39 Study of Human-Structure Dynamic Interactions 393 Fig. 39.1 Test structure at the Virginia Tech Vibration Testing Laboratory 39.3 Modal Testing A full modal test of the floor was conducted using an APS-400 electrodynamic shaker. The shaker was placed at the quarter point from a corner column to excite several lower modes of the floor. A burst chirp excitation with 30 s on and 15 s off and a frequency bandwidth of 3–20 Hz was used. The measured records were used in MEscope VES [22] to estimate the natural frequencies, damping ratios and mode shapes of the structure. The measured natural frequencies and damping ratios for the first three lower modes are as follows: f1 D 4.28 Hz, 1 D 0.264%; f2 D 7.72 Hz, 2 D 0.205%; and f3 D8.91 Hz, 3 D0.227%. Higher modes were not identified since their contributions towards the floor vibrations were limited. Figure 39.3 shows the shaker and accelerometers during one of the stages of the modal test. 39.4 Vibration Tests with Human Subjects To study the effects of a group of people on the dynamic properties of the test floor and estimate the crowd dynamic properties, six groups of human subjects with different sizes from 4 to 41 individuals were placed on the floor in three postures: sitting on chairs, standing erect, and standing with the bent knees. Each individual was weighed before conducting the tests. PCB 393C accelerometers were placed at different locations on the floor and the shaker was placed at the same location as for the floor modal test. A burst chirp excitation with 30 s on and 15 s off and a frequency band of 3–15 Hz was used. Low levels of vibration within the range typically resulted from human movements were applied. The vibration magnitudes were at the lower limits of those used in the biomechanics research. To directly evaluate the effects of humans on the test structure as compared to rigid masses, concrete blocks with equivalent weights to each individual were placed at the subjects’ specific locations on the test structure and the floor response was measured. The same excitation setting and location of the accelerometers were used for consistency. Figure 39.4 shows one of the stages of the vibration tests with 24 human subjects in sitting, standing, and bent-knees postures along with the tests with concrete blocks (weights). Figure 39.5 shows the comparison of the drive point frequency response function (FRF) without people, with 24 people in sitting, standing, and bent-knees positions, and concrete blocks in place of people. This clearly shows that the humans interact dynamically with the floor and they cannot be considered as rigid masses (concrete blocks).
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