EFFECT OF MULTI-LAYER LEANING COLUMN ON DAMAGE CONCENTRATION MITIGATION OF MULTI-STORY STEEL MOMENT RESISTING FRAMES WITH MID-STORY PIN COLUMN BASE SYSTEM

Abstract

 According to current design practices of steel moment-resisting frames, formation of plastic hinges is unavoidable when the frames are supported by conventional column base systems, i.e., exposed-type and embedded-type column base systems, due to unequalized flexural moment demand on the top and base of the first story columns [1]. Kimura et.al [3] has proposed mid-story pin column base system to avoid columns from yielding and to realize formation of beam-yielding mechanism by applying pin connection between the upper steel column and the bottom RC column at the midpoints of the first story. Previous analytical study [3~7] indicated that the frames with the proposed column base system exhibits superior seismic behavior than those with conventional base column systems, mitigating probability of column yielding. However, it is also revealed the difficulty to realize beam yielding mechanism for frames with commonly adopted column-beam strength ratio of 1.4-1.8. Dual structural system is the system that the main structural frame resisting lateral force is backed up by secondary structural system. Previous studies [8~15] shows secondary structural systems, such as a force-demand spreading column [12] and rocking wall [14], sufficiently contribute to redistribution of the inelastic demand over the height of the frames.

 In this paper, mechanical characteristics is examined about a steel moment-resisting frame with mid-story pin column base system upgraded by implementation of continuous column over the height of the frame. The continuous column is named as multi-layered leaning column (M. L. column), hereinafter. A theoretical formulation based on equilibrium equations between dual structural system (the main frame and M. L. column) is presented, in which those of the main frame is formulated by the previously presented modified D-value method [16]. Nonlinear static and time-history analysis is implemented to validate the theoretical observations on the effect of M. L. column on the mechanical characteristics of the main frame (story drift concentration and flexural demand on structural components) and quantify the required flexural stiffness and strength of the M. L. column. Additional theoretical method is also presented to characterize the significant effect of high-order mode responses on flexural demand on the M.L. column based on modal decomposition method.

 Static analysis of three-, six-, nine-story frame validates the theoretical formulation to sufficiently predict mechanical behavior of the frame with M. L. column. The flexural stiffness ratio of M. L. column η is presented, where it is indicated that almost no advantage can be obtained with the M. L. column of η < 0.03, while little further improvement can be expected with the M. L. column of η > 3.3. With the M.L. column of η = 0.33, about the half of the mitigation effect provided by a fully rigid M.L. column (η = 3.3) can be expected. Seismic time-history analysis shows that the maximum story drift concentration rate and flexural demands on the column of the main frame during earthquake excitation are well-predicted by the theoretical formulation with an external force regarding to Ai distribution, while the maximum flexural moment of the M.L. column exceeds the predicted value by 2.0 times. Theoretical formulation with proposed external force considering secondary mode response enhanced by seismic excitation successfully predicts the maximum flexural demand of the columns simulated by dynamic analysis of three to nine-stories frame with the M. L. column of η = 0.03 to 3.3.