COLUMN-TO-BEAM STRENGTH RATIO REQUIRED FOR ENSURING BEAM-COLLAPSE MECHANISMS IN EARTHQUAKE RESPONSES OF STEEL MOMENT FRAMES

摘要

This paper examines: (1) the magnitude of column overstrength (COF) required for ensuring column-elastic behavior in steel moment frames; (2) the uniformity of story drift and beam rotations along the height that can be achieved by such frames; and (3) the change in response for frames having smaller COFs. The COFs required for ensuring column-elastic responses increase steadily with the increase of the ground motion amplitude, reaching about 1.5 for the ground motion amplitude of 0.5 m/s in maximum ground velocity. The steady increase is attributed primarily to higher (second) mode responses that tend to increase linearly despite that many plastic-hinges are formed in the frame. For frames in which column-elastic behavior is ensured, the maximum story drift angle is 1.5 to 2.5 times as large as the maximum overall drift angle. Change in response is gradual with the decrease of the COF, and the maximum overall drift angle and story drift angle remain relatively unchanged for COFs not smaller 1.0. The maximum overall drift angle and maximum story drift angle of a multi story frame are 0.7 to 1.0 and 1.5 to 2.0 times the maximum drift angle of the equivalent SDOF system, respectively.