UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

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    THE EFFECTS OF UNMITIGATED IDLE TIME ON THE PERFORMANCE OF MAGNETORHEOLOGICAL DAMPERS AS A STRUCTURAL PROTECTIVE SYSTEM.
    (2018) Khan, Sami Ur-Rahman; Phillips, Brian; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis discusses the long-term performance degradation of seismic protective systems due to age and inactivity (termed “idle time effects”). Over the lifetime of a structures there is the potential for a significant reduction in ability for the structural control systems to mitigate earthquakes. This can affect the resilience of the structure and lead to uncertainty in engineering judgement when designing seismic protective systems. Further research into these idle time effects could help to create solutions to mitigate age-dependent performance loss. This paper will use magneto-rheological (MR) dampers, which serve as a good analog for other semi-active control devices, to study idle time effects on seismic protection. MR dampers provide controllable damping through the magnetization of small MR particles in a carrier fluid. These particles can settle over time, influencing their performance. Using a model MR fluid, accelerated testing was performed to analyze the consequences of idle time.
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    Development of Approaches to Common Cause Dependencies with Applications to Multi-Unit Nuclear Power Plant
    (2018) Zhou, Taotao; Modarres, Mohammad; Droguett, Enrique López; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The term “common cause dependencies” encompasses the possible mechanisms that directly compromise components performances and ultimately cause degradation or failure of multiple components, referred to as common cause failure (CCF) events. The CCF events have been a major contributor to the risk posed by the nuclear power plants and considerable research efforts have been devoted to model the impacts of CCF based on historical observations and engineering judgment, referred to as CCF models. However, most current probabilistic risk assessment (PRA) studies are restricted to single reactor units and could not appropriately consider the common cause dependencies across reactor units. Recently, the common cause dependencies across reactor units have attracted a lot of attention, especially following the 2011 Fukushima accident in Japan that involved multiple reactor unit damages and radioactive source term releases. To gain an accurate view of a site's risk profile, a site-based risk metric representing the entire site rather than single reactor unit should be considered and evaluated through a multi-unit PRA (MUPRA). However, the multi-unit risk is neither formally nor adequately addressed in either the regulatory or the commercial nuclear environments and there are still gaps in the PRA methods to model such multi-unit events. In particular, external events, especially seismic events, are expected to be very important in the assessment of risks related to multi-unit nuclear plant sites. The objective of this dissertation is to develop three inter-related approaches to address important issues in both external events and internal events in the MUPRA. 1) Develop a general MUPRA framework to identify and characterize the multi-unit events, and ultimately to assess the risk profile of multi-unit sites. 2) Develop an improved approach to seismic MUPRA through identifying and addressing the issues in the current methods for seismic dependency modeling. The proposed approach can also be extended to address other external events involved in the MUPRA. 3) Develop a novel CCF model for components undergoing age-related degradation by superimposing the maintenance impacts on the component degradation evolutions inferred from condition monitoring data. This approach advances the state-of-the-art CCF analysis in general and assists in the studies of internal events of the MUPRA.
