Investigation On SCFs Of Circular Chord And Square Braces K-joints Under Balanced Axial Loading

Steel tubes have been widely used in space truss structures increasingly because of their excellent mechanical performance and their aesthetic appearance.The braces in the multi-planar tubular truss mainly bear the effects of pressure-bending or tension-bending and the chords mainly axial force.Due to the circular steel tubes section of the same moment of inertia in all directions,so in the space truss in the selection of circular tubes as the charge.Square steel tubes because of its strong bending performance were selected as the space truss in the braces.The K-joints are widely used in offshore platform structures under fatigue loading,so it has important significance to study the fatigue behavior of K-joints under fatigue loading.The stress concentration factor is an important parameter in the S-N curve(hot spot stress-fatigue life curve),so in this paper,the stress concentration factor(SCF)of the axial stress of the vertical chord is studied by the combination of experiment and numerical simulation.In this paper,six specimens are designed to test the strain gradient distribution in the intersecting line region of the elastic stage and verify the validity of the finite element model.The boundary conditions of all the test nodes are chosen to be simply supported on the shear frame.The load mode is chosen to be axial pressure applied at the end of the vertical the chord with hydraulic jacks.According to the research of a large number of scholars,it can be concluded that the maximum strain concentration position of K-joints may appear on the intersecting line of weld edge.However,due to the fact that the strain cannot be measured at the edge of the weld during the test,strain extrapolation is used to estimate the strain at the saddle and crown.The results show that the location of the maximum strain concentration is mainly at the crown of the round chord and the brace,and the stress concentration factor(SCFs)of the six groups are less than 0.4.Then the stress concentration factor of this type of joints is analyzed by nonlinear finite element software ABAQUS.Considering various factors,an effective and accurate finite element model is established to carry out a large number of parameter analyses.Based on the finite element model and the finite element analysis of the six test joints,the finite element model is compared with the finite element analysis,and the rationality and applicability of the finite element model are verified.Then,three dimensionless parameter ?(ratio of brace side length to chord diameter),2?(ratio of chord diameter to chord thickness),?(ratio of brace thickness to chord thickness),and125 circular chord and square braces K-joints and 256 concrete-filled in chord of circular chord and square braces of K-joints are designed in this paper.According to the analysis of the results of experiment and numerical simulation,the conclusions are drawn as follows: 2?(ratio of chord diameter to chord thickness)and ?(ratio of brace thickness to chord thickness)to stress concentration factor And ?(ratio of brace side length to chord diameter)has little effect on the stress concentration factor of the K-jointsof circular chord and square braces.The stress concentration factor of the chord is mainly in the vicinity of the saddle point,and the position of the stress concentration coefficient value of the braces mainly depends on the geometric parameters of ?,2 ?,and ?.In the end,the regression formula of the maximum stress concentration coefficient of the circular chord and square braces of the K-joints is obtained and it is proved that the parameter formula is safe and reliable.