Unified method for predicting the fatigue life of pipe–sphere joints in grid structures

Abstract

Pipe–sphere joints (PSJs), which are commonly used in grid structures, are susceptible to fatigue failure under cyclic loading caused by suspended cranes. This paper presents a unified method for predicting the fatigue life of PSJs, including the weld toes on both the pipe and sphere surfaces, based on equivalent structural stress. A decomposition and recombination fitting (DRF) method was proposed to determine the optimal functional form of stress concentration factors (SCFs). A nonlinear regression analysis was conducted on the calculated results of 61 common PSJs to obtain a semi-analytical expression for the structural stress of the weld toes. