Midorikawa et al. 2009
The authors examined steel rocking frames with uplift yielding base plates, which allow rocking and uplift under earthquake motions. The four-winged base plates were installed at the bottom of each column at the first story of the rocking frames. In this study, the seismic response of 1/3 scale 3-story steel frames with columns allowed to uplift were evaluated and compared with that of fixed-base frames by three-dimensional shake table testing. Three different input motion conditions from the 1995 JMA Kobe record were used.
System Concept
The structures in this study were composed of yielding base plates, columns, girders, and bracing members. Base plates act as fuses by yielding due to tension in the columns, and are located at the base of the each column. These plates are composed of 4 wings, and when the base plates yield as a result of column tension during a strong earthquake motion, the columns uplift and allow the building structure to rock.
Experimental Study, Results, and Discussion
1/3 scale, 3-story, 2x1-bay braced test frames were tested on a shake-table for two different base conditions: BPY model and FIX model. In the longitudinal direction, the structure contained two moment-resisting frames with spans of 2 m each. In the transverse direction, the structure contained three braced frames with spans of 2 m.
Horizontal accelerations on the shake table, horizontal accelerations and relative horizontal displacements at each floor level, axial strains of the first story columns and bracing members, and uplift displacements of the first story column bases were measured during the tests. According to the results, when the maximum roof displacements in the BPY model were larger than those in the FIX model in the transverse direction, longitudinal roof displacements were roughly equal or smaller than those in the FIX model. Thus, maximum base shear of the rocking frame was effectively reduced. The axial forces on the columns in the rocking frame were equal or less than those for the fixed base frames, and the response deformation of the rocking frame, excluding the rocking component, was nearly equal to or smaller than the elastic response values of fixed end frames.
Analytical Study
A modified version of the two-dimensional nonlinear dynamic analysis program DRAIN-2DX was used in the analysis of a 6-story frame with the FIX and BYP models. The details of the modeling and the main assumptions were explained for FIX and BYP models separately. Then, three sets of ground accelerations selected for response analyses were explained.
Reference
Midorikawa, M., Ishihara, T., Azuhata T., Hori H., Kusakari T., Toyomaki S., and Asari, T. (2009). “Three-Dimensional Shaking Table Tests On Seismic Response Of Reduced-Scale Steel Rocking Frames,” Journal of Structural and Construction Engineering, 75.647, pp. 213-221.