by Abstract: in order to verify the reliability and mechanical properties of the anchorage structure of the new cast end column beam joint, the finite element modeling is used to analyze the stress state of concrete and reinforcement, concrete cracking damage and hysteretic performance of the column beam joint with superimposed prestress technology under repeated load.
Through stress analysis, the new joint can not only enhance the connection reliability, but also effectively ensure the seismic performance of the joint, and provide a reliable technical path for the connection of cast members.
Introduction the application of precast building technology has expanded from the initial laminated plate to the current pre supported beam, precast grouting, laminated plate and precast wall.
There are also a variety of connection modes between precast components.
The joint connection between precast column and precast beam is particularly important.
Its connection strength and reliability directly affect the safety and seismic performance of the overall structure.
With the development of precast buildings in the direction of volume and height, there are higher requirements for the strength and seismic performance of key connection joints.
The application range of traditional column beam joint anchorage methods is limited.
It is necessary to study and develop new column beam joint anchorage structure and construction technology.
1 precast column beam joint section steel superimposed prestress technology the traditional column beam joint connection methods include bracket support upper cast-in-situ, beam formwork support upper cast-in-situ, column preset lateral projection and beam anchorage, which can meet the strength requirements on the premise of standardized construction operation, but the mechanical performance of negative moment zone is affected by the concrete interface at the connection.
In order to improve the connection reliability of column beam joints and the stress performance in the negative moment zone, the section steel superimposed prestressed connection process for the end column beam joints is designed.
The general idea is to preset the section steel in the column as the anchorage end of the prestressed reinforcement, superimpose the prestressed reinforcement in the upper area of the precast beam and anchor it in the beam with the anchor plate on the basis of the conventional column beam joint connection, Staggered tensioning can be adopted for the tensioning at both ends of the column to form a new joint anchorage structure.
Before the application of the new technology of connection and anchorage, the mechanical and hysteretic properties of the new column beam joints are verified by finite element software modeling and analysis.
2 node structure description: the column beam node at the end of the assembly building is selected as the analysis object.
The size and reinforcement of the frame column and beam body are shown in Figure 1.
Beam a is the beam body at the front end of the column, beam B is the side beam on both sides of the column, and the beam body is embedded into the column for 50mm to simply lay down the beam body in construction.
The reinforcement is HRB400, the concrete strength grade of the column is C50, and the concrete strength grade of the beam body is C40.
Apply prestress to the two preset prestressed reinforcement in the middle of the upper part of beam a (half span along the beam direction).
The prestressed reinforcement is anchored on the embedded T-shaped steel in the precast column, and the steel model is Q235.
The three-dimensional geometric model is established by ABAQUS to investigate the stress state of concrete and reinforcement, concrete cracking damage and hysteretic behavior of the joint under cyclic load.
3-node finite element modeling in abaqus6 The node finite element model is established in 14-4.
The concrete is simulated by solid element c3d8r, the reinforcement is simulated by truss element t3d2, and the section steel is simulated by shell element S4R; The finite element mesh division of the overall modeling analysis is shown in Figure 2.
The boundary condition is that the column bottom is fixed and the beam end is hinged.
3.1 material constitutive model: the prestressed reinforcement is simulated by linear elastic material, and the concrete material is simulated by elastic-plastic damage model.
When the concrete material enters the plastic state, its tensile and compressive stiffness decreases, and the tensile and compressive damage coefficients of concrete are expressed by DT and DC respectively; Recovery of tensile and compressive stiffness of materials under repeated load, such as 3.
When the load changes from tension to compression, the cracks of concrete materials are closed and the compressive stiffness is restored to the original compressive stiffness; When the load changes from compression to tension, the tensile stiffness of concrete material does not recover; The standard values of axial compressive strength and axial tensile strength of concrete materials are adopted according to table 4.1.3 of code for design of concrete structures (gb50010-2010).
The standard value of axial compressive strength of column C50 concrete is 32.4mpa, the standard value of axial tensile strength is 1.89mpa, the standard value of axial compressive strength of beam C40 concrete is 19.1mpa, and the standard value of axial tensile strength is 1.71mpa.
In the analysis, a two fold dynamic hardening model is used to simulate the deformation of steel under repeated load σ − ε The elastic modulus of steel is es and that of strengthening section is 0.01es.
Bauschinger effect of steel under repeated load is considered in the program.
3.2 the loading system mainly analyzes the mechanical performance of the joint under cyclic load.
The loading process is as follows: (1) apply a prestress of 588mpa.
(2) Apply axial pressure, take 0.3 as the control value of axial pressure ratio, and apply axial pressure n on the column top.
N=0.3**fc*A=0.3*23.1*1000*0.6*0.6=2494.8kN。 (3) Apply cyclic load, keep the axial pressure unchanged, and apply X-direction cyclic moving load on the column top until the member is damaged, in which the load of each level is 1.5mm and reciprocating for 3 times.
4 finite element analysis results 4.1 stress state of reinforcement and concrete after prestress is applied.
After prestress is applied to the finite element model, the stress state of reinforcement and concrete is shown in Figure 4.
From the stress cloud diagram, it can be seen that the tensile performance of the beam at the front end of the node is enhanced in the natural state after the prestress is superimposed through section steel anchorage at the upper part, which is conducive to bearing and tension in the later stage.
4.2 after the prestressing force is superimposed in the node area of hysteretic curve and skeleton curve, according to the model analysis results, when the x-direction displacement of column top is increased to 7.5mm, the reaction force reaches the maximum.
When it is increased to 10.5mm, the resistance provided by concrete does not increase, and the reaction force reaches the minimum.
The hysteretic curve and skeleton curve are shown in Figure 5..
