Seismic Behavior of 3D Steel Moment Frame with Biaxial Columns

Abstract

Three-dimensional nonlinear dynamic time-history analyses of a three-story steel moment-resisting frame designed according to the Uniform Building Code for Los Angeles seismicity were carried out. Beams in both directions had moment connections to the hollow rectangular columns. Code drift limits, rather than code considerations for bidirectional horizontal shaking, governed the member sizes. It was found that design level shaking caused the structure to exceed story yield drifts in both directions simultaneously and significant column yielding occurred above the base. Shaking in the direction orthogonal to the main shaking direction increased drifts in the main shaking direction by 46 and 64% for the design level and near-fault records, respectively, indicating that 2D analyses would not estimate the 3D response well. Also, maximum horizontal seismic components of axial force in the corner columns and side columns were 4.87 and 5.30 times the code estimations, respectively. It was shown that by increasing the column strength above the present code levels, that drifts during near-fault shaking were significantly decreased. A methodology to encourage strong-column weak-beam behavior, and to more realistically estimate the column axial forces during design level shaking is described.