Mechanical performance analysis of prestressed socket assembled concrete frame structure relying on computer simulation analysis

In order to improve the mechanical performance analysis of assembled concrete frame structure, this paper combines with computer simulation technology to analyze the mechanical performance of concrete frame structure, and uses the concrete damage plastic constitutive model provided by ABAQUS to define the nonlinear constitutive behavior of concrete. Based on the elastic-plastic model, the damage variables are added to the model, which can better reflect the performance of concrete under cyclic loading, such as stiffness degradation. In addition, this paper uses ABAQUS finite element analysis software to further analyze the structural parameters of this form of joints, and design related prestressed socket assembled concrete frame to study the seismic performance of prestressed socket assembled frame structure. Finally, the effectiveness of this model is verified by computer simulation.


Introduction
The construction industry has always been a labor-intensive industry, which has a series of problems such as waste of resources, environmental pollution, low efficiency, long cycle, high cost, uneven construction quality, etc.The root cause lies in the existing traditional construction methods in China.In actual engineering, concrete pouring is still wet operation on site, with strong labor dependence, different technical level of operators, low production efficiency, chaotic construction site, low utilization rate of resources and serious noise pollution.Compared with the traditional cast-in-place structure system, assembled building structure has the advantages of good construction quality, high construction efficiency, less environmental pollution, improving construction conditions, optimizing industrial structure and reducing engineering costs.
In traditional cast-in-place structures, the construction form of parallel reinforcement is usually used to solve problems such as design and construction difficulties caused by the inability to meet the requirements of steel bar diameter and dense reinforcement.At the connection of prefabricated columns, in order to ensure the continuity of the load-bearing steel bars and meet the requirements of equivalent cast-in-place, it is necessary to use grouting sleeves to connect each longitudinal steel bar.When there are a large number of longitudinal steel bars at the connection, a large number of grouting sleeves are required to ensure the connection of each longitudinal steel bar, which increases grouting time, grouting material dosage, and grouting sleeve construction quality inspection steps.Drawing on the research ideas of cast-in-place structures, the concept of parallel reinforcement is introduced into prefabricated concrete structures, and the equivalent diameter parallel reinforcement connection form is adopted at the longitudinal reinforcement connection to reduce the number of longitudinal reinforcement and reduce construction difficulty [1].
When steel tube concrete composite structures or components are subjected to external vertical loads, the steel tube and concrete will undergo longitudinal compression deformation and circumferential expansion deformation.Due to the different material properties between the two, the radial deformation of concrete filled in steel pipes develops rapidly in the early stage.The outer steel pipes will provide circumferential constraints for the concrete filled in the steel pipes, causing the concrete filled in the steel pipes to be in a three-dimensional stress state, delaying the development of plastic cracks in the concrete material.Concrete changes from brittle material to plastic material, and the failure form of concrete changes from brittle failure to plastic failure, Greatly improved the compressive bearing capacity of concrete [2].The failure of the bearing capacity of steel pipes is mainly due to the susceptibility of steel to instability and damage.The local instability of the steel pipe reduces the bearing capacity of the entire component, and the high-strength performance of the steel cannot be fully utilized.Filling concrete in the steel pipe can greatly improve the local instability problem of the steel pipe and fully utilize the performance of the steel material [3].
At present, steel tube concrete has been widely used in load-bearing structures of industrial and civil buildings, high-rise television towers and transmission towers, bridge piers, columns of rigid frame structures, and pressure bearing structures.The interaction and combination of steel pipes and concrete jointly exert their material properties, which has many advantages.The initial defects of steel have a significant impact on thin-walled steel pipes.Due to the existence of initial defects, the actual bearing capacity of thin-walled steel pipes can only reach 20% -40% of the theoretical calculation value.Filling concrete into the interior of thin-walled steel pipes not only forms a circumferential constraint on the filled concrete, but also avoids local instability of the steel pipes, leveraging the material properties of both, improving the load-bearing capacity of the entire component, and achieving "1+1 ˃ The actual effect of 2 " [4].
The supporting effect of the concrete inside the steel pipe and the confinement effect of the steel pipe on the concrete make the steel pipe concrete structure or component maintain a relatively high bearing capacity platform after reaching its peak bearing capacity, changing the failure mode of each component when acting alone, and demonstrating good plastic deformation ability.From a structural perspective, steel tube concrete has only two simple components; In terms of construction process, compared with traditional reinforced concrete structures, there are no complex processes such as steel bar binding and template making, which simplifies the construction process and accelerates the project progress.Due to its unique structural form, the fire resistance limit of steel tube concrete structures is higher than that of individual components when subjected to fire; After a fire, the damaged bearing capacity of the steel tube concrete structure can be restored to varying degrees with the decrease of external temperature, ensuring the stability of the overall structure [5].
The joint action of steel pipe and concrete fully utilizes the material properties of both, resulting in good economic effects for steel pipe concrete structures or components.A large amount of engineering practice has shown that compared with RC structures, it can reduce the self weight of the structure by half, and the cross-section of the components can also be reduced to half of the original; Compared with steel structures, the amount of steel used can be reduced by half under the same bearing capacity; In practical engineering, the construction process has been simplified, eliminating the process of formwork support and dismantling.The cost has been reduced by about 40%, and the labor force has been reduced by about 38%, resulting in significant economic benefits [6].Prefabricated concrete structure is composed of prefabricated reinforced concrete components connected through reliable connections.This structure has the advantages of high production efficiency, good component quality, less construction waste, and resource conservation, making it a good structural form.Prefabricated buildings have been developed for many years in countries such as Europe and America, and have been widely used.Although the traditional construction model has made a significant contribution to the development of the construction industry, its shortcomings such as uncontrollable quality, high waste, high pollution, high energy consumption, extensive management, soaring labor costs, cracking, and leakage have also become prominent.It has become a bottleneck in the development of the construction industry, and there is an urgent need for a new construction method to replace traditional construction methods [7].Building industrialization refers to the use of modern scientific and technological production methods in buildings to systematically integrate production factors at various stages of the construction process, achieving overall construction uniformity, and achieving orderly assembly of building components in factories through the positioning, on-site assembly, and integration of design and construction of building components.This streamlined production method in factories can improve living quality, production efficiency, and service life, while reducing costs and energy consumption.Prefabricated buildings have advantages that cast-in-place construction cannot surpass, but it cannot be denied that prefabricated nodes are the key to restricting the integrity, safety, and reliability of prefabricated prefabricated structures, and there are still shortcomings compared to cast-in-place construction.In the overall collapse of prefabricated buildings, the less damaged parts are mainly prefabricated components, while the most severely damaged parts are mainly at the connection points of the components, indicating that the weak links of prefabricated structures are mainly at the connection nodes [8].
Prefabricated buildings connect prefabricated components at the construction site.Considering construction operability and node performance under new conditions, the construction of connection nodes is relatively complex and the design difficulty is high [9].Compared to prefabricated building design methods, most structural engineers are more familiar with traditional cast-in-place concrete design methods.The connection nodes between prefabricated components are prone to stress mutations or weak sections, and their connection methods, workers' work level, and construction site environment have a significant impact on the performance of the connection nodes.It is difficult to detect and determine the quality of the joints after construction operations.If the performance of the nodes cannot be guaranteed, the structure is prone to instability [10].Prefabricated architecture refers to the industrial production of components such as shear walls, beams, columns, and stairs.The steel bars are reserved at the joints of prefabricated components when leaving the factory, and then the prefabricated components are transported to the construction site for assembly.Grout and anchor the prefabricated and cast-in-place parts through nodes to form a complete building [11].
In order to improve the mechanical performance analysis of prefabricated concrete frame structure, this paper takes prestressed socket prefabricated concrete frame structure as an example to carry out research, and combines with computer simulation technology to analyze the mechanical performance of concrete frame structure.

