Eatherton et al. 2010
The authors discuss a controlled frame rocking with replaceable structural fuses to provide safe and cost effective resistance to earthquakes. The behavior of the controlled rocking systems is explained through the results of quasi-static cyclic and hybrid simulation tests. A computational model is also presented.
System Concept
This system consists of steel braced frame, vertical PT strands and replaceable energy-dissipating elements. The steel braced frame is design to behave elastically, and is allowed to rock about the column bases. The column base is permitted to uplift but its horizontal motion is restrained by bumpers. Vertical PT strands provided active self-centering forces. The strands are stressed until the point at which additional elastic strain is permitted when the frame rocks. Replaceable structural fuses act as energy-dissipating elements and limit the forces imposed on the rest of the structure. Thin steel plates are used as fuses. Butterfly slits were cut into the steel plates to induce earlier failure of the fuses, and thus early energy dissipation.
The authors determined two important key variables in the design of this controlled rocking system: overturning ratio (OT) and self-centering ratio (SC). OT is intricately associated with the ability of the system to resist lateral loads. SC is the ability of the system to eliminate drift when applied forces are removed. The PT strands and steel shear fuses serve several purposes to address these design variables: they provide overturning resistance, energy dissipation and self-centering capability.
Two configurations of the rocking frame were developed: single and dual frame configurations. In the single frame configuration, the shear fuses are placed at the ground floor connected to a 1-story column at the center of an eccentrically braced frame. When the frame rocks, the center spine of fuses deforms vertically.
In the dual frame configurations, the shear fuses are placed between two rocking frames. When the frames rock, the two inner columns are displaced relative to each other, imposing shear deformation in the fuses. These shear deformations cause energy dissipation, and the vertical PT in the middle of each frame re-centers the dual frames.
The authors postulated a number of fuse and PT configurations. The fuse can be located at the center of the first story bay, with PT at the columns. The locations of the fuse and PT can also be reversed for a second option, with the fuses connected to the columns and the PT in the center of the bay. Finally, both the fuse and PT can be located at the center of the bay.
Experimental Study, Results and Discussion
Quasi-static cyclic tests were conducted on a 3-story prototype building. Steel plate fuse elements were connected to both frames. A milled base plate with rounded bull nose edges was attached to the base plate of the frame, allowing interior column uplift. A Load and Boundary Condition Box was connected to the top of the specimen through a loading beam and was used to apply the load. Seven dual-frame configuration specimens and two single fuse-configurations were tested.
The controlled rocking system for steel-framed buildings was analyzed using experimental testing and analytical modeling. It was found that damage can be concentrated in the replaceable fuse elements while the frame remains relatively elastic. Also residual drifts were minimized after seismic activity.
Reference
Eatherton, M., Hajjar, J., Ma, X., Krawinkler, H., and Deierlein, G. (2010). “Seismic Design and Behavior of Steel Frames with Controlled Rocking – Part I: Concepts and Quasi-Static Subassembly Testing,” Proceedings of the Structures Congress 2010, Orlando, FL, May 12-15.