Eatherton, Ma, Krawinkler, Deierlein, Hajjar, 2014
A controlled rocking braced steel frame is designed to concentrate inelastic deformation post seismic events to a replaceable fuse structural component such to drastically reduce the residual drift. The use of rocking braced frames with steel fuses and self centering vertical post-tensioning strands is experimentally tested using cyclic loads in order to determine the behavior of the system under seismic loads. The study concludes that the system is effective in reducing residual deformations after seismic loading and concentrating the plastic deformation into a replaceable component.
System Concept
The system consists of either a singular or a dual rigid steel rocking frame that is designed to remain elastic throughout the application of lateral forces. The singular steel frame includes an inelastic steel fuse at the bottom and post tensioning vertically along the interior of the frame. The dual frame has steel fuses along the vertical space between the two frames and vertical post tensioning along the center of both frames. Uplifting column bases on the bottom exterior corners of the frames allow them to rock under the application of seismic forces through the use of a 127 mm radius curvature on the gusset plate welded to a base plate that is connected to a strong floor through threaded connections.
The fuses on the system are made of steel plates with elongated diamond shaped cutouts vertically along the center. The cutouts create butterfly shaped links that deform when seismic force is applied. Due to this inelastic deformation, seismic energy is concentrated into the fuse and the fuse is permanently damaged, after which it can easily be replaced by loosening the bolted connections and inserting a new fuse. The fuse is designed to limit structural damage to the elastic frame by absorbing the seismic energy into permanent deformation.
Vertical post tensioning applies a self centering force to the system under lateral forces in order to prevent large lateral drifts. The post tensioning consists of several wires to create four post tensioning strands located at the center of each frame with some initial stress. The strands are anchored at the foundation and to the top of the frame.
Experimental Study, Results and Discussion
The study consists of nine half scale specimens that were loaded with cyclic lateral forces except for two specimens that were loaded with a hybrid simulation. The nine specimens varied in number of frames and each frame had four post tensioning strands, meaning that the dual frames had eight post tensioning strands. The specimens also had different numbers of fuses and initial post tensioning stresses. The goal of the experiment was to test different configurations in order to determine the properties that will exhibit least structural damage in the event of seismic loading.
The loading beam was connected through load cell pins at the upper boundary of the frames, and instrumentation consisted of a load cell at the top of the specimen, two strain gauges at each strut, and string potentiometers on each floor and at the base to measure uplift. The cyclic forces applied to the structures consisted of 6 cycles each at the lower fuse shear strain values and decreasing cycles at the upper fuse shear strain values. Typically the specimens were limited to a 3.1% maximum roof drift to prevent residual deformations so that the frame could be used for other specimens, though specimens A4, A7, B1, and B2 exceeded this drift to a maximum of 4.2% roof drift. Specimens A5 and A6 were loaded with hybrid simulation, and computational analyses were performed for all specimens.
The study found that the dual frame system and the single frame system both exhibited flag shaped hysteretic response curves, and reverted to zero drift after the cyclic loading was concluded, though single frame systems required a larger drift in order to exhibit inelastic deformations at the fuses. Thinner fuses exhibited greater buckling, which in turn increased the self centering forces but reduced the energy dissipation of the system. Individual post tensioning wires experienced yielding and fracture, but overall the post tensioning strands were able to provide self centering forces until a 2.5% drift. The study allows for the definition of limit states that offer a design process for stable seismic design that limits structural damage to a replaceable component.
Reference
Eatherton, M.R., Ma, X., Krawinkler, H., Deierlein, G.G., and Hajjar, J.F. (2014). “Quasi-Static Cyclic Behavior of Controlled Rocking Steel Frames” Journal of Structural Engineering, 140, 11. pp. 5.