Study On Hysteretic Behavior Of Overhanging Steel All-Bolt Assembled RC Beam-Column Joints

Since 2015,the state has issued a n umber of policies to vigorously develop prefabricated buildings.The traditional assembled structure mainly adopts the "wet connection" technology,and uses grouting sleeve,steel bar lapping,anchoring and other methods to post-cast concrete.However,compared with the traditional cast-in-place reinforced concrete structure,the "wet connection" has no obvious advantages in construction speed,quality and safety control,and cost.The "dry connection" technology usually adopts unbonded prestressed tendons,bolts and welding methods,without on-site pouring of concrete,which meets the development requirements of modern prefabricated buildings.In this paper,a new type of all-bolt assembled beam-column joint is proposed for bolt connection.The failure mode,bearing capacity,ductile energy dissipation,strain and rotation performance are studied by low-cycle reciprocating test.On the basis of the test,the finite element software is used to verify the model,and the parameter analysis is carried out for the thickness of the steel plate,the length of the connection area and the weakening degree of the section.Finally,the rotational section of the joint is determined,and a three-line bending moment-rotation mathematical model suitable for the assembled joint is proposed.The main conclusions are as follows:(1)Unlike the conventional cast-in-place node where the plastic zone is located at the root of the beam and column,the damage area of this assembled node is shifted outward to the concrete beam end,and the main cracking area is 1/3 of the net length of the beam,and the flexural deformation of the externally extending steel sections and connection plates occurs,and the plastic zone is formed in 2 sections of the steel connection area and the concrete beam end.(2)The mechanical properties of the new assembled nodes are significantly improved.Compared with the cast-in-place nodes,the yield load is increased by 30%,the ultimate load is increased by 60%,the initial stiffness is increased by more than 50%,its hysteresis loop area is fuller,the ductility coefficient reaches more than 4.0,the equivalent viscous damping coefficient reaches 0.2,and the deformation and energy dissipation capacity is stronger.(3)Analysis of the damage mechanism reveals that the force performance of this new assembled node is greatly influenced by the steel connection zone,which is too stiff and will produce an obvious extrusion effect on the precast beam end and premature yielding of the longitudinal reinforcement at the beam end.Reducing the thickness of steel plate or weakening the connection zone can significantly enhance the rotational deformation capacity of the connection zone,further develop the plastic zone of reinforced concrete beam,enhance the synergistic deformation capacity of steel connection zone and precast members,and improve the force performance of the node.Strain analysis found that:the strain in the steel section zone is much larger than that in the connection cover zone,and the strain value increases gradually as the plastic development zone at the end of the beam increases with the reduction of stiffness;the strain value increases significantly when the bearing capacity is increased by 4%for the weakened specimen in the connection zone.(4)The thickness of the steel plate,the length of the connection zone and the degree of weakening of the section have a large impact on the force performance of the node.When the stiffness ratio of the steel section to concrete beam is greater than 0.4,or the length of the connection zone does not exceed 110mm,or the degree of weakening of the face joint is not greater than 35%,it belongs to the strong nodal connection;conversely,the curve shows an obvious descending section,which belongs to the weak nodal connection.(5)Based on the test and numerical analysis results,the rotation section and damage interface of the assembled node are determined,and the trifold moment-turning angle model suitable for the node is proposed.The mathematical model has a high accuracy with an error within 5%,and can accurately reflect the mechanical characteristics of the new assembled node.