Title:
On Effective Length Factor of Framed Columns in the ACI Building Code
Author(s):
Yu-Xia Hu, Ren-GenN Zhou, Won-Sun King, Lian DuanN, and WaiI-Fah Chen
Publication:
Structural Journal
Volume:
90
Issue:
2
Appears on pages(s):
135-143
Keywords:
beams (supports); columns (supports); frames; girders; stability; reinforced concrete; slenderness ratio; stiffness; Design
DOI:
10.14359/4203
Date:
3/1/1993
Abstract:
Simplified equations and alignment charts are used in the present ACI Building Code (ACI 318-89) for computing the effective length factor of framed columns. The first part of this paper evaluates critically the general validity of the simplified equations by comparisons with computed exact alignment chart solutions. The ACI simplified K-factor equations for unbraced columns are found adequate only for the special case 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, the larger the difference between these two approaches. The ACI simplified equations give a very conservative result when the difference between {PSI} A and {PSI} B of an unbraced column becomes larger, and when end restraints of a braced column are large. To this end, we propose that some comments be added to the present ACI Building Code Commentary concerning the simplified K-factor equations. A modified design method, based on current alignment charts put forward by the ACI Building Code, is then presented for those framed columns restrained by tapered rectangular girders with different far end conditions than those assumed in the present charts. Finally, the modified design procedure is illustrated by two numerical examples.