced vibrations is needed to avoid oversizing an FRP footbridge, which is directly related to the initial budget of a project. Since the first proposal from [PERSON] [25], several models [26; 27] accounting for the dynamic parameters of the human body have been used to consider HSI on the dynamic response of footbridges. Under this approach, the mass, frequency, and damping ratio of the human body together with an external force are employed along with the properties of the structure, leading to a coupled human-structure vibrating system. Among the different HSI models available in literature, a single degree of freedom (SDOF) mass-spring-damper-actuator (MSDA) system is the only one mentioned in a design guidance [28]. Besides the dynamic properties of the human body, this model, which has been used to represent bouncing action, considers an internal harmonic driving force that affects both the pedestrian and the structure. Hence, the ground reaction force transmitted to the structure is comprised by the actuator force and the stiffness and damping forces of the oscillator, depicted though the properties of the pedestrian. In contrast to MF models that use DLFs to describe a periodic action, the internal driving force employs Generated Load Factors (GLFs) [29]. Figure 1: Reduction coefficient \(\psi\) for walking load models in the vertical direction. This research presents an experimental and numerical study related to the dynamic behaviour of a laboratory FRP footbridge, whose linear mass is only 80 kg/m. The lightweight facility exhibits excessive vertical vibrations when the fourth harmonic of a walker is synchronised with the bridge's fundamental frequency. As the first natural frequency of the structure lies above the limits stated in traditional guidelines, a MSDA system is employed herein to represent a pedestrian crossing the bridge and account for HSI. To the best of authors' knowledge, the identified model is the first representation of a single pedestrian walking that considers four harmonics of the human action and interaction phenomenon. To this end, data recorded from experiments involving six test subjects crossing the footbridge is employed. Additionally, an optimization problem aiming to minimise the difference between experimental results and numerical outputs from a coupled human-structure system, based on a closed-loop transfer function (TF), is addressed. After this introduction, the text is organised as follows. The simply supported FRP bridge is described in Section 2, whilst the performed experimental campaign is explained in Section 3. The calibration of a FE model of the structure together with the numerical bridge response under the action of a single pedestrian are presented in Section 4. The identified HSI model, based on a closed-loop TF, is also included in this section. Considering the presented model, a parametric analysis varying the dynamic properties of the human body and the GLFs of the internal driving force is carried out in Section 5. Finally, concluding remarks and future works are drawn in Section 6.