Refined Design Expression for the In-Plane Girder Stiffness of Torsional Beam Bracing Systems

Local and global stability are a major concern during erection and construction of steel bridge systems. Global stability is usually controlled by conventional lateral-torsional buckling (LTB), which is enhanced by reducing the unbraced length of the girders by cross-frames spaced along the length of bridge. Cross-frames fit into the category of torsional bracing since the braces restrain twist of the girder cross-sections. Effective bracing must satisfy both stiffness and strength requirements. The stiffness of cross-frames is a function of several components: including the stiffness of the brace, cross-sectional distortion, and the in-plane stiffness of the girders. While the current bracing provisions in the AISC Specification do not include the in-plane girder stiffness, this stiffness component can dominate the behavior of narrow girder units and may lead to inadequate bracing if not considered. Recently approved provisions in the AASHTO Specification make use of a relatively simple expression developed in the 1990's for evaluating the girder inplane stiffness, however, the accuracy of the expression for many bridge systems is not clear. This paper summarizes an on-going study focused on improved bracing design guidance considering the in-plane stiffness of the girders. The work focuses on the warping rigidity of multi-girder systems and extends findings from previous investigations that have targeted the system-buckling mode of narrow girder systems. The paper focuses on eigenvalue buckling solutions to demonstrate the stiffness behavior of torsional bracing.

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