Effects of Input Direction of Ground Motion and Column Overdesign Factor on Seismic Response of 3D Steel Moment Frames

Abstract eng:
Steel moment frames should be designed to ensure sufficient energy absorption capacity by achieving an entire beam-hinging collapse mechanism under severe earthquakes. Therefore, the column overdesign factor is stipulated in seismic design codes of several countries. Since both orthogonal plane frames are designed as moment resisting frames in Japan, square tube columns are often used for steel building structures. Therefore bi-axial bending moment acts on the columns under bi-direction ground motion. On that occasion, the apparent full plastic moment of the square tube column, which is projected on the direction parallel to the frame plane, under bi-axial bending moment is smaller than the full plastic moment under a uni-axial bending moment. Considering the effect of bi-direction ground motions on the steel moment frames, the specified column overdesign factor is 1.5 or more in Japanese seismic design code. A lot of numerical studies on the required column overdesign factor have been conducted in order to ensure beamhinging responses under uni-direction ground motion. On the other hand, there are few researches about the required column overdesign factor of steel moment resisting frames under bi-direction ground motions. 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 of beam-column connections, number of stories, the planar shape of frames, input direction of earthquake. 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. As a result of earthquake response analysis, cumulative plastic deformation of columns decreases and cumulative plastic deformation of beams increases, when apparent column overdesign factor becomes larger. The profile of maximum value of cumulative plastic deformation of columns to apparent column overdesign factor are nearly identical regardless of number of stories, floor plan, and input direction of ground motion. As an index to evaluate the damage of columns, the plastic deformation capacity of square tube columns depending on local buckling is adopted for different rank of width-thickness ratio, and the damage level of columns is defined as the value obtained by dividing cumulative plastic deformation by plastic deformation capacity. From the relationship between apparent column overdesign factor and the damage level of columns, the required column overdesign factor to keep the damage level below the specified value is proposed. For example, the required apparent column overdesign factor for frames with Ds value of 1/3, under level 2 ground motion, to keep the cumulative plastic deformation below the performance limit is obtained as 1.40 for square tube columns of FA rank width-thickness ratio, and 1.60 for FB rank, 1.83 for FC rank, respectively.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket