Research on seismic behavior of prefabricated concrete beam-column joints

In recent years, with the development of society, the construction method of cast-in-place has been unable to fully adapt to the needs of social development. The construction industry has ushered in new development opportunities, gradually to technology, energy-saving and pollution-free development. Prefabricated buildings have been promoted by reducing construction pollution, saving energy and improving labor efficiency. Different types of joints will directly affect the stability and seismic resistance of the whole structure, and also determine the progress of the prefabricated building. Therefore, the prefabricated reinforced concrete beam and column joints are very necessary to study the seismic performance. In this paper, the seismic performance of assembled concrete frame beam and column joints, assembled concrete beam-buckling column-steel restrained support joints and a new kind of prefabricated pin concrete beam-column joints are summarized and analyzed.


Introduction
The Technical Code for Prefabricated Concrete Structures (JGJ 1-2014) (referred to as the Code for Prefabricated Concrete structures) defines prefabricated concrete structures as those composed of precast concrete members assembled through reliable connections [1].The beams, plates, columns and other structural components of prefabricated concrete structure are prefabricated by factory production.The quality of components is greatly improved compared with cast-in-place concrete components.The key of prefabricated concrete structure technology lies in the connection between components [2].For prefabricated concrete frame structures, beam-column joints bear the pressure from the upper column and transmit and distribute the bending moment at the end of it, which is the main stress part of the frame.Therefore, the reliability of the beam-column connection, especially seismic performance of beam-column joints has always been a hot topic in the research of scholars.Over the past decades, researchers around the world have designed a large number of different forms of prefabricated beamcolumn joint connection structures, conducted experimental research and theoretical analysis, and formed different prefabricated concrete frame structure systems based on different beam-column joint connection methods [3].The current research and application of prefabricated concrete structure systems mainly includes cast-in-place column structure systems and cast-in-place joint structure systems.Precast prestressed concrete frame structure system, world structure system, etc.
At present, the cast-in-place column and the node structure system are most widely used at home and abroad because of the convenience of construction and the cognition of people that they are closer to the cast-in-place concrete structure [4].Compared with cast-in-place column structure system, cast-in-place joint structure system has higher concrete precast rate.According to the "Prefabricated Code", for the beam-column joints in the middle layer of the frame, the longitudinal reinforcement bars at the bottom of the beam on both sides of the joints should be anchored in the area of the post-cast joints, and can also be directly connected by mechanical connection or welding.In practical engineering, direct connection is difficult to achieve, so bending anchorage or anchorage plate anchorage is usually used [5].This structure is easy to cause reinforcement congestion conflict in the core region of the joint, resulting in prefabricated components can not be installed, this problem is particularly prominent in the bidirectional frame [6].In addition, the reinforcement in the joint core area is too dense, which is easy to lead to insufficient concrete binding force, and it is difficult to meet the reinforced anchorage requirements.Xu Qigong et al. [7] Two types of prefabricated beamcolumn joints are designed in the core region of the joint to avoid the problem of reinforcement congestion at the joints.One is a casting joint of the core area and the beam, and the beam top is strengthened longitudinally and the beam bottom are continuously connected at the joint; The other is the core area and the lower column integration casting node, the core area reserved for the beam bottom longitudinal reinforcement, through the casting section and prefabricated beam connection.Deng Yang et al. [8]A new kind of beam-column joint is proposed form consisting of pin shaft, ear plate and beamcolumn end plate, and designed and fabricated cast-in-place beam-column joint and prefabricated beamcolumn joint.In this paper, three kinds of beam-column joint structures are compared and summarized: cast-in-place beam and column joint, joint with core area, and joint with lower column and prefabricated pin beam joint.The purpose is to summarize the precast concrete beam and column joints with good seismic performance.

The joint core area and the beam are cast together and prefabricated
The composite beams with concave section and rectangular section are used in the beam-column joints of prefabricated concrete structures.The influence of the composite beams with two different structures on the seismic performance of the joints is studied.

