Seismic Collapse Response of Steel Moment Frames with Deep Columns

Abstract

The seismic collapse behavior of 4-story and 8-story steel special moment frames (SMFs) with deep columns is investigated using computational simulation. The models used are capable of modeling local and global instabilities and explicitly representing both sidesway and vertical collapse behaviors. Three key factors that affect the collapse potential of the frames are studied: (1) column lateral bracing; (2) level of column gravity load; and (3) column section properties. It is shown that, even when they satisfy current seismic provisions, deep columns can suffer early global instability, leading to vertical system collapse at relatively low drift levels. The findings indicate that the performance of moment frames can be improved by limiting the axial load levels on exterior columns, carefully selecting member sizes to limit depth-thickness and overall slenderness of the columns, and providing adequate lateral bracing. It is suggested that column shortening, in itself, is a benign effect that does not compromise serviceability or contribute to the collapse of a well-designed frame.