Computer finite element simulation analysis
The constitutive relation of materials is a set of relations that relate strain tensor and stress tensor, which can fully express the macroscopic mechanical properties of materials.The model proposed by this paper is composed of steel plate, bolt, steel bar, stirrup and concrete, and the stress state is complex.Therefore, accurately determining the constitutive model of steel and concrete is the premise of structural nonlinear analysis.
Reasonable determination of constitutive relation of material is the basis of linear calculation of model in finite element simulation analysis.Concrete material is the most basic raw material in engineering structure, which is mixed by water, cement, sand, stone and a small amount of admixtures according to a certain proportion.Moreover, it has the characteristics of diversity of composition materials, uneven internal structure, numerous micro-cracks, compressive strength far greater than tensile strength, and is also a typical brittle material.In this paper, the damage plastic constitutive model of concrete provided by ABAQUS is used to define the nonlinear constitutive behavior of concrete.Based on the elastic-plastic model, the damage variables are added to the model, which better reflects the stiffness degradation of concrete under cyclic loading.
Uniaxial constitutive relation is the basis of plastic damage model.The constitutive relation of concrete under uniaxial compression is shown in Figure 1.When determining the stress-strain relationship of concrete under uniaxial compression in ABAQUS, the stress-plastic strain relationship curve is adopted.In the curve shown in Figure 1, the strain ε needs to be converted into plastic strain pl ε , and the specific formula is  , where 0 E is the initial elastic modulus.In uniaxial tensile constitutive relation, the ultimate tensile strain of concrete is 0.001, the residual strength is t 0.1 f , and the value of t f is referred to the measured c f .The constitutive relation of concrete under uniaxial tension is shown in Figure 2. The function expression is shown in formula (1), where c f is taken according to the measured strength, and the other parameters are taken according to the Code for Design of Concrete Structures. under tension or pressure can be described as the tension constitutive relationship, The elastic strain el t 0t 0 σ ε E  in the formula is the initial elastic model of concrete.
The tensile plastic strain after considering damage is: The effective tensile stress is: In the formula,  As can be seen from Figure 4, before the concrete reaches the yield stress c0 σ , the relationship curve is a straight line, which shows that the concrete stress should become proportional.Then, the loading continues.After the concrete reaches the ultimate stress cu σ , the concrete enters the softening stage.The calculation formula of inelastic strain in t ε is as follows: The elastic strain el 0c ε is calculated as follows: The plastic strain pl c ε can be seen from Figure 3 as: From this, the calculation formula of compressive stress c σ can be obtained as follows: Therefore, the calculation formula of effective compressive stress σ is as follows: In the formula,  Compared with concrete materials, the mechanical properties of steel are more uniform and stable, and the stress-strain relationship is generally obtained by tensile test, and the of relation model is relatively mature.In this paper, the ideal elastic-plastic model is adopted for ordinary tensile steel bars and steel plates.The stress-strain relationship of steel bars is shown in Figure 5.The mathematical expression is shown in formula (11).

