Experimental Assessment of the Individual-to-Crowd Consistency of a Stationary Human–Structure Interaction Model

Stationary human–structure interaction (HSI) can significantly modify the vibration characteristics of lightweight structures, yet it remains unclear whether a human model identified for an individual can be extended consistently to crowd conditions. This study investigates this issue through sweep-frequency tests on a single-degree-of-freedom (SDOF) rig under single-person standing, single-person seated and two- to five-person standing group conditions. The measured acceleration frequency response functions (FRFs) were fitted using a two-degree-of-freedom (2DOF) integrated HSI model to identify equivalent human/crowd and structural parameters. The experimental program involved 20 participants and 180 tests, yielding 40 averaged individual datasets and 50 averaged group-combination datasets. Clear double-peak FRF characteristics were observed under all occupied conditions, indicating coupled 2DOF behavior rather than a simple added-mass effect. Compared with seated occupants, standing occupants showed a slightly higher equivalent human natural frequency (6.76 Hz versus 6.45 Hz), but lower human damping ratio (23.37% versus 31.73%) and body mass ratio (BMR) (0.621 versus 0.729). For standing groups, the equivalent crowd damping ratio increased from 24.60% for the two-person group to 29.10% for the five-person group, and BMR increased from 0.66 to 0.80, whereas the equivalent crowd natural frequency remained within 6.74–7.02 Hz. These results support a unified but parameter-dependent 2DOF framework for stationary individuals and crowds, provided that posture- and crowd-dependent parameters are identified from the overall coupled-system FRF rather than obtained by direct averaging of individual properties.