Abstract: This study analyzed the effective length factors of frame columns in a structure where columns and beams are connected back-to-back with bolted joints， and these connections were simplified as rotational springs between the beams and columns. Since both the columns and beams are continuous at the joint， each joint involves two unknown variables： the column rotation and the beam rotation， making such frames different from conventional frames. The results showed that the traditional effective length factor formula can be applied to calculate the effective length of columns in such frames， provided that a reduction factor is introduced to modify the combined linear stiffness of the left and right beams， and this reduced beam stiffness can be used to define the stiffness parameter.