ON EFFECTIVE LENGTH FACTOR OF FRAMED COLUMNS IN THE ACI BUILDING CODE

Simplified equations and alignment charts are used in the present ACI Building Code (ACI 318-89) for computing the effective length factor o f framed columns. The first part of this paper evaluates critically th e general validity of the simplified equations by comparisons with com puted exact alignment chart solutions. The ACI simplified K- factor eq uations for unbraced columns are found adequate only for the special c ase of equal relative joint bending stiffness ratios PSI(A) = PSI(B). It is found that the larger the difference between PSI(A) and PSI(B) t he larger the difference between these two approaches. The ACI simplif ied equations give a very conservative result when the difference betw een PSI(A) and PSI(B) of an unbraced column becomes larger, and when e nd restraints of a braced column are large. To this end, we propose th at some comments be added to the present ACI Building Code Commentary concerning the simplified K-factor equations. A modified design method , based on the current alignment charts put forward by the ACI Buildin g Code, is then presented for those framed columns restrained by taper ed rectangular girders with different far end conditions than those as sumed in the present charts. Finally, the modified design procedure is illustrated by two numerical examples.