## 5 Conclusions  
For structural vibration issues, knowledge and availability of computationally efficient and realistic modelling strategies represent a strategic task. Over the years, several formulations and proposals have been theoretically elaborated in the literature and validated towards experimental investigations. These efforts supported a more realistic analysis of vibration issues and Human-Structure Interaction (HSI) phenomena, especially for low-frequency structural systems (like bridges and walkways) characterized by high sensitivity to vibrations. In this paper, among others, the attention was focused on a literature selection of Spring-Mass-Damper (SMD), Single Degree of Freedom (SDOF) approaches to describe the vertical loads induced by a single pedestrian on a given structural system (SMD-1 to SMD-4), in terms of calibrated equivalent mass \(m\), spring stiffness \(k\) and damping coefficient \(c\). The parametric numerical analysis (inclusive of 100 different configurations) was focused on four different floors (F#1 to F#4) with various combinations of vibration frequency \(f_{1}\) and structural mass \(M_{s}\), compared to the walking pedestrian (with mass \(M\) and moving at \(f_{p}\)). From the selected SMD models, it was shown, for example, that calibrated (\(m\), \(k\), \(c\)) parameters may strongly differ from each other, even to describe the same walking pedestrian/walking setup. Accordingly, modifications can also be expected in terms of predicted structural behaviours and performance indicators of the examined floors (like acceleration peak, RMS acceleration, CREST factor, etc. ), which are of significant interest for vibration serviceability assessment. Furthermore, such sensitivity can be further affected by the intrinsic dynamic parameters of those floor systems most sensitive to human-induced loads. Through the parametric numerical analysis carried out in ABAQUS, the attention was thus focused on two concrete floors with medium or low vibration frequency but relatively high structural mass compared to the occupant (F#1 and F#2, with mass ratio \(>1/40\)th). The third and fourth systems (F#3 and F#4) were made of glass and characterized by low or medium vibration frequency but still high or rather small mass respectively, compared to the pedestrian (\(\approx\)1/4 th the mass ratio for F#4 system). The parametric study proved that SMD models are more stable and less scattered in dynamic estimates on the structural side as far as they are applied to low-frequency systems with high mass. Besides, for low-mass systems, such as modular slabs composed of glass, which are characterized by specific intrinsic features compared to traditional structural solutions, the numerical predictions emphasized a relatively high scatter of reference performance indicators based on various SMD models of literature. Also, the SMD-based sensitivity of structural parameters under the effects of the same pedestrian was measured at up to 30% from the average for F#2 and F#4 systems, thus denoting a primary role of low-frequency for floor sensitivity, but also low-mass, as a critical additional influencing parameter, regardless the vibration frequency. As such, additional investigations and dedicated calibrations are needed for this special typology of pedestrian systems. Methodology, C.B. and F.A.S. ; Software, C.B. ; Formal analysis, C.B. ; Investigation, C.B. and F.A.S. ; Data curation, C.B. ; Writing--original draft, C.B. and F.A.S. All authors have read and agreed to the published version of the manuscript. This research activity has been carried out at Department of Civil Engineering and Architecture of University of Trieste, and financially supported in the framework of \"ComBioDyn\" Microgrants 2022 project. Part of this research activity has been carried out at Civil Engineering Research and Innovation for Sustainability (CERIS), with funding by Fundacao para a Ciencia e a Tecnologia (FCT), UIDB/04625/2020 project. Not applicable. Data Availability Statement: Data will be shared upon request.