Mechanical property analysis
Considering the convergence and time cost of frame calculation, the grid of concrete, steel bar and prestressed steel strand set to 80mm, the grid of beam-column steel sleeve is set to 50mm, the grid of rigid cushion block is set to 100mm, and the grid of bolt is set to 100mm.
When low cyclic loading is applied, the boundary condition of each frame is simplified as setting the column bottom of the frame to be completely fixed in the initial analysis step, and the displacement and rotation angle in X, Y and Z directions are restricted.When seismic wave is applied, gravity load and seismic wave analysis step are added after preload analysis step, and in seismic wave analysis step, the constraint of complete consolidation of column bottom is changed into the constraint of displacement in Y and Z directions and rotation angle in X, Y and z directions to release displacement in x direction.The finite element model is shown in Figure 6.

Figure 6. Finite element model of frame
The limit value of elastic and plastic interstory displacement angle is 1/550 and 1/50 respectively.The story height of the frame model in this paper is 2.2 m, and the limit value of elastic and plastic interstory displacement is 4mm and 44mm respectively.When the horizontal load is lower than 85% of the peak load, the seismic performance of the frame can be better studied by appropriately amplifying the maximum inter-story displacement.Horizontal displacements are applied at the beam-column joints of the second story, and the displacements are loaded to 56mm by 4mm, 8mm and 12mm increments.
Seismic wave is a typical dynamic load.Under the action of earthquake, the seismic response of the structure is related to the natural vibration characteristics of the structure itself.The natural vibration characteristics of the structure are also called modal, and the free vibration analysis is also called modal analysis, and modal analysis is the basis of seismic wave loading.Then, modal analysis of prestressed socket assembled concrete frame optimized by this model is carried out.
This test refers to the code "Code for Seismic Test of Buildings" (JGJ/T101-2015) to adopt the loading system of force-displacement hybrid control.Force control is adopted before the specimen yields, and displacement loading control is adopted after the specimen yields.The controlled displacement is the displacement when the specimen yields, and the increment of displacement loading is an integer multiple of the yield displacement.In the force control stage, the load of each stage is cycled once, the first three stages in the displacement loading stage are cycled three times, and the other stages are cycled only twice until the specimen is finally destroyed.The loading system is shown in Figure 7, and the specific loading process is as follows: (1) The application of axial force at the top of the column: the same static test.(2) Application of low cyclic loading at beam end: Before loading, in order to check whether the test equipment works normally, 2kN load is applied at beam end in advance and repeated twice.During the formal loading, the MTS hydraulic actuator is used for low cycle repeated loading, and the loading system is as mentioned above.In the force control stage, the load increment of each stage is 4kN.The criterion for judging the yield state of specimens is based on the comprehensive judgment of yield load and yield displacement obtained from static test and load-displacement curve during test (the curve is obviously bent).
The test ends when one of the following conditions is met: a.The beam is seriously buckled and the load value drops to 85% of the maximum load; b.The concrete exfoliates in a large range and enters a large deformation state; c.Other accidents involving safety.8 shows the loss under prestressed loading.Combined with the experimental analysis of this paper, the following points are summarized.There are three main failure modes of joints under repeated loading.The first is shear failure in the core area of the joint when the joint does not yield; The second is that there is no shear failure in the core area of the joint during yielding, but shear failure gradually occurs after yielding.The last one is that there is no shear failure in the core area of the joint, but the beam or column yields or the section fails.The beam-column joint specimens in this test are designed according to "strong column and weak beam", so no cracks are observed in the core areas of beams and columns, and all of them are damaged by beam hinge mechanism, and they belong to the third failure mode, and the goal of "strong column and weak beam are stronger joints" is achieved.