New pin shaft concrete beam-column joint
The invention relates to a new kind of beam-column joint consisting of a pin, a lug plate, and a beamcolumn end plate.The cast-in-place beam-column joints and prefabricated pin beam-column joints were designed and fabricated, and the low-cycle reciprocating load test and finite element simulation were carried out.The failure mode, hysteretic capacity, stiffness degradation and skeleton curve of the joints were studied.The influence of the thickness of ear plate and beam-column end plate on the damage control ability of nodal non-energy consuming members is analyzed.The ductility of the two specimens is evaluated by the displacement ductility coefficient, which is the ratio of yield displacement △y to ultimate displacement △u.Table 1 shows the displacement ductility coefficient of the two specimens.It can be seen from the table that the ductility of specimen LZ-B is less than that of Specimen LZ-a.Under positive loading, the Ductility of specimen LZ-B is 7.7% lower than that of test piece LZ-a, and the bottom of the right beam of specimen LZ-B is under compression.Under reverse loading, the ductility of specimen LZ-B was 13.7% lower than that of specimen LZ-a, and the bottom of the right beam of specimen LZ-B was under tension.The ductility of specimen LZ-B is lower than that of specimen LZ-A for two reasons.One is that a full grouting sleeve is set at the bottom of the beam end of specimen LZ-B, and the stiffness of the sleeve is much higher than that of the steel bar, so the length of the plastic deformation range of the steel bar under tensile action is small.Second, a large diameter steel bar was set at the bottom of the beam end of the right beam, and the height of the compression zone of the section was too large during tension.The skeleton curves of each node under different parameters were analyzed, and the stiffness changes of each node under different loading displacements were calculated.The results are shown in Figure 2. According to the figure, the initial stiffness of node S6 is the largest, and the initial secant stiffness is about 16 kN/mm, indicating that increasing the thickness of the ear plate and end plate can increase the initial stiffness of the node.When the loading displacement is less than 20 mm, the elastic deformation of the new pin beam-column node is dominant, and the stiffness of the node does not degrade significantly.When the loading displacement is between 20 mm and 60mm, the end plate begins to buckle, and there is a slight rotation between the pin shaft and the ear plate, which indicates that the new pin beam-column joint has plastic deformation and rigid body rotational deformation.Therefore, the secant stiffness decreases continuously and the stiffness degradation rate is relatively stable.When the loading displacement is greater than 60 mm, the end plate plasticity of the new pin beam-to-column joint continues to develop, resulting in the gradual degradation of the secant stiffness.The joint ductility coefficient can be calculated as follows

New pin shaft concrete beam-column joint
The Δu, Δy are the ultimate displacement and yield displacement of the node, respectively.
The joint ductility coefficient and maximum equivalent plastic strain under different end-plate and lug plate parameters are shown in Table3.It can be seen from the table that the increase of the thickness of end plate and ear plate will increase the ductility coefficient of the node.When the thickness of the ear plate and the beam-column end plate are 25 mm and 10 mm (node S5), separately, the point ductility coefficient reaches the maximum, reaching 4.7.The thickness of the beam-column end plate is 8 mm, the maximum equivalent plastic strain increases slightly with the increase of the thickness of the ear plate.When the thickness of the beam-column end plate is 10 mm, the maximum equivalent plastic strain decreases first and then increases with the increase of the thickness of the ear plate.Therefore, the overall stiffness of joint S6 is larger, which leads to the increase of plastic damage of the concrete at the beam end.
In conclusion, when the thickness of the beam-column end plate is 10 mm and the thickness of the ear plate is 25 mm, the ductility of the new fabricated pin beam-column joint is the best, and the plastic damage of the non-energy dissipation member is low.

The joint core area and the beam are cast together and prefabricated
Beam-column joint refers to the intersection point of the column and beam of the frame structure.For the prefabricated concrete frame structure, the beam-column joint not only bears the pressure from the upper column, but also transmits and distributes the beam-column end moment of bending the, which is the main force structure in the frame.Therefore, the reliability of the beam-column connection, especially the seismic performance of the beam-column joint.The beam-column members of the frame structure are conducive to standardization, and it is easier to adopt the integrated structure, so as to shorten the construction period; The integrity and stiffness of cast-in-place concrete frame structure are good, and if the design and treatment are good, it can also achieve better seismic effect, and the beam or column can be cast into any required section shape.The concrete frame structure is widely used in buildings, but also be based on need for prestressing concrete beam or plate, in order to apply to the larger span.However, compared with the cast-in-place concrete frame structure, the beam-column joint bearing capacity, energy consumption capacity and structural integrity of the prefabricated structure are poor, which makes the seismic performance of the structure poor.After the earthquake, the prefabricated beam-column joints suffered significant plastic damage to the beam and column components, making it difficult to fully utilize their energy dissipation.The damage control ability needs to be improved, and the energy dissipation components are difficult to be repaired or replaced quickly after the earthquake.
Based on these defects, Qigong Xu, Wendong Chen By extending reinforcement bars with large diameter at the joints, the flexural strength of the beam-column joint surface is improved to achieve strong connection.When the bottom of the beam is strained, the plastic hinge is transferred from the beam-column joint surface to the outer end of the sleeve joint, and the length of the longitudinal bar at the bottom of the beam where plastic deformation takes place is increased, thus improving the ductility of the beam-column joint [9].The results show that this structure should be considered under repeated loads of concrete compressive strength by node, and that the plastic hinge from the beam-column joint surface is transferred to the outer end socket joint to achieve a strong connection.Therefore, the longitudinal reinforcement ratio of beam top and beam bottom within the sleeve area should be increased at the same time.

