Effective torsional bracing for stability must satisfy both stiffness and strength requirements. The design philosophy for most stability bracing is to provide twice the "ideal" stiffness that often limits the member deformation to a value equal to the initial imperfection. For beam torsional bracing, computational studies have shown that the current AISC Appendix 6 provisions for stiffness do not meet this requirement. This paper outlines the results of a detailed numerical parametric study on the stiffness requirements for stability bracing to control the corresponding strength requirements. Previous research work has outlined the necessary "ideal" stiffness; however, some of the work has focused on single I-girders and the applicability of the developed design criteria has not been validated for multi-girder systems. Therefore, a numerical research program was executed to investigate the stiffness requirement of multi-girder systems. The results of the investigation highlight that the calculation method of the ideal stiffness related to single I-girders and multi-girder bridges are different and the application of design equations developed for single I-girders can lead to significant overestimation of the required ideal stiffness for twin or multi-girder bridges. However, the increment on the ideal stiffness to control deformations is often larger than twice the ideal stiffness. The paper presents result from the parametric studies and introduces improved design equations for the determination of the stiffness requirements to control brace forces and out-of-plane deformations.
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