EFFECTS OF INPUT DIRECTION ON COLLAPSE MECHANISM AND STORY DRIFT FOR LOW AND MIDDLE-RISE STEEL MOMENT FRAMES

Abstract

 Many Low and Middle-rise steel moment frames are verified for seismic safety by horizontal load capacity calculation with push-over analysis assuming one-directional force. In this case, it is a general rule to design the column's strength to be sufficiently larger than that of the beam or panel so that an overall sway mechanism in which the beams or panels yield prior to the columns is formed. However, under bi-directional input, the column strength decreases compared to the one-directional input because of bi-axial bending moment and additional axial force by over turning moment, and it becomes easy to form the weak column mechanism. When the weak column mechanism is formed, there is concern about the strength deterioration of column due to local buckling and the accompanying excessive story drift.

 When evaluating the effects of horizontal bi-directional input, it is useful to conduct seismic response analysis using a three-dimensional frame model that considers the effects of biaxial bending and varying axial forces of columns. In this case, two-component simultaneous input with different phases or 45° uni-directional input is assumed as the input wave, but it is not sufficiently clear how different the responses are under each assumption.

 In this paper, in order to grasp the effects of input direction for Low and Middle-rise steel moment frames, seismic response analyses with three-dimensional frame model that can evaluate the effect of varying axial forces and the strength deterioration of column under uni-directional input on the structural plane, 45° uni-directional input and two-component simultaneous input are conducted. The difference of the collapse mechanism and the story drift depending on the input direction is studied under various conditions with the strength ratio of columns to beams and panels and width-thickness ratio of columns as parameters.

 From the analytical results, the following knowledge was obtained.

 1) The column damage and the maximum story drift angle are likely to be the largest in the case of 45° uni-directional input as compared with the uni-directional input on the structural plane and two-component simultaneous input. This tendency is particularly remarkable under conditions where the plastic deformation of the column is likely to proceed, such as when the external force level is large, the column strength ratio γ is small, or the column width-thickness ratio is large. In addition, at a response level where the plastic deformation is so small that the column does not deteriorate, there is no significant difference in column damage and story drift between 45° uni-direction input and two-component simultaneous input.

 2) For the frame targeted in this paper, by setting the column strength ratio γ to 1.5 or more, or γ to 1.25 and the column width-thickness ratio to 22 or less, week column mechanism and deterioration of column can be avoided under Lv. 2 earthquake motions (the maximum earthquake motions to be considered by Building Standards Law of Japan). On the other hand, in a frame with a column strength ratio γ = 1.0 and a column width-thickness ratio of 29 or more, there is a high possibility that the columns deteriorate at Lv. 2 input and a story drift angle of 1/20 rad or more occurs under Lv. 3 earthquake motions (1.5 times Lv. 2).