Browsing by Subject "Torsion"

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    Left ventricular mechanics during right ventricular pacing.
    (2010-07) Burns, Kevin Victor
    Despite the routine use of right ventricle (RV) pacemakers to treat sinus node dysfunction, atrioventricular (AV) block, and other electrical abnormalities in the heart, recent studies have demonstrated that chronically RV-paced patients have an increased the risk of hospitalization, heart failure (HF), and death. Dyssynchronous electrical and mechanical activation of the left ventricle (LV) has been demonstrated in animal models of acute RV pacing. In humans, acute and chronic RV pacing have been demonstrated to impair LV systolic function and induce longitudinal or radial mechanical dyssynchrony. However, the 3-dimensional LV mechanics that result from acute and chronic RV pacing have not been fully explained. Recent advances in echocardiographic image analysis, including tissue Doppler imaging (TDI) and speckle tracking echocardiography (STE), can quantify LV motion in multiple planes throughout the LV. This dissertation will examine the effects of RV pacing on LV mechanics and synchrony in longitudinal, radial, and rotational planes of motion. We hypothesize that acute RV pacing will result in reduced and dyssynchronous longitudinal, radial and rotational LV function. We further hypothesize that these alterations to normal LV function may lead to HF during chronic RV pacing, and that this type of HF is structurally and functionally different than HF due to other causes. These results may provide insight into the mechanisms responsible for pacing-induced LV dysfunction, and enable physicians to better track cardiac function in paced patients, and modify treatments or design new therapies for patients requiring ventricular pacing.
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    Load Rating of Composite Steel Curved I-Girder Bridges through Load Testing with Heavy Trucks
    (Minnesota Department of Transportation, 2006-10) Krzmarzick, Dan P.; Hajjar, Jerome F.
    Current techniques for rating of horizontally curved composite steel I-girder bridges often use approximate methods of analysis based on assessment of individual straight girders with altered properties to account for member curvature. This project investigates the behavior and rating of these bridges through load testing with heavy trucks. A five-span continuous two-girder horizontally curved steel I-girder bridge was load tested. Strain and displacement measurements were obtained for the main girders, diaphragms, lateral wind bracing, bearings, composite interaction, and areas of high strain concentrations near stiffener details. Forty-three static tests with different truck load patterns were conducted along with thirteen dynamic tests to assess the bridge response. A linear elastic grillage-based model of the bridge was used to simulate the load patterns. A sensitivity study was carried out based on the tested bridge along with simulations of two other bridges previously tested elsewhere so as to assess the robustness of grillage analysis for use in load rating of horizontally curved steel I-girder bridges. Recommendations are made outlining rating of horizontally curved composite steel I-girder bridges through the use of grillage-based analysis, with and without the use of load testing, and within the context of the AASHTO Load and Resistance Factor Rating (LRFR) and Load Factor Rating (LFR) procedures.
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    Stresses in Steel Curved Girder Bridges
    (Minnesota Department of Transportation, 1996-08) Galambos, Theodore V.; Hajjar, Jerome F.; Leon, Roberto T.; Huang, Wen-Hsen; Pulver, Brian E.; Rudie, Brian J.
    Steel curved I-girder bridge systems may be more susceptible to instability during construction than bridges constructed of straight I-girders. The primary goal of this project is to study the behavior of the steel superstructure of curved steel Igirder bridge systems during all phases of construction, and to ascertain whether the linear elastic analysis software used by Mn/DOT during the design process represents well the actual stresses in the bridge. Sixty vibrating wire strain gages were applied to a two-span, four-girder bridge, and the resulting stresses and deflections were compared to computational results for the full construction sequence of the bridge. The computational results from the Mn/DOT analysis software were first shown to compare well with results from a program developed specifically for this project (called the "UM program"), since the latter permits more detailed specification of actual loading conditions on the bridge during construction. The UM program, in turn, correlated well with the field measurements, especially for the primary flexural stresses. Warping stresses induced in the girders, and the stresses in the crossframes, were more erratic, but showed reasonable correlation. It is concluded that Mn/DOT's analysis software captures the behavior well for these types of curved girder bridge systems, and that the stresses in these bridges may be relatively low if their design is controlled largely by stiffness.