Required Column Overdesign Factor of 3D Steel Moment Frames with Square Tube Columns

Steel moment frames are designed to ensure sufficient energy absorption capacity by achieving an entire beam-hinging collapse mechanism against severe earthquakes. Therefore, the column overdesign factor is stipulated in seismic design codes in some countries. For example in Japanese seismic design code, the specified column overdesign factor is 1.5 or more for steel moment frames with square tube columns. And this paper describes seismic response by 3D analysis of steel moment frames, and presents seismic demand for the column overdesign factor to keep the damage of square tube columns below the specified limit of plastic deformation. The major parameters are column overdesign factor, horizontal load bearing capacity, shape of frames and input direction of ground motion. In order to investigate 3D behavior of frames and correlation between plastic deformation of columns and column over design factor, apparent column overdesign factor, which is defined as the ratio of full plastic moment of the column (s) to the full plastic moment of the beam (s) projected in the input direction of the ground motion, is introduced. From the earthquake response analysis, it is clarified that the profile of maximum value of cumulative plastic deformation of columns to apparent column overdesign factor, with the similar horizontal load bearing capacity, are nearly identical regardless of number of stories, floor plan, and input direction of ground motion. As a result, the required column overdesign factor to keep the damage of columns below the limit of plastic deformation is proposed under the reliability index of 2.