Granular aggregates of quartz subjected to triaxial compression under constant effective pressures (Pe) undergo macroscopic failure at critical stress states that depend on the effective mean stress. Although the mode of failure and mechanical response vary systematically with mean stress at failure, prefailure loading at subcritical stress states may induce yielding, and subcritical load paths may influence behavior at failure. Here, I investigate how the failure of quartz aggregates at conditions favoring compaction depends on consolidation history and load path in the transitional and ductile deformation regimes in terms of strain localization and microfracture fabric. Three distinct non-standard triaxial compression load paths were employed; the paths involve different preconsolidation of the aggregates at subcritical isotropic stress followed by differential loading with increasing or decreasing confining pressure. Deformed aggregates were injected with epoxy and studied using optical microscopy techniques to determine microscopic damage evolution for the different load paths. Microfracture data show that preconsolidation at subcritical isotropic loads facilitates formation of campaction bands during subsequent triaxial compression in the transitional regime. The preferred orientation of intragranular cracks evolves from near random fabrics for isotropic loading to strongly preferred orientations parallel to the maximum principal compression direction for differential loading, with the strongest preferred orientation within the compaction bands. Aside from the preconsolidation, different load paths have only a minor effect on the mechanical response during macroscopic failure.

Load transfer analysis deals with an important function of engineering structure, which is the ability of structure in transferring imposed loads to the supporting points. Although stress value has proved to be an efficient index for performing the failure analysis, the necessity of defining an index for evaluation of structure stiffness has led to the introduction of the U* index theory. The U* index characterizes the internal stiffness distributions, as an indicator of the load transfer in the structure. Although U* index theory have been proven to be useful in design, it is missing necessary steps toward becoming a mature theory for structural analysis. Firstly, the U* index theory needed to be examined and validated by experimental testing. Therefore, an experimental setup was proposed and tested, and U* index theory was validated through comparison of results. Secondly, a systematic comparison between the conventional stresses analysis and the U* index analysis was lacking. Such comparison was made for structural analyses of a vehicle component. The results, also compared to observations of experimental testing showed that in some cases, application of conventional stress analysis might be limited or less precise. Thirdly, design modification capability is a significant feature of the U* index theory, and it was necessary to demonstrate that real life problems can benefit from this potential. In this study, sample structures representing the components of multiple passengers carrying vehicles were selected and analyzed by U* index theory and design modifications were proposed and implemented on the structure. Lastly, the U* index theory should be applicable to different types of problems, including nonlinear domain. Hence, to remove the limitations of linear analysis that is a part of the original theory, an extension of U* index theory to the nonlinear domain was proposed and tested. In summary, U* index theory provides an understandable explanation of load transfer in the structure and provides a general awareness regarding structural performance. He presented work showed that the existing methods of structural analysis have limitations in certain aspects that can be overcome by combining the perspective of U* index analysis to the existing structural analysis paradigm. / February 2017

Joining of uniformly-curved composite sandwich panel segments, typical in state of the art aerospace launch vehicles, should be mass-efficient. Adhesively bonded joints can provide increased mass-efficiency over mechanically-fastened joints. But, due to manufacturing sensitivities and certification requirements, conventional bonded joints can be improved upon by introducing structural redundancy. A longitudinal, durable redundant joint (DRJ) architecture featuring multiple adhesive load-paths, via a novel composite preform insert, was proposed to join composite sandwich panel segments of the interstage element for NASA's Ares V launch vehicle. A series of twenty-five static linear-elastic finite element models with plane strain solutions were developed to assess certain characteristics of a joint's structural response when subjected to a simplified circumferential hoop loading convention. Shear and normal stress distributions at the adherend-adhesive interface along the splice plate bondline of the DRJ are compared with those from a conventional splice joint (CSJ) configuration for a series of linearly increasing bondlines thicknesses and joint overlap lengths. The parameter studies indicate the DRJ configuration's adhesive peak stresses are independent of the joint overlap length at the joint edges. Also, simulated bonding defects, in the form of local adhesive gaps, due to manufacturing processes are investigated to determine the load path redistribution for the DRJ and CSJ configurations. Results for pristine versions of both configurations are included. The defective CSJ joint exhibits severe overloading of certain laminates, while the defective DRJ load redistributions are relatively mild. Between the two primary types of bondline gaps considered for the DRJ configuration, the gap corresponding to the splice plate, a more mature manufacturing operation and also a more easily inspected location than the insert-to-face sheet interface, is noted to be more severe. A direct joint-to-joint mass-comparison reveals a 164% increase in mass, per unit thickness, between the CSJ and DRJ. To put this in perspective, a second comparison is made using a four-segment sandwich panel barrel. A 3.51% increase in mass is observed between the CSJ and DRJ-based cylinders. Also, for a simplified sizing philosophy, based solely on the peak stresses in the adhesive domain, a CSJ may require a 1.5-inch longer joint overlap than a DRJ. The mass-estimate is recomputed, and the mass percent-increase of the segmented cylinder is reduced to 2.61% over a CSJ configuration. / Master of Science

