Hayashi, Skalomenos, Inamasu, Luo, 2018
The self-centering rocking composite frame proposed in the study uses double-skin concrete-filled steel tube (CFT) columns and energy-dissipating fuses. The system was subjected to cyclic loading history with the aim of determining its seismic performance such as post-yielding stiffness, flexural strength, energy dissipation, and deformations of the system. A shear-transfer mechanism is proposed in this system to prevent column-based slippage and reduce any permanent deformations. The CFT columns ensure sufficient self-recentering forces and the replaceable steel fuses attached at the floor levels facilitate energy dissipation at multiple locations. This well distributed energy-dissipating system ensures greater control of the total dissipation energy in cases with large loads. This work also includes equations that were developed to estimate the behavior of frame rotation, maximum flexural strength and post uplifting stiffness of the proposed system. The results show that the equations predicted the behavior with close accuracy and the proposed self-centering rocking system reduces permanent deformation.
System Concept
The self-centering rocking system described in the study involves a braced frame with double-skin concrete-filled tube columns made of ultra-high strength steel (HS), replaceable steel fuses, all-steel moment-resisting frames (MRF), and a steel basement beam. When post-tensioned force is applied from the beam to the columns, the columns detach from the basement beam to experience rocking. When lateral forces are applied, one column raises while the other rotates and transfers the force to the steel beam. This ensures effective rocking of column bases without slippage.
Another intention of the proposed self-centering mechanism is to avoid the large strength demands required in the design of the beam and steel braces. This is accomplished when the large restoring forces from the structure are directly transferred to the CFT columns and post-tensioned (PT) bars are directly connected to the pile foundations. The fuses are also designed and shaped specifically to focus inelasticity in the deformable section.
Experimental Study, Results, and Discussion
The purpose of the experimental study is to evaluate the performance of the proposed self-centering rocking frame system. Six one-quarter-scale specimens were tested, each with a
different combination of low-yield point steel or conventional steel fuses, base plates and stoppers. A hydraulic jack was used to apply cyclic loading history exerting incremental lateral displacements to the specimens.
The results show that specimens with a combination of self-centering mechanism and low-yield point steel (LY) fuses reduce residual deformation of the entire frame system and develop no fractures until the end of the procedure. In all of the test specimens, the steel sections of the CFT columns exhibit elastic behavior, as the maximum strain measured was less than the yield strain of the HS steel.
The shear-transfer mechanism prevented column base slippage, while the LY steel fuses increased the energy dissipation capacity of the system. Provided that the PT bars remained elastic, the proposed system exhibited a flag-shaped hysteresis loop with no residual deformation. This indicates that the proposed self-centering rocking frame system reduces permanent deformations and achieves a high seismic performance.
Reference
Kazuhiro, H., Skalomenos, A. K., Inamasu, H., and Liu, Y. (2018). “Self-Centering Rocking Composite Frame Using Double-Skin Concrete-Filled Steel Tube Columns and Energy-Dissipating Fuses in Multiple Locations,” Journal of Structural Engineering, 144. 9, pp. 18.