Study On The Bearing Capacity Of Concrete-filled Steel Tubular With Second Order Effect And Void

Concrete-filled steel tube(CFST)structure has the characteristics of high bearing capacity and good ductility,at the same time,the CFST structure is convenient for construction,which is widely used in bridges and high-rise buildings.In practical engineering,the void of concrete is normal and the second-order effect has a great influence on the stability of the structure.The ultimate bearing capacity analysis methods include incremental nonlinear finite element method(INFEM)and elastic modulus adjustment procedures(EMAPs).The latter determines the ultimate bearing capacity of the structure through linear elastic iterative analysis,which can effectively overcome the defects caused by the nonlinear iterative of INFEM,has higher calculation efficiency and accuracy.However,EMAPs are usually used in homogeneous steel structures,and it is difficult to consider the void and the second-order effect of the structure is not considered.Therefore,the elastic modulus reduction method for ultimate bearing capacity analysis considering second-order effect and void was established.The main contents are as follows:(1)The model of CFST material constitutive relationship was selected,which is suitable for bearing capacity analysis.Based on the experimental data of CFST components and structures,the influence of different CFST material constitutions on the calculation results of ultimate bearing capacity was studied and analyzed,and the suitable CFST material constitutions were selected.(2)The homogeneous generalized yield function of CFST member with second-order effect was studied and established.Based on a large number of component test data,the accuracy of 6 kinds of CFST codes was compared,from which the calculation formulas of axial compression,pure bending and compression bending bearing capacity of circular and rectangular CFST members were determined.By introducing moment amplification factor and stability factor,the influence of second-order effect on the stable bearing capacity of CFST members was considered.Furthermore,fiber model method and finite element method are used to analyze dumbbell-shaped CFST components.Based on the study of interpolation function of hoop coefficient,the homogeneous generalized yield function of rectangular,circular and dumbbell shaped CFST members was established by regression analysis.The experimental results show that the homogeneous generalized yield function has high accuracy.(3)The elastic modulus reduction method for the ultimate bearing capacity analysis of CFST structures with second-order effect is established.By using the homogeneous generalized yield function to define the element bearing ratio,the structural stiffness degradation in the high stress region was simulated by strategically reducing the elastic modulus of the high bearing element in the CFST structure,and then the ultimate bearing capacity of the CFST structure was determined by linear elastic iterative analysis.Then,the elastic modulus reduction method for ultimate bearing capacity analysis of CFST structures considering the effect of second order effect was established.Finally,by comparing the calculation results with the incremental nonlinear finite element method and the experimental results,it is verified that the proposed method has higher calculation accuracy and efficiency.(4)The influence of spatial load on the ultimate bearing capacity of CFST structures is studied.Firstly,finite element method and fiber model method were used to study the flexural capacity of different types of CFST around major and minor axes,and then the homogeneous generalized yield functions of CFST flexural members with different types of sections under spatial load are established.Furthermore,the EMRM and INFEM were used to study the bearing capacity of CFST frame and arch under spatial load.Finally,the influence of non-directional force effect on INFEM model and EMRM model is studied by comparing with experimental data.The results show that the INFEM model is affected by non-directional force,and the influence of loading device on the ultimate bearing capacity of arch must be considered.However,the EMRM established in this paper overcomes the influence of non-directional force,and the effect of loading steel cable can be ignored in the calculation model,and the load can be directly loaded on the arch rib.Therefore,the calculation results can reflect the bearing capacity of CFST arch under spatial load,and has higher calculation accuracy and efficiency.(5)The influence of the void of core concrete on the ultimate bearing capacity of CFST arch is analyzed.Firstly,the computational models of CFST under compression and pure bending were established by using the fiber model method.Then,the homogeneous generalized yield function of the void CFST member with the second-order effect was established by regression analysis.The ultimate bearing capacity of the void CFST arch with second-order effect was analyzed.Furthermore,the influence of different void rates and location on the ultimate bearing capacity of CFST arch is analyzed by numerical method.(6)The application of elastic modulus reduction method for ultimate bearing capacity analysis of CFST structure established in this paper is carried out in a 430-meter CFST arch bridge on the Qinghai-Tibet Plateau,and the bearing capacity of members and systems of CFST arch bridge is analyzed.The research shows that the EMRM in this paper can obtain the strength coefficient of the members in the elastic state at the first step of iteration,and obtain the strength coefficient of member before the structure enters the plastic state or even the failure state at the last step of iteration,so as to identify the high and low bearing members correctly.For the members with low bearing ratio in elastic stage and high bearing ratio in elastoplastic stage,the traditional design method usually reduces the cross-section strength of such members according to the calculation results in elastic stage,thus affecting the ultimate bearing capacity of the whole bridge.However,the method in this paper can correctly identify this type of member,and design the section of this type of member,so as to avoid the problem of conventional design,and ensure the bearing capacity and safety of CFST arch bridge.