The effect of an adjacent member on the buckling of columns in the frame is examined. Formulas for equivalent stiffness of the adjacent column affecting the focused column is presented. The effect of the adjacent column is classified in three groups, that is the focused member is perfectly braced, buckle simultaneously and the adjacent member is perfectly braced, according to the critical flexural stiffness ratio. Relations between the equivalent stiffness and above mentioned 3 groups are discussed, and the flexural stiffness ratio characterizing the groups are shown, taking the height ratio, axial load ratio as the parameter.
