Low back disorders and their prevention is of great importance for companies and their employees. Whole-body vibration is a risk factor for low back disorders, but the neuromuscular, biomechanical, and/or physiological mechanisms responsible for this increased risk are unclear. These studies investigated changes in the biomechanics and control of the trunk in order to further the understanding of the mechanisms responsible for this increased risk. The purpose of the first study was to measure the acute effect of seated whole-body vibration on the postural control of the trunk during unstable seated balance. The findings show that whole-body vibration impaired the postural control of the trunk as evidenced by increased kinematic variance and non-linear stability control measures during unstable sitting. These findings imply an impairment in spinal stability control. The purpose of the second study was to measure the effect of seated whole-body vibration on the parameters of spinal stability control: passive stiffness, active stiffness, and neuromuscular reflexes. The findings show that whole-body vibration altered trunk stiffness (passive stiffness and equivalent reflex stiffness) as well as reflex dynamics. There was no evidence of compensation by active muscle co-contraction recruitment for the decreased trunk stiffness and reflex gain. The purpose of the third study was to measure the changes in the natural frequency characteristics of the trunk (which can be related to trunk stiffness and damping) during exposure to seated whole-body vibration. The findings show that whole-body vibration caused a decrease in natural frequency suggesting a decrease in the trunk stiffness, and also an increase in the peak amplitude of the frequency response functions suggesting a decrease in overall trunk damping. The rate of change of the natural frequency characteristics suggest that the majority of effects happen within the first 10 minutes of vibration exposure. These findings reveal changes in the biomechanical properties of the trunk with exposure to seated whole body vibration, and a mechanism by which vibration may increase the risk of low back injury. / Ph. D.

Le contexte automobile actuel pousse les industriels à innover pour réduire à la fois l’impact énergétique et la consommation en hydrocarbure de leurs véhicules. Un accent particulier a donc été mis sur l’allègement structural des véhicules. Dans le cadre des véhicules récréatifs, cet allègement se traduit par l’augmentation de l’utilisation des tôles formées notamment dans les zones de transmission d’effort et amène de nouveaux besoins pour la conception. En effet, des comportements vibratoires problématiques en basses fréquences de ces tôles ont été mis en avant sur des véhicules récréatifs au stade de prototype. Il est alors nécessaire de proposer une méthode de conception de ces tôles réceptrices d’excitation, pour améliorer le comportement vibratoire en basses fréquences d’une structure au stade de prototype. Pour cela trois outils techniques sont développés, le premier pour la prise en compte des efforts à l’aide d’une cartographie de mobilité, le second pour guider les modifications de raideur des tôles avec une cartographie de contraintes différentielles et le dernier avec un outil de mise en forme pour appliquer ces modifications de raideur par embossage. La méthode est alors présentée et appliquée à partir de ces trois outils sur des cas d’application statiques et dynamiques. Pour une tôle seule encastrée excitée dynamiquement, les résultats démontrent une diminution de l’énergie cinétique stockée de 76.07%, ceci sur une bande de basses fréquences ([0,500]Hz) par rapport à un cas de référence. Un cas de véhicule récréatif simplifié est aussi réalisé sous excitations moteur réelles sur une bande fréquentielle ([0,500]Hz), l’assemblage de tôles couplées présente alors une diminution de 13.51% de l’énergie cinétique stockée après l’application de la méthode proposée. L’application de la méthode permet donc d’améliorer le comportement vibratoire des structures considérées en basses fréquences.

With the development of lightweight concrete and design optimizations, floor vibration problems are becoming a serious serviceability problem. The castellated beam is a prime example and was the focus of this study. The vibration and flexural strength performance were verified in this paper. The vibration characteristics of castellated beams were examined using experimental and analytical test methods. The effective moment of inertia is essential to accurately predict the frequency and deflection of a floor system due to human occupancy. Since castellated beams have non-prismatic cross-sections, their effective moment of inertia is an uncertainty and was verified in this study. This paper confirmed the accuracy of the AISC Design Guide procedures used in for prismatic beam, when applied to castellated beams. The flexural strength of various composite castellated beam were studied. Three full-scale specimens were tested to failure to evaluate their yield and maximum applied load. Each specimen's moment strength was verified based on span, beam properties, concrete slab, and amount of shear connection. / Master of Science

No description available.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

The purpose of this study was to compare major theories of laryngeal vibration. The basic hypothesis of the study was that the differences and similarities between the major theories of laryngeal vibration could be made evident and clear through a comparative study. It was assumed that there are two or more theories of laryngeal vibration and that all the major theories of laryngeal vibration from 1945 to the present have been described in written form in English.

