Cross-frame forces in a straight bridge with normal supports

Access full-text files




Moore, Matthew Eric

Journal Title

Journal ISSN

Volume Title



Cross-frame and diaphragm systems are important structural elements in steel I-girder bridges. These members enhance the lateral-torsional buckling resistance in straight girder systems by reducing the unbraced length. The critical stage for the braces from a stability perspective often occurs during construction of the concrete bridge deck when the non-composite steel girders must resist the entire construction load. The braces not only enhance the lateral-torsional buckling resistance of the girders, but are also necessary to resist the torsion applied to the girders due to the deck overhang construction, and distribute lateral loads across the structure from sources such as wind. In horizontally curved bridges, the braces are primary structural elements in the superstructure and engage the girders across the width of the bridge to behave as a structural system to resist the torsion that develops as a result of the curved geometry. For straight girder systems, the specifications of the American Association of State Highway and Transportation Officials (AASHTO) have generally provided little guidance in the sizing of the braces other than recommended connection plate (web stiffener) details or slenderness limits. While there have been many advances in recent years towards improving the understanding of the behavior of cross-frame systems, there has not been sufficient research carried out on the proper loading conditions for assessing the fatigue performance of cross-frames. The work outlined in this thesis is part of a larger study on the behavior of cross-frames in steel bridge system. The study is funded by the National Cooperative Highway Research Program (NCHRP 12-113). The fundamental goals of the research investigation are to produce methodologies and design guidelines for the following: evaluation of fatigue design stresses in cross-frames in straight and horizontally curved steel I-girder bridges; calculation of minimum cross-frame strength and stiffness requirements for stability bracing of I-girders during construction and in-service; development of improved methods to account for the influence of end connection details on cross-frame stiffness that extend beyond and improve upon the suggested guidance currently provided in Article C4. of the AASHTO LRFD Bridge Design Specifications. This work includes field monitoring and parametric FEA studies. The field studies are focused on three bridges: 1) a straight bridge with normal supports, 2) a straight bridge with skewed supports, and 3) a horizontally curved bridge. The field studies include rainflow monitoring of fatigue induced stresses in select cross-frames and the girders for a period of approximately 1 month as well as live load tests using trucks of known weights. This thesis focuses on some of the background studies, a survey of bridge owners around the United States, as well as the instrumentation and live load tests on the straight bridge with normal supports


LCSH Subject Headings