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Sponsor: NCHRP

PI: Todd Helwig
Co-PI: Michael Engelhardt

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 a number of 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 fundamental goals of this research 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, including the effect of support skew;
  • calculation of minimum cross-frame strength and stiffness requirements for stability bracing of I-girders during construction and in-service, including cases with bottom flanges in compression, bracing behavior of non-prismatic girders, and approaches for combining force effects from stability, wind loads, skew effects, construction loads, and in-service loads;
  • 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 C4.6.3.3.4 of the AASHTO LRFD Bridge Design Specifications.

One of the products of this research will be developing draft specification and commentary language for proposed changes to some sections of the AASHTO LRFD Bridge Design Specifications (henceforth referred to as AASHTO). The results of this research will lead to improved methods for cross-frame analysis and design for steel I-girder bridges. These improved methods will lead to improved reliability and economy of cross-frames.