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    PERFORMANCE EVALUATION OF TWO NEW SEISMIC RESISTANT DIAGRID FRAMING SYSTEMS
    (2011) Moghaddasi Bonab, Nasim Sadat; Zhang, Yunfeng; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The diagrid system offers a visually appealing and structurally efficient structural system for gravity load bearing. The architectural elegancy and high structural redundancy of the diagrid structure makes it a desirable choice for tall building design. However diagrid structure is prone to high inelastic deformation demand during strong earthquakes. To address this issue of limited ductility and energy dissipation capacity in conventional diagrid framing, two new types of seismic resistant diagrid structural systems termed highly energy-dissipative ductile (HED) diagrid and hybrid diagrid framing systems are proposed in this research and their seismic performance is assessed. The proposed HED diagrid framing system provides a competitive design option in high seismic regions with its high ductility and improved energy dissipation capacity enabled by incorporating replaceable shear links interconnecting the diagonal members at their nodes. A parametric study has been conducted to investigate the effect of different design parameters on the seismic performance of this system. A new type of composite brace comprised of glass fiber reinforced polymer (GFRP)-tube confined concrete, steel core and post-tensioned tendons, is developed for self-centering diagrid members. The hysteretic behavior of a self-centering chevron assembly comprised of two inclined composite braces is subsequently examined. Constitutive modeling of GFRP-tube confined concrete with high confinement volumetric ratio is conducted with experimental data calibration under monotonic and cyclic compression. The constitutive model is implemented into a finite element analysis platform OpenSees to enable nonlinear analysis of complex structures utilizing this type of confined concrete elements. The self-centering chevrons are implemented in the lower stories of the hybrid diagrid framing system to form base diagonals with large stiffness, enhanced ductility and energy dissipation capability and enable a rocking behavior for the diagrid system. The structural characteristics and seismic behavior of these two new seismic resistant systems are demonstrated with a prototype 21-story building subjected to nonlinear static and dynamic analysis. The findings from nonlinear time history analysis verify that satisfactory seismic performance can be achieved by these structural systems subjected to design basis earthquakes in California, specifically showing re-centering behavior while all main structural elements remain elastic in both systems.
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    Seismic assessment of curved bridges using modal pushover analysis
    (2010) Ahmed, Mohamed Salah; Fu, Chung C; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The assessment of existing bridge structures against earthquake threat has become a major issue lately, motivated by the maturity of seismic design of new structures, on one side, and by the recognition of the inadequate level of seismic protection, the aging and the constant need of maintenance of the existing ones, on the other. While nonlinear time history analysis (NL-THA) is the most rigorous procedure to compute seismic demands, many seismic-prone countries, such as United States, New Zealand, Japan and Italy, have recently released standards for the assessment of buildings, all of which include the use of the non-linear static analysis procedure (NSP), the so-called pushover. Recently Chopra and Goel (2002) proposed the modal pushover analysis (MPA) procedure that considers the effect of higher modes on the behavior of buildings. This research investigation is intended to evaluate the accuracy of the modal pushover analysis (MPA) procedure in estimating seismic demands for curved bridges after proposing some modifications that would render the MPA procedure applicable for bridges. For verification purpose, the nonlinear time history analysis (NL-THA) is also performed in order to quantify the accuracy of MPA. Three bridges were analyzed using both the MPA and NL-THA in addition to the standard pushover analysis (SPA). Maximum Demand displacements, total base shear and plastic rotations obtained from SPA and MPA are compared with the corresponding values resulting from the NL-THA. Comparison shows a good agreement between MPA and NL-THA results and MPA is deemed to be accurate enough for practical use. Furthermore, to evaluate the applicability of the MPA method for a wide range of bridges, a parametric study using both the MPA and NL-THA is performed. Results from the MPA for demand displacement and base shear are compared with results from the NL-THA. Also, the influence of different parameters on the behavior of curved bridges is studied. Parameters included the girder cross section (steel I vs. steel BOX), span length, number of spans, radius of curvature, and pier height. Pier height is found to have the most significant effect on bridge behavior as well as span length, while radius of curvature is found to have less influence on the behavior of curved bridges.
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    AN EVALUATION OF FLOOR RESPONSE SPECTRA FOR ACCELERATION-SENSITIVE NONSTRUCTURAL COMPONENTS SUPPORTED ON REGULAR FRAME STRUCTURES
    (2004-05-12) Kingston, Kevin M.; Medina, Ricardo A.; Civil Engineering
    This study evaluates the acceleration response of elastic nonstructural components (NSCs) subjected to earthquake-induced supporting structure motions. The objective is to provide insight into the development of the floor response spectrum (FRS) and its dependence on critical ground motion and structural system parameters such as the ground motion intensity, modal periods of the supporting structure, fundamental period of the NSC, strength of the structure, and location of the NSC with respect to the height of the supporting structure. The focus is on NSCs supported on regular moment-resisting frames. Results indicate that the FRS is highly dependent on the ratio of the period of the NSC to the modal periods of the supporting frame, the level of inelastic behavior of the frames and the location of the NSC. This study demonstrates that in several cases these effects are not adequately represented in floor design spectra recommended by current building codes.