Hysteretic Behavior And Application Of Unbonded Steel Plate Brace Encased In Reinforced Concrete Panel

The hysteretic behavior of steel braces has a direct influence on the aseismic performance of the concentrically braced steel frames. After buckling of the conventional steel braces in compression under a severe earthquake, their hysteretic behavior will deteriorate, namely their energy dissipation capacity and compressive strength can not be developed efficiently, and it is disadvantageous to the aseismic performance of the structures. In order to avoid buckling of the conventional steel braces and improve their energy dissipation capacity, the steel brace can be equipped with a lateral restraining part, which is a so-called buckling-restrained brace (BRB). The unbonded steel plate brace encased in reinforced concrete panel is a panel type buckling-restrained brace (Panel BRB), which is composed of one or two steel plate braces and an encasing reinforced concrete panel. An unbonded material or clearance is generally employed between the brace and the hole of the encasing panel to prevent from the adhesion and the lateral expanding force of the compressive brace due to Poisson's effects. As a consequence of the fact that the large amplitude flexural buckling of the encased braces is prevented, the Panel BRB can yield under both tension and compression. Thus, the strength and energy dissipation capacity of the steel can be developed efficiently. The Panel BRB has a good prospect of application in the buildings that more partition walls are required. In this paper, a systematic study has been carried out to investigate the hysteretic behavior, constructional measures and other relevant problems for the Panel BRBs by test researches and numerical simulations. In addition, the aseismic performance of the buckling-restrained braced frames (BRBFs) in which the Panel BRBs are used as the lateral bracing members has been studied. Based on the analysis results, methods and some suggestions for the aseismic design of this type structure were put forward.Quasi-static tests for seven pieces of single-diagonal Panel BRBs and six pieces of chevron Panel BRBs were conducted, and the effects of unbonded material and clearance between the panel and the plate brace, constructional measures in the panel, effective width panel, etc. on the hysteretic behavior of Panel BRBs have been investigated. The test results indicate that the compression steel braces buckle with multiple waves in the holes of panel, and the hysteretic behavior of Panel BRBs is desirable when the evener and smaller clearance between the steel brace and the hole of the panel is provided. A specimen with the effective width panel also exhibits good hysteretic behavior when it has the same constructional details as the specimen with the normal weight concrete panel. However, this kind of Panel BRB would be advantageous to panel installation and aseismic performance of the buildings because of its lighter weight. All specimens of Panel BRB exhibit full stable performance under the quasi-static loading until the local failure of panels occurs by either flexure or punching shear. After the steel brace yields, the resistance of the specimen exceeds its yield load capacity due to the strain hardening and frictional action. Skeleton curves of the specimens are close to the bilinear models. Based on the test results, the hysteretic model and the simplified simulation method for the Panel BRBs have been put forward, which can provide a reference for the elasto-plastic aseismic analysis of the structures.The hysteretic behavior of the Panel BRB specimens has been simulated by the ABAQUS program in which the material, geometrical and contact nonlinearity are modeled in order to examine the working behavior of specimens and the interaction between the panel and the encased brace under a reversal axial loading. The effects of the clearance and the frictional action on the hysteretic behavior of specimens were mainly studied, and a suggestion of the appropriate clearance was put forward based on the simulation results. In addition, A parametric analysis was performed for the purpose of examining the effects of the initial flexural deformation of steel braces, thickness, as well as strength, of the brace and the panel, width of the brace, width to thickness ratio of the steel plate brace, clearance across the thickness of the plate brace, friction between the panel and the brace, ribs of the brace, etc. on the working behavior of the diagonal Panel BRBs. The analysis results reveal that the number of the buckling waves increases and punching shear forces applied to the panel by the brace are amplified with the dimension of the clearance increasing. Moreover, it is found that the ribs of the steel plate brace can prevent the ends of panel from failure by punching shear when the appropriate clearance is provided, whereas, the crack and local failure of the ends of panels occur earlier when the ribs are omitted. For the purpose of reducing the number of buckling waves of the brace, decreasing the punching shear force applied to the panel by the brace and preventing the local buckling of the plate brace occurring earlier, the width to thickness ratio of the plate brace should not be larger than 19. At the same time, in order to avoid crack and failure of the flexural panel occurring earlier, it is necessary to control the maximum initial flexural deformation of the brace, which should not be larger than 0.001 times the length of the brace. The analysis results also indicate that the smallest thickness ratio between the panel and the brace, which is required to avoid the overall buckling of Panel BRBs, is influenced by thickness, as well as strength, of the panel and the steel brace, etc. Based on a large amount of parametric analysis results, the regression formulae to determine the smallest thickness ratio within the range of the analysis parameters were provided.The aseismic performance of steel frames in which the chevron or V-shaped Panel BRBs are used as the lateral bracing members has been investigated by the ANSYS program in which the material and geometrical nonlinearity are modeled. The analysis results indicate that the brace actually provides vertical support action for the braced beam under the frequent excitation of the earthquakes, whereas, this support action decreases greatly under the severe excitation due to the cumulative plastic deformation of the postyield braces. The analysis results validate that, when the design of braced beams is performed by taking account of the support action of braces but omitting the overstrength of postyield braces, the deflection and plastic defomation of the braced beams, as well as the cumulative plastic deformation of the braces, are all large. Whereas, when the design is carried out by taking account of the overstrength but omitting the support action of the braces, the deflection and plastic deformation of the braced beams, as well as the cumulative plastic deformation of the braces, are all small. The latter design method is in accordance with the design purpose in which the braces mainly resist the lateral forces. The extent of plastic deformation of columns is decreased when the columns connected with the Panel BRBs are designed according to the axial force distribution of the Panel BRBs under the first-mode response of the structures. It is found that the design methods mentioned above are helpful for realizing the aseismic design purpose in which the Panel BRBs can mainly dissipate energy but the frame nearly remain elastic under a severe earthquake. Moreover, the chevron Panel BRBs with different yield strength all exhibit good energy dissipation capacity, which depends on the response of the structures.Analysis results on the aseismic performance of the panel bucking-restrained braced frame (BRBF), the special concentrically braced frame (SCBF) and the ordinary concentrically braced frame (OCBF) indicate that the base shear forces and the amount of steel used for the BRBF and SCBF are obviously smaller than those for the OCBF when lateral drifts of the three structures are all smaller than the limits. In addition, large vertical shear forces are applied to the braced beams by the ordinary concentrically braces and the special concentrically braces when the large amplitude overall buckling of those braces occurs, whereas, those shear forces applied by the large amplitude yielded Panel BRBs are very smaller.The aseismic analysis for the dual systems in which the Panel BRBs are used as the lateral bracing members indicates that the inter-story deformation, as well as the development of plastic deformation in members, decreases but the amount of steel used increases when sections of the frame members are enlarged. The analysis results validate that the aseismic design for the dual systems should be performed to ensure that the frame can independently resist 25 percent of the total base shear force of the structure.