Saturation: above acceleration levels of around 1-1.2 m/s\({}^{2}\)[21], some pedestrians feel uncomfortable and choose to change their gait or stop. The growth of the structural response slows down. It can therefore be concluded that the human-structure interaction (HSI) phenomenon is widely accepted in the existing literature as the main cause of the lock-in and postlock-in behaviours. However, there are different hypotheses about how to formulate the pedestrian loads and, in consequence, different techniques to evaluate the footbridge response. The pedestrian load is assumed to be a random load with large intra-subject variability (changes in the load exerted by the same person at different times) and inter-subject variability (differences between subjects). Some recommendations and design guidelines (e.g., Setra [22], Hivoss [12], Fib [23], Eurocode 5 [24] and Iso-10137 [25]) use a deterministic load model to evaluate the footbridge response. Given that the load takes approximately the same value at equal slots of time periods, and neglecting the intra-subject variability, the single pedestrian action is considered to be a harmonic load, in resonance with the bridge vibration. The effect of a group of pedestrians is taken into account by considering an effective number of perfectly synchronised pedestrians [12; 22; 23]. The main advantage of the deterministic model is that it is simple to use for design purposes. However, as pointed out by [PERSON] [7] and [PERSON] [26], the deterministic methods are not reliable for groups of pedestrians as a result of the great scatter of effective numbers of pedestrians given by the different guidelines. For example, for 100 pedestrians, the effective number of pedestrians would range from 10 to 19 [7; 26]. Furthermore, the deterministic methods fail to consider the HSI phenomenon. Even though [PERSON] [12] and [PERSON] [22] introduce the modification of the modal frequency due to the HSI by modelling the pedestrians as passive masses, they fail to account for the influence on modal damping. In addition, current design recommendations lack an expression for the auto-induced component of the pedestrian action and, in consequence, it is not possible to evaluate the footbridge degree of comfort once the lock-in effect has developed. In addition, the codes also have to introduce a criterion for checking the lateral lock-in. [PERSON]'s stability criterion [3] is the most commonly accepted, but it fails to agree with some experimental results. There are other criteria, such as [PERSON]'s [27], [PERSON]'s [28] and [PERSON] [29]. [PERSON] [26] has compared the different formulations, finding again a large scatter in the results. A different approach is to define the threshold acceleration amplitude beyond which the phenomenon develops. Most of the design guides set it between 0.1 and 0.15 m/s\({}^{2}\)[12; 22; 30]. A different methodology, which is formulated in the frequency domain, is to define the pedestrian load as a stationary random process through its power spectral density (PSD), treating the pedestrian flow probabilistically and thus considering the intra- and inter-subject variability of the load. The stationary response is obtained by calculating the root mean square (RMS) value of the modal acceleration. [PERSON] and [PERSON] [11] and [PERSON] et al. [31] have proposed a characteristic PSD for the first five harmonics of the pedestrian lateral action that has been obtained measuring the lateral pedestrian forces on an instrumented treadmill. HSI is included through an experimental coefficient, usually being noted as \(c_{p}\) for the auto-induced in-phase component of the pedestrian force and \(q_{p}\) for the out-of-phase component [31; 32], thus considering the change of the modal parameters of the structure in a simplified way. Nevertheless, the spectral methods apply to a stationary and uniform pedestrian crowd sufficiently low to omit the human-human interaction.