Stress Concentration Factor Of Large Eccentricity Tubular N-joints Under Axial Compressive Loading In Lap Brace

Large eccentricity tubular joints contain two parts:negative large eccentricity tubular joints and positive large eccentricity tubular N-joints. Negative large eccentricity tubular N-joints are used in welded trusses and offshore structures under fatigue loading. The stress concentration factors (SCFs) are main parameters in the fatigue reliability analysis of negative large eccentricity tubular N-joints. There are four negative large eccentricity tubular N-joint specimens were separately tested for the confirmation and standardization of finite element (FE) models by considering axial compression in lap brace case. Both joints were simply hinge supported at their chord and through brace end. We measure elastic strain distributions near the crown and saddle areas, and strain concentration factors (SNCFs) at hot spots were computed to utilize an extrapolation approach. The maximum stress concentration factor (SCF) of the welded joints occurred at saddle or crown point under axial compressive loading. For the FE analysis, hexahedral solid elements and certain mesh size were suitable for modeling the welded joints. Effect of β(brace to chord diameter ratio),2y(chord diameter to wall thickness ratio), τ(brace to chord wall thickness ratio) and ε(eccentricity ratio) on the SCF distribution of the negative large eccentricity tubular N-joints were numerically investigated. SCF linearly increased with the increment of eccentricity ratio at the through brace and lap brace intersection near through brace and at chord and through brace intersection near through brace. The effect of eccentricity ratio on SCF equation of tubular non-eccentricity and small eccentricity N-joints was not considered in current code. A group of parametric equations considering eccentricity ratio was purposed for predicting the SCF of negative large eccentricity tubular N-joints ground on 256 FE models. Based on the experimental and numerical investigations, the following conclusions can be drawn:the maximum SCF at the through brace and lap brace intersection near the lap brace or the through brace occurred on the saddle point; the maximum SCF at the chord and through brace intersection near the chord or the through brace occurred on the crown point; the effect of these parameters (β,2y, τ, and ε) on SCF is complex for the negative large eccentricity tubular N-joints under axial compressive loading in lap brace; SCF linearly increased with increment of eccentricity ratio at the through brace and lap brace intersection near through brace and at chord and through brace intersection near through brace; SCF formulae for the negative large eccentricity tubular N-joints under axial compressive loading in the lap brace at two intersection were proposed.This paper put forward the importance of stress concentration factors (SCF) of positive large eccentricity CHS N-joints under compression in vertical brace although the static behavior of the N-joints has been researched. The four positive large eccentricity CHS N-joints were first tested for the verification and calibration of finite element (FE) models. As eccentricity increased, the experimental results and FE analysis showed that the position of maximum SCF moved from the crown heel point to the saddle point at the chord and vertical brace intersection. At the inclined brace and chord intersection, the position of maximum SCF moved from the saddle point to the crown toe point with the decrease of eccentricity. A set of parametric formulae was proposed to predict the SCFs of positive large eccentricity CHS N-joints based on 256 FE models. For positive large eccentricity CHS N-joints, SCF increased as τ increased. As β and 2γ increased, SCF distribution was complex. SCF linearly increased with the increase of eccentricity to chord diameter ratio (ε) at the chord and vertical brace intersection welding seam. SCF of N-joint around inclined brace and chord intersection increased when the value of θ increased. The assessment of the proposed formulae and CIDECT formulae were based on the database of FE analysis results. Simple method for predicting SCFs of positive large eccentricity CHS N-joints given in CIDECT was unreliable. The new formulae of SCFs were proposed for positive large eccentricity CHS N-joints under axial loading, which were verified to be accurate and reliable. According to the experimental and numerical investigations, main concludes as follows: As eccentricity decreased, the position of maximum SCF moved from the saddle point to the crown heel point at the chord and vertical brace intersection welding seam; At the inclined brace and chord intersection welding seam, the position of maximum SCF moved from the saddle point to the crown toe point with the decrease of eccentricity; SCF linearly increased with the increase of eccentricity ratio at the chord and vertical brace intersection welding seam; The fatigue life of positive large eccentricity CHS N-joints obtained by using simple method in CIDECT at the chord and vertical brace intersection welding seam was dangerous, while at the chord and vertical brace intersection welding seam was conservative; SCF formulae for the positive large eccentricity CHS N-joints under axial loading are reliable and safe in the defined boundary conditions.