Passive vibration mitigation via multi-stable, mechanical means is relatively unexplored. In addition, achieving vibration suppression through avoiding resonance is at the forefront of up and coming research. This thesis investigates the application of a purely mechanical, bistable device as a passive method of vibration suppression. A purely mechanical device does not require power, multiple materials, or electrical circuits, and a passive device does not require external interaction or control. Therefore, a passive, mechanical device could be implemented with ease even in physically constrained environments with large dynamic loads, such as turbomachinery. The purely mechanical, bistable device presented herein replicates the two switches per resonance crossing evident in semi-active Resonance Frequency Detuning method. This work explores two different bistable, mass-spring models. The first is a single degree of freedom nonlinear mass spring model aiming to utilize asymmetry in the potential function to change the stiffness of the overall system. The second model is a coupled, two degree of freedom system that combines the nonlinear softening and hardening spring characteristics with the unique stiffnesses of two stable states. The performance is verified by targeting the first mode of a cantilever beam, with the device shifting the resonance away from the excitation frequency. Future research could apply these idealized models to complex, rotating structures and replicate the performance of the passive, mechanical devices in a physical geometry that could be manufactured as a part of a target structure.

MPH, Occupational Hygiene, Faculty of Health Sciences, University of the Witwatersrand / Introduction Locomotive operators in the mining industry are exposed to high levels of noise and vibration. There is currently limited information indicating whole-body vibration exposure levels conducted over an 8-hour time weighted average (TWA) exposure period; most of the available data are based on instantaneous measurements. The 10-Ton New Era locomotive was specifically designed with the focus on areas such as ergonomics, safety, future automation, productivity and flexibility of use. The locomotive has a single cab and can be driven and controlled with maximum visibility in the direction of travel. Most mining houses are converting from the use of the 10 Ton Goodman battery operated locomotive to the 10Ton New Era locomotive, hence the focus of noise and vibration measurements on the latter. Objectives This research report documents a project to measure noise and whole-body vibration exposure levels of locomotive operators working in an underground platinum mine. The objectives of the study are: • to describe the eight hour time weighted average occupational noise exposure levels of locomotive operators operating the 10-Ton New Era locomotive in an underground platinum mine over 2008 and 2009; • to describe personal whole-body vibration exposure levels of locomotive operators operating the 10-Ton New Era locomotive in an underground platinum mine over 2008 and 2009; and 5 • to determine whether personal noise and whole-body vibration exposure levels of locomotive operators operating the 10-Ton New Era locomotive in an underground platinum mine over 2008 and 2009 comply with national and international standards. Methods Personal noise and whole-body vibration exposure measurements were obtained from 21 underground locomotive operators. Measurements were conducted in accordance with the procedures described in the SANS 10083 standard for personal noise dosimetry and the ISO 2631-1 standard for whole-body vibration. Determination of likely health risks for the operators were based on a comparison of the measured time-weighted noise exposure levels with the South African OEL and the ACGIH threshold limit value; whole-body vibration levels were compared with the HGCZ limits presented in Annex B of the ISO 2631-1 standard and the EU directive daily exposure limits. Results The measured noise and whole-body vibration levels taken over an 8-hour TWA exposure period were higher when compared to national and international standards. The mean LTWA levels for noise was 66.5 dB(A) with 12.5% of the measurements exceeding the South African OEL of 85dB(A). 45% of the wholebody vibration measurements fell within the HGCZ indicating that whole-body vibration exposure on locomotive operators presents a moderate probability for an adverse health outcome. Discussion and Conclusion Locomotive operators are exposed to potentially harmful levels of noise and whole-body vibration. The Mine Health and Safety Act requires an employer to assess the health and safety risks that hazards pose to their employees, and to take reasonably practicable steps towards eliminating or controlling those risks. Like any other risks at a workplace, noise and whole-body vibration needs to be 6 identified and controlled, and the approach to be taken is one of a risk management

