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In the analysis of lightweight footbridges subjected to pedestrian actions, the vertical dynamic response is often overestimated when Human-Structure Interaction (HSI) is omitted. To account for the phenomenon, a Single- Degree of Freedom (DOF) system has been employed to represent a person. When dealing with a stream of people, this approach may lead to an expensive computational problem as several DOFs have to be managed. An alternative to overcome this issue is modelling the crowd as a distributed Mass-Spring- Damper-Actuator system acting on the structure. Hence, an equivalent time-invariant system with two DOFs can be obtained while considering HSI. This paper proposes a procedure to determine the resonant response of pedestrian structures subjected to a flow of walking pedestrians. Through the construction of a Transfer Function in the frequency domain, accounting for the parameters of the crowd and the structure, the dynamic analysis can be addressed by computing a simple algebraic multiplication. The proposed approach is applied to a lightweight Fibre Reinforced Polymer footbridge, which has been recently designed and built by the authors at the School of Civil Engineering – Universidad Politécnica de Madrid. A weak traffic class (0.2 pedestrians/m²) is the considered load scenario for the assessment of the structural acceleration response. As a good agreement between experimental and numerical results is shown, the proposal may be employed for the fast prediction of the dynamic response of other lightweight pedestrian structures at design stage.