Browsing by Author "Hajjar, Jerome F."

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    Fatigue of Stud Shear Connectors in the Negative Moment Region of Steel Girder Bridges
    (Center for Transportation Studies, University of Minnesota, 2000-06-01) Carlsson, Magnus; Hajjar, Jerome F.
    In a simply supported composite bridge girder the concrete is in compression over the full length of the bridge, whereas in a continuous multispan bridge some regions of the bridge are subjected to negative bending moment, in which case the concrete goes into tension. With respect to fatigue behavior of stud shear connectors, one key difference between placing studs in the positive and negative moment regions is the stress state of the base metal of the beam flange. The fact that the beam flange is in tension at the base of the studs in the negative moment region raises questions about whether this affects their fatigue endurance. This report summarizes the research behind the fatigue design provisions for shear connectors in the negative moment region in the American Association of State and Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Specification.
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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.
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    Transverse Cracking in Concrete Bridge Decks
    (Minnesota Department of Transportation, 1999-01-01) French, Catherine E.; Eppers, Laurice J.; Le, Quoc T.; Hajjar, Jerome F.
    This study sought to determine the dominant parameters that lead to premature transverse cracking in bridge decks and to make recommendations that help reduce cracking tendency in bridge decks. The project includes two main parts: a field study and a parametric study. The field study identified 72 bridges in the Minneapolis/St. Paul area and explored the correlation between the observed cracking of those bridges and available design, material, and construction-related data. The parametric study investigated the relative influence of the factors that affect transverse deck cracking through a controlled nonlinear analysis study. Variables included: shrinkage, end restraint, girder stiffness, supplemental steel bar cutoff, cross frames, splices, deck concrete modulus of elasticity, and temperature history. In addition, four bridges from the companion field study were modeled to compare the analytical results with the actual crack patterns. Based on these results and correlation with other research, the study identified the following dominant factors affecting transfer cracking: shrinkage, longitudinal restraint, deck thickness, top transverse bar size, cement content, aggregate type and quantity, air content, and ambient air temperature at deck placement. Recommended practical improvements to bridge deck construction, in order of importance, include: using additives to reduce shrinkage of the deck concrete, using better curing practices, and minimizing continuity over interior spans.