Title: Stability Problems of Columns and Frames

Author(s): J. Dario Aristizabal-Ochoa

Publication: Structural Journal

Volume: 94

Issue: 4

Appears on pages(s): 389-398

Keywords: buildings; column design; computer applications; earthquake design; lateral loads; moment magnification; reinforced concrete; slenderness effects; structural analysis; stability;

DOI: 10.14359/490

Date: 7/1/1997

Abstract:
In spite of the alignment charts' limitations to be valid only to fully symmetrical rectangular frames with rigid/hinged connections (i.e., frames with identical columns under equal axial loads and boundary conditions), they have been indiscriminately used in all types of framed structures. Construction codes and the technical literature have promoted their use since their inception more than forty years ago, and only recently some codes have tried to address certain inadequacies. This indiscriminate use has created the following major misconceptions: 1.) columns and frames are classified into braced and not braced cases only; 2.) the critical load and effective length k-factor for each column in a frame is a function of its ends' flexural conditions only; 3.) the k-factor for columns with lateral sidesway shall be greater than one (k>1); 4.) pin-ended columns with sidesway have zero critical axial capacity; 5.) the total critical axial load of a frame with sidesway buckling is independent of the axial load distribution among the columns; and 6.) the stability behavior of framed structures can be analyzed by breaking the structure into plane frames and is independent of the column layout. Because of its extreme importance this last misconception is analyzed in a separate paper.1 The first five misconceptions are thoroughly discussed and their solutions presented in this paper, and a new stability classification for column systems is proposed. The proposed criteria and equations not only overcome the misconceptions and paradoxes of the classical alignment charts, but they are more general and simpler to apply than any other known available criteria. Examples are presented that demonstrate the effectiveness and accuracy of the proposed criteria.