Conclusion
Prefabricated buildings have become the development trend of the construction industry because of their advantages of energy saving, environmental protection and short construction period.Moreover, with the introduction of various policies on prefabricated buildings, prefabricated structures have emerged.In order to improve the mechanical performance analysis of prefabricated concrete frame structure, this paper combines computer simulation technology to analyze the mechanical performance of concrete frame structure, and based on the experimental study of prestressed socket prefabricated concrete frame structure joints, this paper uses ABAQUS finite element analysis software to further analyze the structural parameters of this form of joints.In addition, this paper designs related prestressed socket assembled concrete frame to study the seismic performance of prestressed socket assembled concrete frame structure.Finally, this paper directly finds the maximum load, failure load, and corresponding displacement of the specimen through the load-displacement curve or skeleton curve, and verifies the effectiveness of the model proposed in this paper through computer simulation research.

Figure 1 .Figure 2 .
Figure 1.Stress-strain curve of concrete under uniaxial compression complete damage of concrete.

Figure 3 .
Figure 3. Stress-strain relationship curve of concrete tensile cracking The compressive constitutive relation is shown in Figure 4.As can be seen from Figure4, before the concrete reaches the yield stress complete damage of concrete.

Figure 4 .
Figure 4. Inelastic stress-lesion relationship curve of concrete under compressionCompared with concrete materials, the mechanical properties of steel are more uniform and stable, and the stress-strain relationship is generally obtained by tensile test, and the of relation model is relatively mature.In this paper, the ideal elastic-plastic model is adopted for ordinary tensile steel bars and steel plates.The stress-strain relationship of steel bars is shown in Figure5.The mathematical expression is shown in formula(11).

Figure 5 .
Figure 5. Stress-strain curve of steel bar and steel plate

Figure 8 .
Figure 8. Loss under prestress loading Figure8shows the loss under prestressed loading.Combined with the experimental analysis of this paper, the following points are summarized.There are three main failure modes of joints under repeated loading.The first is shear failure in the core area of the joint when the joint does not yield; The second is that there is no shear failure in the core area of the joint during yielding, but shear failure gradually occurs after yielding.The last one is that there is no shear failure in the core area of the joint, but the beam or column yields or the section fails.The beam-column joint specimens in this test are designed according to "strong column and weak beam", so no cracks are observed in the core areas of beams and columns, and all of them are damaged by beam hinge mechanism, and they belong to the third failure mode, and the goal of "strong column and weak beam are stronger joints" is achieved.4.ConclusionPrefabricated buildings have become the development trend of the construction industry because of their advantages of energy saving, environmental protection and short construction period.Moreover, with the introduction of various policies on prefabricated buildings, prefabricated structures have emerged.In order to improve the mechanical performance analysis of prefabricated concrete frame structure, this paper combines computer simulation technology to analyze the mechanical performance of concrete frame structure, and based on the experimental study of prestressed socket prefabricated concrete frame structure joints, this paper uses ABAQUS finite element analysis software to further analyze the structural parameters of this form of joints.In addition, this paper designs related prestressed socket assembled concrete frame to study the seismic performance of prestressed socket assembled concrete frame structure.Finally, this paper directly finds the maximum load, failure load, and corresponding displacement of the specimen through the load-displacement curve or skeleton curve, and verifies the effectiveness of the model proposed in this paper through computer simulation research. )