New concrete beam-column joint with pin shaft
New fabricated pin concrete beam-column joint, connect prefabricated beam and prefabricated column through ear plate and pin shaft.The beam and column are symmetrical reinforcement, column longitudinal reinforcement diameter of 20 mm, beam tension steel bar diameter of 22 mm, compression steel bar diameter of 20 mm, stirrup diameter of 8 mm.The pin shaft energy dissipation node connects the beam end to the column end on both sides of the ear plate mainly through two pin shafts inserted into the ear plate, and the two pin shafts bear the shear force and bending moment transmitted by the beam.The column end is connected by the plug welding of the column end which is connected with the inner anchor bar and the ear plate, and the beam end is directly connected with the plug welding of the beam end by the anchor steel bar in the beam.The ear plate is connected with the beam and column end plate by welding, in which the thickness of the ear plate is 25 mm, and the thickness of the beam and column end plate is 10 mm.In order to prevent the weld cracking between the ear plate and the beam and column end plate, a trapezoidal stiffening rib plate is welded at the joint above the ear plate, with a thickness of 8 mm and an area of 17500 mm 2 .Experimental results show that the energy consumption of this new assembly pin beam-column joint is mainly through the buckling of the end plate and the rotation of the ear plate and the pin shaft.Compared with cast-in-place beam-column joints.The new joints reduce the plastic damage of concrete beams and columns without reducing the bearing capacity and initial stiffness.The failure mode of the joints changes from shear failure in the core area to buckling failure of the end plates of the beams and columns [10].The ductility and energy dissipation capacity of the joints can be improved by using pins and lug plates.Reduce node damage.Parameters were analyzed for the thickness of nodal ear plate and end plate.The viscous damping coefficient, ductility coefficient and plastic strain at the end of the beam under different conditions were investigated.The results show that.When the thickness of ear plate is 25 mm and the thickness of beam-column end plate is 10 mm, the ductility and energy dissipation capacity of the new pin-shaft energy-dissipating node are the best, and the plastic damage of the beam end is the least.

Conclusion
The integrity of the specimen LZ-A by Xu Qigong et al. is good, and the test results show that the failure of the sitting slurry layer at the joint core area and the connection between the upper and lower columns is not obvious, and the shear failure of the joint core area is slight.Therefore, it can be considered that the seismic performance of the joint structure with the joint core area and the beam cast together is the same as that of the cast-in-place structure, and its ductility coefficient is between 2.72 and 3.28.And Deng Yang put forward by a pin shaft, ear plate, rizzano plate constitute a new form of beam-column joints, when rizzano, plate thickness, ear plate thickness is 25 mm to 10 mm, the new type of prefabricated pin beam-column joints ductility is best, the ductility coefficient can reach 4.7, and lower than wall-beam and plastic damage, is superior to the general structure of cast-in-situ.In the process of comparative experiments, it is difficult to make a horizontal comparison between different node experiments, and only a general summary can be made.Its rigor needs to be considered.

Figure 1 .
Figure 1.The joint core area and the beam are cast together and prefabricated.

Figure 2 . 3 .Figure 3 .
Figure 2. New pin shaft concrete beam-column joint.3.Comparison of experimental data3.1.The joint core area and the beam are cast together and prefabricatedNode failure Characteristics

Figure 4 .
Figure 4. Failure characteristics of the new prefabricated pin beam-column joint during loading.Deng Yang et al. used ABAQUS software to establish the pintle energy-consuming node and analyzed its parameters to study the influence of the thickness of the ear plate and end plate on the energyconsuming capacity of the node.The dimensions of the auricular plate and endplate components are shown in Table2.

Table 2 .
New type assembly pin beam column joint dimensions mm.

Table 3 .
Ductility coefficient of joint and maximum equivalent plastic strain of concrete.