Verification of the existence of human-induced horizontal forces due to vertical jumping

It is well known that jumping of humans on horizontal surfaces creates considerable dynamic forces in the vertical direction. However, recent measurements have demonstrated that sustained periodic jumping has the ability to create not only vertical but also horizontal near-periodic dynamic forces. Little is known about these forces, but they are crucial when checking vibration serviceability of grandstands which may be under a considerable load due to suddenly jumping crowds. These forces are likely to be generated by the inability of humans to perfectly jump from and land on the same spot which means that slight horizontal movement of the human body has the ability to generate horizontal dynamic forces. A similar phenomenon related to the horizontal sway forces, this time due to human walking, was recently observed in the UK on the Millennium Bridge in London which had to be closed immediately after its opening in June 2000 due to large sway vibrations cased by crowds of pedestrians walking in unison. Three components of the jumping forces, one vertical and two horizontal (in the sway left-right and front-to-back directions), were induced by a test-subject of an average weight and measured using a standard Kistler 9268 force measurement platform mounted on solid ground. The same test subject was then asked to perform jumping on a purposely designed and built simply supported concrete beam weighing 15 tonnes. Both the vertical and lateral modes of the beam's vibration were measured as well as its accelerations during the vertical jumping of the test subject. It was observed that in addition to the vertical, considerable horizontal acceleration responses of the beam were measured as a result of vertical jumping. These were then used, together with the measured modal properties, in an inverse analysis to determine indirectly the magnitude of the horizontal forcing functions, which were verified by comparing them with their directly measured counterparts using a force plate.