Highly Effective And Accurate Linear Elastic Iteration Method For Ultimate Bearing Capacity Analysis Of Concrete-filled Steel Tubular Structures

Concrete-filled steel tubular(CFST)is widely used in long-span bridges and high-rise buildings for its superior mechanical property,convenient construction,low cost and good durability.The ultimate bearing capacity analysis is the basis for safety evaluation and design of CFST structures,and has been widely recognized in engineering and academia.However,the current analysis is mainly based on the incremental nonlinear finite element method(INFEM),which depends on the loading path and modifies the material constitutive relations to simulate the failure process of structures.Hence,INFEM involves a large number of nonlinear iterative analysis on material level,which leads INFEM to be time-consuming,poorly stable and sensitive to calculation parameters.For this,through establishing homogeneous generalized yield functions(HGYF)for CFST members with different cross sections,a highly effective and accurate linear elastic iteration method is developed for evaluating the ultimate bearing capacity of CFST structures.The main works are as follows:1)Through collecting and selecting the test results of CFST members and structures at home and abroad,a test database for ultimate bearing capacity is developed including 1708 CFST members and 23 structures.Among them,the number of members with rectangular,circular and dumbbell-shaped cross sections are 720,934 and 54,respectively,and the number of truss and arch structures are 11 and 12,respectively.The test database provide an objective basis for verifying the analysis method for CFST structures.2)The modified constitutive relations of steel tube are presented by taking into account the different stress states of steel tube and the effect of steel strength.Meanwhile,considering the enhancement of concrete strength caused by the steel constraint,the expression for peak stress of core concrete is derived.Then the modified constitutive relation of core concrete is developed by determining the up and down curves,peak strain and residual stress coefficient.By comparing with the test data and the existing constitutive relations,the proposed modified constitutive relations are verified to achieve wide applicability and higher accuracy;3)The compression strength are determined by means of test database for CFST members with rectangular,circular and dumbbell-shaped sections.Meanwhile,taking into account the ultimate limit state of CFST members under pure bending and the strain harden of steel,the bearing capacity failure criterion is proposed,and then the plastic developing coefficient and bending strength formulas are derived on the basis of regression analysis and fiber model method.By comparing with the existing failure criteria and bending strength formulas,the proposed failure criterion is demonstrated to represent the bending capacity more precisely,and the proposed bending strength formula can achieve higher accuracy;4)Taking the effect of hooping and stability coefficients into consideration,the homogeneous generalized yield functions(HGYF)under compression and bending for rectangular,circular and dumbbell-shaped CFST members are presented on the basis of fiber model method.The results of HGYF agree well with the test data under compression and bending.Taking the influence of rise to span ratio into consideration,a modified stability coefficient for parabolic single-tube arch is developed,on the basis of which the HGYF suitable for the stability bearing capacity analysis of arches is obtained.Moreover,the HGYF can overcome the drawbacks of the generalized yield function whose results vary with the initial loading,and can avoid the complex material level nonlinear iteration analysis;5)The homogeneous generalized yield function is used to define the element bearing ratio.The elastic modulus adjustment strategy is derived from the conservation principle of deformation energy,and the elastic moduli of highly stressed elements are reduced to simulate the internal force redistribution of structures.Then,the linear elastic iteration method is developed for the ultimate bearing capacity analysis of CFST structures with various cross sections,whose results agree well with test results and can achieve higher efficiency and stability than incremental nonlinear finite element method;6)The self-adaptive criteria for identifying the highly and lowly stressed elements is developed by means of the reference of element bearing ratio.The element bearing ratio and maximum element bearing ratio are adopted to define the member and structure safety factors,on the basis of which the quantitative relation between the two-level safety factors is presented.Hence,the proposed method overcomes the drawbacks of current design procedure that cannot distinguish the second kind highly stressed elements precisely and lacks the two level safety factor relations.And then,the proposed method is adopted to evaluate the component and structural safety of an under-constructed 430m half-through CFST arch bridge and lays a foundation for its future optimization.