Strong-Axis Stability of Wide Flange Steel Columns in the Presence of Weak-Axis Flexure

Abstract

Column stability during seismic events is a critical consideration for all steel-framed structures, but is of particular importance for a unique class of steel frames called multitiered braced frames (MT-BFs). A MT-BF is created by subdividing a tall single-story braced frame into two or more braced tiers with intermediate horizontal struts in the plane of the frame. Since out-of-plane bracing is not provided at the tier levels, under pure axial compression, a MT-BF column may buckle in plane over one tier or out of plane over the full story height. Typically, wide flange shapes with weak axis oriented in the plane of the frame are used for MT-BF columns. During a seismic event, brace inelasticity may result in differential tier drifts that induce weak-axis flexure in the wide flange columns. However, this flexural demand is not considered in current braced frame seismic design procedures, which only require column design for axial force. To facilitate development of seismic design guidelines for MT-BF columns, their inelastic stability behavior was explored using three-dimensional finite-element analysis. This paper describes studies of isolated columns subjected to compressive loads in combination with varying magnitudes of imposed weak-axis rotation. The location of the imposed rotation and the axial force distribution were also varied. In addition, the influence of torsional bracing at the tier levels on buckling strength and buckling limit states was examined. The results from the computational simulations indicate that strong-axis buckling strength can decrease significantly due to the presence of weak-axis flexure, if the imposed rotation is large and torsional restraint is not provided at the tier levels. Furthermore, the performance of many columns was enhanced by providing torsional bracing at the tier levels. Finally, weak-axis flexure at column midheight was found to be more detrimental to strong-axis buckling strength than weak axis flexure closer to the column end. The results from this study provide guidance for design provisions and will be used to inform the development of three-dimensional finite-element models of complete MT-BFs.