Each year there is an increasing incidence of slip and fall accidents, especially among the elderly population. Existing evidence has identified several aging effects related to slip and fall accidents, yet, the causes of these accidents with advancing age are still little known. The objective of this research was to investigate the factors influencing the initial phase of unexpected slips and falls in younger and older individuals. More specifically, the relationship between ankle strength, the ankle joint power to transfer the whole body center-of-mass during normal gait, and the likelihood of slip-induced falls was identified. The walking experiment and the ankle strength tests were conducted in the Locomotion Research Laboratory, Virginia Tech. Fourteen old (67-79 years old) and 14 young (19-35 years old) individuals participated in this study (7 male and 7 female for each age group).Within a subsequent 20-minute session of natural walking on a linear track, kinematic and kinetic data were collected synchronously. A slippery surface was introduced to the participant on the purpose of unexpected slip event. The ankle strength tests were performed using a dynamometer. The results indicated that ankle strength degradation in older individuals was related to the outcome of slips (i.e., higher frequency of falls). The results also indicated that older individuals' RCOF was less than their younger counterparts. However, older individuals fell more often than younger individuals. It is concluded that friction demand characteristics may not be a total deterministic factor of fall accidents. Thus, the further research should focus not only on the dynamic of slips, but also on the dynamics of falls.</p> / Master of Science

Deterioration in the mechanical properties of concrete, masonry and steel structures are usually observed under repeated cyclic loading in the inelastic response range. Therefore such a behavior becomes critical when these types of structures are subjected to ground motions with specific characteristics. The objective of this study is to address the influence of degrading behavior on simple systems. The Structural Performance Database on the PEER web site, which contains the results of cyclic, lateral-load tests of reinforced concrete columns, are employed to quantify the degradation characteristics of simple systems by calibrating the selected degrading model parameters for unloading stiffness, strength and pinching of a previously developed hysteresis model. The obtained values of parameters from cyclic test results are compared with the recommended values in literature. In the last part of the study, response of SDOF systems with various degradation characteristics are investigated using a set of seismic excitations recorded during some major earthquakes. The results indicate that when all the degradation components are combined in a structural system, the effect of degradation on response values becomes much more pronounced.

Although clays are generally considered stable materials under seismic conditions, recent failures initiated in clay layers after earthquakes have emphasized the need to study the cyclic and post-cyclic behavior of these materials. Moreover, if strength loss as a result of cyclic loading were to occur in the material comprising the dam and/or dam foundation, the consequences of failure could be substantial. The objective of this study is to evaluate the effect of plasticity characteristics, mineralogical composition, and accumulated energy on the cyclic behavior, post-cyclic shear strength and the degradation in shear strength due to cyclic loading in normally consolidated clays. Seventeen soil samples prepared in the laboratory from kaolinite, montmorillonite, and quartz were tested using static and cyclic simple shear apparatuses. In addition, the results of cyclic simple shear tests on twelve natural samples were provided by Fugro Consultants, Inc. in Houston, TX. Using the results, cyclic strength curves were developed to represent 2.5%, 5% and 10% double amplitude shear strains. These curves were used to examine the influences of mineralogical composition, plasticity characteristics and shear strain on the cyclic resistance of soil samples. A power function was used to represent the cyclic strength curves. The samples were found to become increasingly resistant to cyclic loading as the plasticity index increased. Moreover, the soils with montmorillonite as the clay mineral were noted to have consistently higher cyclic resistances than the soils with kaolinite as the clay mineral. By examining the power functions, it was found that the cyclic strength curve approaches linearity as the plasticity index increases in soils having kaolinite as the clay mineral. However, the opposite trend is observed in soils having montmorillonite as the clay mineral. The study shows that the post-cyclic shear strength increases with increasing plasticity index. Moreover, the post-cyclic shear strengths of soils with montmorillonite as the clay mineral were significantly higher than the post-cyclic shear strengths of soils with kaolinite as the clay mineral. The degradation in shear strength due to cyclic loading appeared unaffected by mineralogy, but a greater reduction in strength was noted with decreasing plasticity index. The post-cyclic shear strength was also found to reduce as the number of cycles required to cause 10% double amplitude shear strain increased. The energy approach considering the accumulated energy per unit volume in the soil mass as a result of cyclic loading was also utilized in this study. The results from the energy approach were independent of the cyclic wave form, but were still dependent on the amplitude of the cyclic load used during the testing. An increase in the amplitude of the cyclic loading function results in a decrease in the accumulated energy per unit volume. Furthermore, an increase in the liquid limit and/or plasticity index of the soils containing kaolinite as the clay mineral shows an increase in the accumulated energy, whereas an increase in plasticity of the soils containing montmorillonite as the clay mineral results in a decrease in the amount of accumulated energy. In both types of materials, the amount of accumulated energy per unit volume is found to increase with increasing double amplitude shear strain. Relationship between the ratio of post-cyclic undrained shear strength to the baseline undrained shear strength and the accumulated energy is also determined. / Ph. D.

The demand for flat glass is high and increasing significantly in the building industry as a direct result of architectural requirements for lightness, transparency and natural light. Current architectural trends require glass in curvilinear forms for smooth free-form façades. Two principal challenges arise from this: to cost-effectively produce the desired curvature and; to ensure its safe performance after exposure to ageing. The recent availability of high strength glass provides an opportunity to address the first challenge by developing cold bent glass. Cold bending involves the straining of relatively thin glass components, at ambient temperatures, and is a low energy and cost effective manner of creating curvilinear forms. However, cold bending is not yet widely established as a reliable method. The aim of this thesis is to develop the understanding of cold bent glass during the bending process and to evaluate its post-ageing performance. This thesis, firstly, investigates the mechanical response of monolithic glass plates during the cold bending process. The stability of cold bent glass is investigated experimentally by bending it in double curved anticlastic shapes. A parametric numerical analysis involves different boundary conditions, geometrical plate characteristics and bending parameters. The principal outcome is that a local instability, now termed cold bending distortion, occurs when certain displacement limits are exceeded and could degrade the optical quality of the glass. An evaluation procedure is also formulated to set limits and aid designers/manufacturers to predict the mechanical response and the optical quality of the glass. Cold bent glass is subjected to permanent bending stresses throughout its service life and therefore, its strength degradation after ageing needs to be quantified. Analytical, experimental and numerical investigations are undertaken in this thesis to identify the most effective method for estimating glass strength (evaluation of destructive tests, required number of specimens, statistical analysis methods and sub-critical crack growth). The limited availability of naturally aged toughened glass and the absence of a reliable ageing standard impede the evaluation of its aged performance. Therefore, a parametric experimental investigation of artificial ageing methods on glass is undertaken in this thesis. A procedure for the evaluation of the strength of aged glass is finally, formulated to allow the selection of artificial ageing parameters that correspond to a target level of erosion. The knowledge on artificial ageing and strength prediction acquired above is finally implemented on different types of glass to determine their strength after ageing and assess their safe use in cold bending / load bearing applications. The investigation showed that fully toughened glass has a superior performance to chemically toughened or annealed glass. Overall, the research presented in this thesis demonstrates that high quality cold bent toughened glass can be created when certain applied displacement limits are respected. These can be used as a safe, cost-effective and energy efficient replacement to the more conventional hot bent glass. However, cold bending / load bearing applications in which the stressed glass surface is exposed to ageing, require glass with a relatively high case depth such as fully toughened or bi-tempered glass.