Seismic Behavior And Design Method Of RC Sandwich Beam-column Joints

It is prevalent that concrete strength of column is stronger than concrete strength of beam in reinforced concrete high-rise buildings. However, casting joint core area with weaker strength concrete of beam have some advantages, such as simplifying the construction process, reducing construction difficulty, improving construction high quality and shortening construction duration and so on, this joint is called sandwich joint in this paper. And seismic behavior and design method of sandwich joint aren't investigated systemically until now; scientific problems, such as force-transferring mechanism, failure pattern and so on, are not identified. Problems above are investigated in this paper, the works are as followed:(1)18 sandwich joints with middle and high shear-compress ratio are tested under quasi-static load, integrated seismic performance of these specimens are studied, such as crack development and impairment process of members, failure model, displacement ductility, compress and shear resistant capacity of core area, energy dissipation behavior, beam bars anchorage capacity and shear deformation of core area and so on.(2)Contrasts between strengthened joints (6 plane sandwich interior joints, 3 space sandwich joints) and traditional joints (with the same dimension and parameters) are provided, and the differences between them are clarified.(3)Failure characteristics and failure models of sandwich joints are summarized, the possible failure models and conditions of its occurrence are analyzed, and transferring mechanism of core area for sandwich joints is investigated.(3)Failure characteristics and failure models of sandwich joints are summarized, the possible failure models and conditions of its occurrence are analyzed, and transferring mechanism of core area for sandwich joints is investigated.(4)Based on the experimental results, checking contents and design regulations are provided. The limit of design parameters are presented through the contrasts of seismic behavior between sandwich joints and traditional joints. Checking formula is presented, based on the resistance capacity of members and the nonlinear finite element simulation.Seismic behavior, scope of application and design method, the main conclusions are followed: (1)If certain shear compress ratio, axial compress ratio, the concrete strength of beams and columns, and stirrups ratio in core area are satisfied, the displacement ductility of the majority of sandwich joints is more than 3.5, their seismic behavior or their seismic behavior after certain fortification measures satisfies the requirements of structure seismic design. Therefore, adapting the sandwich joints in engineering is feasible.(2)Shear compress ratio, axial compress ratio, strength ratio of beam to column, style of stirrups, confinement effect in core area, fortification measures, stirrup ratio in core area are influencing factors of seismic behavior for sandwich joints, and the shear compress ratio is most significant influencing factor, is the key factor determining the failure model of members.(3)Displacement ductility of sandwich joints is no less than traditional joints, the performance of energy dissipation is equivalent with traditional joints, but the others'performances are worse than traditional joints, such as bearing capacity of beam-end, compress performance and shear performance of core area, anchorage performance of beams'bars and so on.(4)Failure pattern of frame joints casted with weaker concrete strength of beam doesn't change apparently. Failure pattern of high shear compress ratio joints is mostly caused by the shear failure of core area; failure pattern of middle and low shear compress ratio joints is mostly caused by beam-end failure; it is possible that compress failure of core area happens in sandwich joints, but failure of bias compress can't occur.(5)In the design process of sandwich joint, shear compress ratio, axial compress, strength ratio of beam to column, stirrup ratio of core area, anchorage length of beam-end should be controlled, in order to satisfy the requirement of displacement and bearing capacity. And checking compress capacity and shear capacity of core area, checking bearing capacity of column-end should be carried out to satisfy the requirement of bearing capacity; strengthening measures, such as addition of vertical dowels, addition of diagonal bar and enhancement of joint constraint and so on, should be brought into force.(6)Confinement effect differences of different joint style should be considered, when checking compress capacity of sandwich joint is calculated; And when checking shear capacity of sandwich joint is calculated, as for middle and high shear compress ratio members, stirrups of core area should be added to offset the impairment caused by the normal concrete strength.