The influences of human occupants on the dynamic properties of slender structures
This thesis describes a combined analytical and experimental investigation into the influence of human occupants on the dynamic properties of civil engineering structures. This is an increasingly important issue in the design of assembly structures against human-induced vibrations. An analytical parametric study demonstrated that a damped single degree of freedom (SDOF) model of one or more human occupants can explain (1) natural frequency increases and decreases, (2) additional natural frequencies, and (3) response reductions, reported in the literature. Experimental investigations employed a lightly damped simply supported prestressed concrete beam spanning 11 m and weighing 15 tonnes. The influence of up to five stationary humans on the modal properties of this laboratory based, but realistic, full-scale test structure was quantified. For this purpose, natural frequencies, damping ratios, mode shapes, and modal masses were estimated by curve-fitting of measured frequency response functions. It was shown that the occupants affected the three investigated vertical bending modes of the test structure (at about 4.5 Hz, 17 Hz, and 38 Hz). The occupants most significantly increased damping and it was established that the location, the posture and the number of occupants were important. Within the range of low-level vibrations studied, the level of vibration of the structure had only little effect. The combined analytical studies and experiments demonstrated that the presence of groups of stationary humans can be modelled by a damped 'human' SDOF system attached to the 'structural' SDOF system representing a well separated mode of an empty assembly structure. Based on the obtained experimental data, the mass, frequency and damping properties (mH, f and ýH) of a damped SDOF model of groups of sitting occupants were derived. It was established that these properties varied with the natural frequency of the structural system. It was found that mH decreases while fH and ýH increase with increasing natural frequency of the empty structural system. Based on these findings, mH should be assumed to be greater than 60% of the total mass of occupants, fH smaller than approximately 9 Hz and ýH less than 40% in the case of empty structures with natural frequencies below about 17 Hz. The derived damped SDOF human model was used to quantify the influence of occupants on the dynamic response of a range of structures modelled as SDOF systems. These data are provided in the form of design charts. They can, until further information becomes available, be used to estimate dynamic responses of civil engineering structures occupied by sitting humans to sinusoidal excitation.