| It is necessary to use reinforcement steel bar with high strength and good comprehensive performance and high toughness concrete in beam-column joint for environment protection and energy saving in China.It's important to study the mechanical performance,seismic performance and bond deterioration of beam bar of beam-column joint with high strength reinforcement steel bar and high toughness concrete.Adopting high toughness concrete instead of common concrete or X-shaped reinforcement to the weak part of beam-column joint is proposed and investigated.Moreover,the experiment and research foundation were provided for high performance structural,which was better to generalize further.The beam-column joint with common reinforcement steel bar and high strength concrete,the joint with 600MPa high strength reinforcement steel bar and high strength concrete,the joint with 600MPa high strength reinforcement steel bar and high toughness concrete,the joint reinforced by X-shaped reinforcement and the joint with high toughness concrete instead of common concrete in core area were tested under low cyclic loading.The damage characteristic,stirrup strain,X-shaped reinforcement strain,concrete strain variation of core area,shear deformation of core area and plastic hinges deformation of beam-end of beam-column joint were compared.The results in terms of bearing capacity,deformability,displacement ductility,hysteretic curve,rigidity degradation,energy dissipation and cumulated damage were also analyzed.The experimental results on the behavior in beam reinforcing bar strain,column reinforcing bar strain,measured steel slip and bond deterioration of beam bar in the range of column depth were presented.The effect of axial compression ratio,shear compression ratio,stirrup characteristic value,column depth-beam reinforcing bar diameter ratiosh_cd_b,beam-column steel strength and concrete type on mechanical performance,seismic performance and bond deterioration of beam bar of beam-column joint were compared.The conclusion is as follows.With the increase of stirrup characteristic value,column depth-beam reinforcing bar diameter ratiosh_cd_b,beam-column steel strength and with the decrease of axial compression ratio and shear compression ratio as well as with high toughness concrete instead of common concrete,the behavior of beam-column joints were improved in terms of improved concrete failing and damage characteristic,decreased shear deformation of core area and increased plastic hinges deformation of beam-end,enhanced deformation behavior,ductility and energy dissipation,improved hysteretic characteristic,lightened cumulated damage and bond deterioration of beam reinforcing bar.Adopting high toughness concrete instead of common concrete or X-shaped reinforcement is an effective measure for improving the behavior of beam-column joints in the following aspects.decrease crack width and improve damage characteristic,decrease stirrup stress of core area in earlier stages,reduce shear deformation of core area and increase plastic hinges deformation of beam-end,enhance deformation behavior,ductility and energy dissipation,improve hysteretic characteristic,lighten cumulated damage and bond deterioration of beam reinforcing bar.The seismic behavior and mechanics characteristic of beam-column joint reinforced by X-shaped reinforcement are better than that of adopting high toughness concrete instead of common concrete in terms of enhanced bearing capacity and shear-bearing capacity,improved hysteretic characteristic,increased energy dissipation,slowed the stiffness degradation and lightened cumulated damage.While the seismic behavior and mechanics characteristic of beam-column joint by adopting high toughness concrete instead of common concrete are better than that of reinforced by X-shaped reinforcement in terms of improved damage characteristic,increased principal tensile strain of concrete and enhanced ductility.For the coordination coefficient of 600MPa high strength reinforcement steel bar and high toughness concrete as well as enhancement function of X-shaped reinforcement,the beam-column joint show better mechanical performance,seismic performance and beam reinforcing bar bond deterioration.The proposal for determining the shear strength of beam-column joint with 600MPa high strength reinforcement steel bar and high toughness concrete for seismic resistance is made.The softened strut-and-tie is based on strut-and-tie concept and derived to satisfy equilibrium,constitutive laws of cracked reinforced concrete and deformation compatibility principle.The softened strut-and-tie model of beam-column joints reinforced by X-shaped reinforcement was also presented.The simplified softened strut-and-tie model is more practical to calculate the shear capacity of beam-column joints.Furthermore,the suggested model was verified using test data of beam-column joints.Good agreement between test results and prediction results was achieved.The contribution of X-shaped reinforcement and high toughness concrete to shearing capacity of beam-column joint was analyzed.The confined compression strut mechanism and the truss mechanism was proposed for determining the shear strength of beam-column joint considering the beam longitudinal bar bond and slip,stress and bond strength as well as the proportion of those two shear-resistance mechanisms in the total shear-resistance mechanism.The function of X-shaped reinforcement or high toughness concrete for the weak part of beam-column joint was explicit.Furthermore,the suggested model was verified using test data of beam-column joints.Good agreement between test results and prediction results was achieved.On the basis of mechanical mechanism and design method of beam-column joint,the effect of reinforcement by X-shaped reinforcement or adopting high toughness concrete instead of common concrete are considered,the shearing capacity formula of beam-column joints reinforced by X-shaped reinforcement or adopting high toughness concrete instead of common concrete was proposed.It is shown that the shear capacity of beam-column joint reinforced by X-shaped reinforcement or adopting high toughness concrete instead of common concrete is also higher than that of without reinforcement.The X-shaped reinforcement or high toughness concrete has the function of reinforcement for the weak part of beam-column joint with 600MPa high strength reinforcement steel bar and high strength concrete.In addition,the calculated values are smaller than test values,which indicates that the formula is safe for design of beam-column joins with 600MPa high strength reinforcement steel bar and high toughness concrete and with slight high reliability. |
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