A lateral bracing system is installed at the top flange level of a steel tub girder to form a quasi-closed box, thereby increasing torsional resistance and retaining the original box shape. A single diagonal (SD) type or crossed diagonal type is typically adopted along with intermediate internal bracing systems (also called internal cross frames) that are placed at either even or odd numbered spacing. A significant coupling action between SD bracing members and internal cross-frame members has been found to occur in the case of a SD lateral bracing system and internal cross frames placed at odd numbered panel spacing. The member forces developed in lateral bracing-are 25-30 percent higher than those in structures with internal cross frames installed at even numbered panel spacing. Matrix equations were formulated to compute the brace forces developed in both SD lateral bracing and K-shaped internal cross-frame members subjected to bending action. Member forces computed from the new equations were compared with those from three-dimensional finite-element analysis, and an excellent agreement between the two solutions was found.
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