| Unbonded steel plate brace encased in reinforced concrete panel is one of the bracing members being used in multi-story and high-rise buildings. The unbonded material is painted onto the steel plate brace to remove the bond stress between the brace and the panel, so the axial load is only applied on the brace. The panel is only designed to prevent the encased brace from buckling so that the brace can dissipate seismic energy through axial yielding under both tensile and compressive forces. With the excellent energy dissipating capacity provided by this panel type of buckling-restrained brace which acts as a hysteretic damper, the damage to the main structure can be greatly alleviated under the strong motion earthquakes, however the encased braces will perform as the concectrical steel braces under the smaller earthquakes or wind load. As for the layout of the encased braces, it can be divided into diagonal-shaped braces and chevron-shaped braces, which are mainly used in the structures with many partition walls.Recently, buckling-restrained braces have been attached much importance to by many scholars and engineers, but there are few papers dealing with the panel type of buckling-restrained brace(BRB) in which some questions such as the interaction between the braces and the panel are still unclear. So the main work and conclusions on the panel type of BRB are listed as follows.Pseudo-static tests including seven pieces of panel type of diagonal-shaped buckling-restrained braces(DBRB) and six pieces of chevron-shaped buckling-restained braces(CBRB) have been carried out to investigate the hysteretic performance, in which the layout and width to thickness ratio of the encased steel plate brace, the thickness and reinforcement ratio of the panel, the edge reinforcement of the panel and the unbonded materials are considered. The tests indicate that the punching shear failure near the edge of panel and the local bending failure near the middle of panel are two major failure modes. The constructional reinforcement of panel and unbonded material are two important factors influencing the hysteretic performance of panel type of BRB. With smaller grids of mat reinforcement near the edge of braces and in the middle of panel and with smoother and thinner unbonded materials painted onto the steel plate braces, the panel type of BRB exhibit more stable hysteretic performance. Local buckling is prevented with the ribs welded at the end of braces. The split failure at the edge of panel is also prevented by the angle reinforcement or anchored slab reinforcement. It is found that local failure of the panel is due to the buckling in higher order mode in the yielding portion of brace.The analysis results reveal that the unbonded materials do well in removing the bond stress between the brace and the panel. During elastic stage, the horizontal stiffness of the panel type of BRB almost amounts to that of the encased brace without buckling which suggests that the unbonded materials can also diminish the friction force in between. The panel type of BRB exhibit good ductility and stable performance with well energy dissipating capacity before failure. The hysteretic curves can be simplified by the bilinear hysteretic model which agrees well with the real curves. The skeleton curves also exhibit bilinear performance.The working performance, the buckling behavior at the end of brace and the brace-beam connection design are also analyzed. The design formula on the overall stiffening requirement of the panel type of BRB is modified. The effective width of panel is derived from the principle of equal bending capacity. According to the failure mode, the stiffening force between the brace and the panel and the stiffening requirement preventing the edge of panel from punching shear failure are also obtained. Finally, in order to supply a reference for engineering application, the reasonable construction method and suggestion of the panel type of BRB are also summarized. |
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