In this thesis, a load path caused by arching action in reinforced concrete slabs is described and validated using a three-dimensional model. Currently, the CHBDC enforces a 4 meter girder spacing requirement in the design of deck slabs. The aim of this thesis is to investigate the load path induced by arching action in deck slabs with a wide range of girder spacing. To do this, a two-dimensional model was developed to examine the path of horizontal stress and was validated using a FEM. A parametric study showed that girder spacing does not affect the development of restraining stress while cantilever width does. As well, cracking of the slab is necessary for arching action to occur. These results help with future development of a rational model to be used by bridge designers.

See appended files.

In this thesis, a load path caused by arching action in reinforced concrete slabs is described and validated using a three-dimensional model. Currently, the CHBDC enforces a 4 meter girder spacing requirement in the design of deck slabs. The aim of this thesis is to investigate the load path induced by arching action in deck slabs with a wide range of girder spacing. To do this, a two-dimensional model was developed to examine the path of horizontal stress and was validated using a FEM. A parametric study showed that girder spacing does not affect the development of restraining stress while cantilever width does. As well, cracking of the slab is necessary for arching action to occur. These results help with future development of a rational model to be used by bridge designers.

Precast concrete multistory buildings are used in an attempt to optimize the available construction space and reduce costs. However, little is known about predicting their capacity in a brittle response mode due to the sudden loss of a critical element that could induce a Progressive Collapse Scenario. Therefore, the National Institute for Standards and Technology (NIST) developed an explicit approach in the design of precast concrete systems that is intended to mitigate a progressive collapse by enhancing the rotational capacity of joints and the robustness of the structural system. A full-scale experiment was conducted to investigate the structural performance of a prototype design under a column-removal scenario. The test assembly frame, consisting of three columns and two beams, was subjected to a displacement controlled vertical force acting at the center to characterize the failure modes and collapse mechanisms. Brittleailures of critical structural elements were observed and significantly impacted the performance.

A structure subjected to extreme load due to explosion or human error may lead to progressive collapse. One of the direct methods specified by design guidelines for assessing progressive collapse is the Alternate Load Path method which involves removal of a structural member and analysing the structure to assess its potential of bridging over the removed member without collapse. The use of this method in assessing progressive collapse therefore requires that the vertical load resistance function of the bridging beam assembly, which for a typical laterally restrained reinforced concrete (RC) beams include flexural, compressive arching action and catenary action, be accurately predicted. In this thesis, a comprehensive study on a reliable prediction of the resistance function for the bridging RC beam assemblies is conducted, with a particular focus on a) the arching effect, and b) the catenary effect considering strength degradations. A critical analysis of the effect of axial restraint, flexural reinforcement ratio and span-depth ratio on compressive arching action are evaluated in quantitative terms. A more detailed theoretical model for the prediction of load-displacement behaviour of RC beam assemblies within the compressive arching response regime is presented. The proposed model takes into account the compounding effect of bending and arching from both the deformation and force points of view. Comparisons with experimental results show good agreement. Following the compressive arching action, catenary action can develop at a much larger displacement regime, and this action could help address collapse. A complete resistance function should adequately account for the catenary action as well as the arching effect. To this end, a generic catenary model which takes into consideration the strength degradation due to local failure events (e.g. rupture of bottom rebar or fracture of a steel weld) and the eventual failure limit is proposed. The application of the model in predicting the resistance function in beam assemblies with strength degradations is discussed. The validity of the proposed model is checked against predictions from finite element model and experimental tests. The result indicate that strength degradation can be accurately captured by the model. Finally, the above developed model framework is employed in investigative studies to demonstrate the application of the resistance functions in a dynamic analysis procedure, as well as the significance of the compressive arching effect and the catenary action in the progressive collapse resistance in different designs. The importance of an accurate prediction of the arching effect and the limiting displacement for the catenary action is highlighted.

