Stability of Columns in Unbraced Frames

Abstract

A new design model is presented to derive expressions for the elastic effective length factors for columns in unbraced multistory frames. This model takes into account the effects of boundary conditions at the far ends of the columns above and below the column in question. According to the relative bending stiffness of the girders and columns framing at the far ends, the far end conditions can be modeled as rigid, hinged, or fixed. Because the alignment chart model is based on rigid-far-end conditions, it is not applicable in designing columns with either hinged or fixed-far-end conditions. This refined model rectifies inherent problems associated with currently available solutions caused by both the invalid assumption of constant story drift angles and the errors in mathematical formulation. Using the background of extensive numerical studies, a modified design procedure based on the alignment chart is suggested. The proposed design procedure is characterized by both simplicity and accuracy.