Self-centering buckling-restrained brace (SC-BRB) has both a good energy dissipation capability and self-centering effect, which can effectively improve the reparability of the structure after a strong earthquake. However, for the preexisting SC-BRB, the pre-tensioned tendons may yield to fracture, which would affect the safety and reparability of the structure during strong earthquake events. In addition, SC-BRB frames may exhibit a certain residual deformation when pre-tensioned tendons can not meet the structural deformation demand and the adjacent resisting-moment frame yields. At present, the design of SC-BRB frames adopts the displacement-based seismic design method that aims to control the peak story drift, rather than the residual drift. This method can not ensure the reparability of the structure after earthquakes.
For the above problems, this paper has made the following works.
(1) Concept and configuration of an innovative SC-BRB with pre-tensioned tendons, which can increase deformation capacity and initial stiffness of braces, have been proposed. Due to the constructional limitation of the preexisting SC-BRB, a high elastic elongation of the pre-tensioned tendon was demanded, which would result in a low deformation capability of SC-BRB. Besides, the issue that the tube length error would cause the practical initial stiffness of SC-BRB smaller than the theoretical value, was worth addressing. This paper had provided some measures to deal with these issues, such as the friction fuse or series SC-BRB for enhancing the deformation capability, and the rubber sandwich end plate for strengthening the initial stiffness controllability. Thus, an innovative SC-BRB with pre-tensioned tendons was proposed. After deeply analyzing the working mechanism and mechanical behavior, the design method and procedure for the proposed SC-BRB were developed.
(2) Experimental investigations on the pre-tensioned tendons and the friction fuse. The experimental investigation of pre-tensioned tendons made by basalt fiber-reinforced polymer (referred as BFRP) and friction fuse devices were conducted. The experimental results showed that the ultimate elongation of BFRP rebar exceeded 2.5% and this material exhibited a stable performance. However the epoxy deformation in the anchorage would lead to a relative displacement between the BFRP rebar and the anchorage device during the cycle tension of pre-tensioned tendons. Based on the experimental results, this paper provided the regression formula describing the relationship between the epoxy deformation and the stress difference, which could be used to evaluate the loss of pre-stressed force in SC-BRB after earthquakes. Supposing that the friction fuse was equipped with a large friction area and no rustiness in the surface, the loading rate or displacement amplitudes would almost have no influence on the sliding friction force. Moreover, the sliding friction force showed an unobvious variation and exhibited a stable performance after several cyclic loadings.
(3) Quasi-static tests of SC-BRB specimens. Four SC-BRB specimens with different improvement measures were designed and manufactured. The test results showed that for SC-BRB without improving the deformation capability, the bearing force would drop steeply and the residual deformation would increase to 90% of the displacement amplitude when the pre-tensioned tendons fractured due to exceeding the strain limit. The friction fuse could protect SC-BRB specimens from being damaged when the specimens experienced an axial deformation corresponding to the 4% inter-story drift. And the SC-BRB specimens possessed a certain self-centering ability. When the axial deformation of the brace is the same, the configuration of the series SC-BRB would meet the demands to reduce half of the pre-tensioned tendon’s elastic elongation. At the same time, the self-centering capacity would not be affected. The use of the rubber sandwich end plate could effectively reduce the influence of the tube length errors and improve the controllability of the initial stiffness of SC-BRB.
(4) Numerical simulations of the hysteretic behavior of SC-BRB. Based on the ABAQUS finite element analysis software, the fine analytical model of SC-BRB was established and the accuracy was verified by comparing numerical and experimental results. The influences of several key parameters on the hysteretic behavior of SC-BRB were investigated. In addition, based on the rheological analysis theory, this paper established an elastic-plastic analysis model to consider the influences of both the rubber sandwich end plate and the tube length errors on the hysteretic performance of SC-BRB with different pre-tensioned tendon materials.
(5) Parametric analysis on the seismic performance of SC-BRB frames. The key parameters that influence the seismic response of SC-BRB hinged steel frame and SC-BRB dual system frame were determined based on the structural hysteretic model and the dynamic equation. The two simplified model of SC-BRB frames were established via the OpenSees software. The influence of key parameters on the seismic performance of structures was conducted by time history analyses and the parameter values in structural design were suggested.
(6) Establishment of the nonlinear drift ratio spectrum and the residual drift ratio spectrum of SC-BRB frames. Based on the single degree-of-freedom model of SC-BRB frames, the influences of key parameters on both the nonlinear drift ratio and the residual drift ratio were investigated. 162000 and 486000 time history analysis for the single freedom system of SC-BRB hinged steel frame and SC-BRB dual system frame were conducted. The numerous regression analysis on the time history analysis results of the single-degree-freedom system, the nonlinear drift ratio formula, as well as the residual drift ratio formula, were established respectively for SC-BRB hinged steel frames and SC-BRB dual system frames. In addition, the displacement evaluation and structure seismic design were conducted.
(7) A seismic design method based on the post-earthquake reparability of the structure which considers both the peak story drift and the residual drift as the controllable index. The influences of high modal and P-Δ effect on the seismic displacement response of SC-BRB frames with different floors were studied. The regression formulas of the high modal amplification factor of the peak story drift and residual drift, which could be applicable for SC-BRB frames and SC-BRB dual system frames, were given respectively, as well as the referenced amplification factor of the P-Δ effect. Finally, selecting the peak story drift and residual drift as the controllable index and according to the formula of nonlinear displacement ratio and residual displacement ratio, this paper established a seismic design method based on the post-earthquake reparability of the structure, and this method was applicable for SC-BRB frame. The design procedure of SC-BRB steel frames and SC-BRB dual system frames were given.