COLLAPSE MECHANISM AND DISPLACEMENT OF LOW AND MIDDLE-RISE STEEL MOMENT FRAMES UNDER HORIZONTAL BI-DIRECTIONAL GROUND MOTIONS

Low and middle-rise steel moment frames are generally verified for seismic safety by horizontal load capacity calculation with push-over analysis assuming one-directional force. However, during an actual earthquake, the dynamic behavior and the horizontal bi-directional input may cause the building behavior different from the design assumption. Therefore, to evaluate the seismic performance of a steel moment frame, it is important to grasp the collapse mechanism and response displacement under bi-directional dynamic input. Under bi-directional input, in a moment frame, bi-axial bending moment and additional axial force by overturning moment act on the column, which lower the column strength and causes to form the weak column mechanism as compared with one-directional input. While overall sway mechanism in which the beams or panels yield prior leads to a stable plastic deformation behavior where member plasticization is dispersed throughout the building, the week column mechanism leads to damage concentration of the specific story and brittle collapse. Previous studies have shown that not only bi-directional input, but also the deterioration of column's restoring force due to local buckling and composite effect of beams by concrete slab are factors in the weak column mechanism. In this paper, as a starting point of research to grasp response behavior of low and middle-rise steel moment frame under horizontal bi-directional input, seismic response analyzed under various conditions for 4-story 2 x 1 span moment frame. For the analysis, to evaluate the collapse mechanism and the maximum story deformation precisely, a three-dimensional frame model was used that can evaluate composite effect of beams and panels by concrete slab and the deterioration of column's restoring force due to local buckling. From the analytical results, the following knowledge was obtained. 1) The response story drift angle of 1st story is in negative proportion to the column strength ratio γ, and the story in which γ is 1.0 can be deformed by more than 1/40rad under horizontal bi-directional input. The smaller γ of the story, the more bi-directional input affects, and the story in which γ is 1.5 or more is hardly affected because plasticization of the beam and panel precedes. 2) Even in a frame designed to form overall sway mechanism, there is a high possibility that the story collapse will proceed beyond the design assumption due to horizontal bi-directional input and composite effect