Stress concentration factors of negative large eccentricity tubular N-joints under axial compressive loading in vertical brace

Abstract

Negative large eccentricity tubular N-joints sometimes are used in welded trusses and offshore structures under fatigue loading. It is imperative that researches should be done on fatigue behavior of negative large eccentricity tubular N-joints under fatigue loading, though static behavior of negative large eccentricity tubular N-joints has been researched. The stress concentration factors (SCFs) are the main parameters in the fatigue behavior analysis of negative large eccentricity tubular N-joints. Four negative large eccentricity tubular N-joints specimens were separately tested for calibration and verification of finite element analysis models by considering axial compression in vertical brace case. All experimental joints were simply hinge supported at one end of chord and inclined brace end. Elastic gradient strain distributions along the intersection of joints were measured. Strain concentration factors (SNCFs) at hot spots including saddle and crown points were computed taking advantage of an extrapolation approach. As eccentricity decreased, the position of maximum SCF moved from the saddle point to the crown point at the inclined brace and vertical brace intersection near the vertical brace or the inclined brace. The maximum SCF at the chord and inclined brace intersection near the chord or the inclined brace occurred on the crown point. For the FE analysis, solid elements and certain mesh size were used to model the N-joints. Effects of brace diameter to chord diameter ratio (β), chord diameter to thickness of chord ratio (2γ), brace wall thickness to chord wall thickness ratio (τ) and eccentricity ratio (ε) on the SCFs distribution of the negative large eccentricity tubular N-joints were numerically investigated. SCF linearly increased with the increase of eccentricity ratio at the inclined brace and vertical brace intersection near inclined brace and at chord and inclined brace intersection near inclined brace. The effect of eccentricity ratio on SCF equations of small eccentricity tubular N-joints was not considered in current code. A series of parametric equations considering eccentricity ratio were proposed for calculating the SCFs of negative large eccentricity tubular N-joints based on 256 joints FEA models calculation result.