Bracing Requirements to Improve System Buckling of Narrow Girder Systems

Recent research has demonstrated the propensity of I-girder systems with a relatively large length/width ratio (ie. narrow girder systems) to fail by a system mode of buckling that is not sensitive to the spacing between traditional torsional bracing systems. The mode is particularly common in bridge systems that make use of cross frames or plate diaphragms for bracing. Several field problems have occurred involving the system mode of buckling in recent years and design equations have been developed to determine system buckling resistance. However, accurate design methods to improve the buckling resistance has not been developed. This paper outlines a parametric FEA study focused on the design of top lateral truss to adequately raise the buckling strength of the girder system. The requirements include three criteria: bracing system should have enough stiffness (i) to reach the critical moment level necessary for the obtained design load level, (ii) to eliminate excessive out-of-plane deformation of the girders as a results from geometric imperfections, and (iii) the bracing system must possess adequate strength which are a function of the initial geometric imperfections. The current paper focuses on the check of all these three requirements. The computational studies were executed by the authors to investigate the stiffness and the strength requirements for narrow girder systems, which are the most sensitive against global lateral torsional buckling. The current paper presents the results of the executed numerical research program and introduces improved mechanical models and design equations for the determination of the required stiffness and strength.

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