Behavior of three-span braced columns with equal and unequal spans
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Columns with three spans separated by elastic braces are analyzed. The influences of translational and rotational resistance at the braces, various end conditions, and the bracing locations for the perfect columns with equal and unequal spans subjected to uniform and nonuniform axial load are investigated. For imperfect columns with equal or unequal spans subjected to uniform compression, the effects of various end conditions at the top and various initial deflections are studied. "Ideal stiffness" and "full bracing" only exist for these columns with equal spans and translational restraints with identical spring stiffness at the braces. Rotational restraints at the braces or at the ends improve the load-carrying capacity of the column. For the imperfect column, three types of initial deflection on the columns with equal and unequal spans and various end conditions at the top are analyzed. Based on a linear elastic analysis, the bracing force induced by a cubic initial deflection for a three-span column often exceeds 20/0 of the axial load. The violation of this rule of thumb in practice is readily revealed. Design curves for the effect of the bracing stiffness on the deflection ratio and bracing force percentage are created for the determination of bracing requirements.
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