Railway induced ground-borne vibration is among the most common and widespread sources of perceptible environmental vibration, adversely impacting on human activity and the operation of sensitive equipment. The rising demand for building new railway lines or upgrading existing lines in order to meet increasing traffic flows has furthered the need for adequate vibration assessment tools during scheme planning and design. In recent years many studies of rail and ground dynamics have produced many vibration prediction techniques which have given rise to a variety of procedures for estimating rail-induced vibration on adjacent buildings. Each method shows potential for application at different levels of complexity and at different stages of a scheme. However, for the majority of the procedures significant challenges arise in obtaining the required input data, which can compromise their routine use in Environmental Impact Assessment (EIA). Moreover, as the majority of prediction procedures do not provide levels of uncertainty (i.e. expected spread of data), little is available on their effectiveness. Additionally, some procedures are restricted in that they require specific modelling approaches or proprietary software. Therefore, from an industrial point of view there is a need for a robust and flexible rail-induced vibration EIA procedure that can be routinely used with a degree of confidence. Based on an existing framework for assessing rail-induced vibration offered by the USA department of transportation (FTA) this project investigates, revises and establishes an empirical procedure capable of predicting rail-induced vibration in nearby buildings that can be routinely applied by the sponsoring company. Special attention is given to the degree of variability inherent to rail-induced vibration prediction, bringing forward the degrees of uncertainty, at all levels (i.e. measuring, analysis and scenario characterisation) that may impact on the procedure performance. The research shows a diminishing confidence when predicting rail-induced absolute vibration levels. It was found that ground-to-transducer coupling method, which is a critical step for acquiring data for characterising the ground, can impact on the results by as much as 10 dB. The ground decay rate, when derived through transfer functions, also showed to vary significantly in accordance to the assessment approach. Here it is shown the extent to which track conditions, which are difficult to account for, can affect predictions; variability in vibration levels of up to 10 dB, at some frequency bands, was found to occur simply due to track issues. The thesis offers general curves that represent modern UK buildings; however, a 15 dB variation should be expected. For urban areas, where the ground structure is significantly heterogeneous, the thesis proposes an empirical modelling technique capable of shortening the FTA procedure, whilst maintain the uncertainty levels within limits. Based on the finding and acknowledging the inherent degree of variability mentioned above, this study proposes a resilient empirical vibration analysis model, where its flexibility is established by balancing the significance of each modelling component with the uncertainty levels likely to arise due to randomness in the system.

Vehicle occupants are typically exposed to unpleasant whole-body vibration (WBV) for extended period of time. It is well known that the transmission of unwanted vibration to the human body can lead to fatigue and discomfort. Moreover, the unwanted vibration normally distributed in the low-frequency range has been found as the main risk factor for lower back pain and lumbago, which seriously affect the health and working performance of occupants. Thus vibration cancellation on seats has attracted considerable interest in recent years. So far, for most vehicle seats, vibration isolation is achieved passively by using seat cushions and conventional energy absorbers, which have very limited performance in the low-frequency range. The work presented in this thesis forms a successful development and experimental study of an active seat and control algorithm for occupants’ WBV reduction under low frequency excitations. Firstly, a modelling study of the seat human subjects (SHS) and an extensive experimental measurement of the vibration transmissibility of a test dummy and vehicle seat are carried out. The biodynamic responses of SHS exposed to uncoupled vertical and fore-and-aft WBV is modelled. A comparison with the existing models is made and the results show that an improved fit with the aggregated experimental data is achieved. Secondly, an active seat is developed based upon the observations and understanding of the SHS and seat system. The characteristics of the active seat dynamics are identified through experimental tests found suitable for the development of an active seat to attenuate the vibration experienced by vehicle occupants. The vibration cancellation performance of the active seat is initially examined by feedforward plus proportional-integral (PI) control tests. Through these tests, the effectiveness of the actuators control authority is verified, but the limitations are also revealed. Because the active seat system is subject to non-linear and time-varying behaviour, a self-tuning fully adaptive algorithm is a prime requirement. The Filtered-x Least-Mean-Square (FXLMS) algorithm with the Fast-block LMS (FBLMS) system identification technique is found suitable for this application and is investigated through experimental tests. Substantial vibration reductions are achieved for a variety of input vibration profiles. An excellent capability of the active seat and control system for efficiently reducing the vibration level of seated occupants under low-frequency WBV is demonstrated.