EFFECTIVE LENGTH FACTOR OF COLUMNS IN SEMIRIGID AND UNBRACED FRAMES

In current engineering practice, to evaluate the stability of columns in frames with rigid and semirigid connections, one must estimate the effective length factor (K-factor) of columns considering the effects of the nonlinear moment-rotation characteristics of beam-to-column con nections. Herein, using the alignment chart approach and based on a re cent study on the stability of semirigid frames and the concept of Sha nley's inelastic column buckling theory, the governing equations on th e K-factor of columns for various boundary conditions with rigid and s emirigid beam-to-column connections are derived for unbraced frames. W ith a proper evaluation of the tangent connection stiffness for semiri gid beam-to-column connections at buckling state and with the introduc tion of the modified relative stiffness factors, the alignment chart i n the present American Institute of Steel Construction-load and resist ance factor design specification can be applied to find the correspond ing column K-factor in semirigid frames. This is described in the pres ent paper. Furthermore, using a semirigid and unbraced portal frame ex ample, the effects of the nonlinear moment-rotation characteristics of connections on the K-factor are numerically studied and the K-factors obtained by two simplified methods are also compared with the present method.