Proposed in this thesis is a methodology to enable yacht designers to develop innovative structural concepts, even when the loads experienced by the yacht are highly uncertain, and has been implemented in sufficient detail to confirm the feasibility of this new approach. The new approach is required because today??s yachts are generally lighter, getting larger and going faster. The question arises as to how far the design envelope can be pushed with the highly uncertain loads experienced by the structure? What are the effects of this uncertainty and what trade-offs in the structural design will best meet the overall design objectives? The new approach provides yacht designers with a means of developing innovative structural solutions that accommodate high levels of uncertainty, but still focus on best meeting design objectives constrained by trade-offs in weight, safety and cost. The designer??s preferences have a large, and not always intuitive, influence on the necessary design trade-offs. This in turn invites research into ways to formally integrate decision algorithms into knowledge-based design systems. A lean and robust design system has been achieved by developing a set of tools which are blanketed by a fuzzy decision algorithm. The underlying tool set includes costing, material optimisation and safety analysis. Central to this is the innovative way in which the system allows non-discrete variables to be utilized along with new subjective measures of structural reliability based on load path algorithms and topological (shape) optimisation. The originality in this work is the development of a knowledge-based framework and methodology that uses a fuzzy decision making tool to navigate through a design space and address trade-offs between high level objectives when faced with limited design detail and uncertainty. In so doing, this work introduces the use of topological optimisation and load path theory to the structural design of yachts as a means of overcoming the historical focus of knowledge-based systems and to ensure that innovative solutions can still evolve. A sensitivity analysis is also presented which can quantify a design??s robustness in a system that focuses on a global approach to the measurement of objectives such as cost, weight and safety. Results from the application of this system show new and innovative structural solutions evolving that take into account the designers preferences regarding cost, weight and safety while accommodating uncertain parameters such as the loading experienced by the hull.

Severe seismic events in urban regions during the last two decades revealed that the structures constructed before the development of modern seismic codes are the most vulnerable to earthquakes. Sub-standard reinforced concrete buildings constitute an important part of this highly vulnerable urban building stock. There is urgent need for the development and improvement of methods for seismic performance assessment of existing reinforced concrete structures. As an alternative to current conventional force-based assessment methods, a performance evaluation procedure for structural members, mainly reinforced concrete columns is proposed in this study, by using an energy-based approach combined with the low cycle fatigue concept. An energy-based hysteresis model is further introduced for representing the inelastic response of column members under severe seismic excitations. The shape of the hysteresis loops are controlled by the dissipated cumulative energy whereas the ultimate strength is governed by the low cycle fatigue behavior. These two basic characteristics are obtained experimentally from full scale specimens tested under constant and variable amplitude displacement cycles. The first phase of the experimental program presented in the study constitutes of testing sub-standard non-conforming column specimens. The second phase of testing was conducted on standard, code compliant reinforced concrete columns. A total number of 13 specimens were tested. The behavior of these specimens was observed individually and comparatively according to the performance based objectives. The results obtained from the experiments were employed for developing relations between the energy dissipation capacity of specimens, the specimen properties as well as the imposed displacement history. Moreover, the measured rotation capacities at the plastic regions are evaluated comparatively with the limits proposed by modern displacement-based seismic design and assessment provisions.