Experimental study of human-induced dynamic forces due to jumping on a perceptibly moving structure

Abstract

This paper describes the first ever direct measurements of human-induced dynamic forces due to jumping on a force platform which is moving. Therefore, the paper addresses the increasingly important issue of jumping on a flexible structure that can move perceptibly. A unique test rig, developed to permit a person to jump on an idealised single-degree-of-freedom system with variable natural frequency and mass, is described and the test methodology explained. A set of representative results, for different rig natural frequencies (2–6 and 16 Hz) and a range of achievable jumping frequencies (1–3.5 Hz), is presented. This clearly demonstrates the effect of the flexibility of the structure on the levels of dynamic response and force that can potentially be generated by humans when they feel the motion of the structure on which they jump. The acceleration and displacement responses show significant peaks when the jumping frequency is in the region of half the natural frequency and of the natural frequency itself. This indicates that the first and second harmonic of the human-induced forcing functions are exciting resonant response, as would be expected. However, it is also shown that, for the test rig configuration chosen, it was not physically possible to jump at frequencies close to the natural frequency when the structural motion was significant. This is a new finding thought to be due to the limitations imposed by the projectile motion of the human test subject. It is also apparent that the contact ratio (ratio of time in contact with the platform/period of jumping), determined from the measured jumping force time history, increases in the regions of peak response and does not ever fall below a value of 0.5. This is a considerably higher contact ratio value than was established in the past in similar jumping tests performed on stiff and not perceptibly moving surfaces. As a consequence of the variation of contact ratio, the amplitudes of the force harmonics do not vary only with the jumping frequency, which is a widely known fact, but also with the ratio of jumping to natural frequency, which is linked to the amount of motion of the test rig.