Sifiso Nhleko

Doctor of Philosophy, New College, Oxford University, Trinity Term, 2011

Human-induced Lateral Excitation of Public Assembly Structures

Excessive human-induced vibrations of assembly structures have been reported more frequently over the past two decades. In the automotive industry, the evaluation of the response of the human body to vibration is an important and active research area due to its significance in design safety. For example, the results of on-going research has led to the establishment of a number of models for predicting the effects of sitting posture, vibration magnitude and direction on the response of the human body when it is exposed to different types of vibration motion. However, research focusing on structural engineering applications remains scarce, leading to inadequate design standards. For example, current structural design guidelines focus on human-induced vertical forces and assume linear structural behaviour, however, the most widely publicized problems have involved horizontal vibrations and many real-life structures are characterized by nonlinear behaviour. Following a brief investigation of vibration perception and comfort for subjects occupying a vibrating rig-structure, this dissertation focuses on humaninduced horizontal forces and examines the effect of nonlinear structural behaviour.

Dynamic horizontal loads of individuals performing predefined manoeuvres such as swaying and vertical jumping were measured in a laboratory setting. The fundamental force due to swaying occurred at the activity frequency. By contrast, the fundamental horizontal force due to vertical jumping did not always occur at the activity frequency. Furthermore, tests conducted for swaying were used to establish the relationship between the side-to-side force and the velocity of the subject‟s centre of mass. A customized footswitch system was also developed to monitor synchronization among individuals performing as a group in order to form a crowd loading model.

Models of analytic forces were derived based on measured data and used to evaluate structural response by focusing on a finite element model of a demountable grandstand characterized by nonlinear structural behaviour. The frequency spectra of displacement and acceleration responses showed clear peaks at the fundamental and the third harmonic of the swaying force, demonstrating the capability of the horizontal force to excite resonance. The resonant frequency decreased at higher levels of excitation, indicating a reduction in the stiffness due to the onset of nonlinear behaviour. Finally, load cases assuming synchronized and perfectly periodic group forces produced a significantly higher response compared to unsynchronized and imperfect group loads.

Thesis (13.3Mb, pdf)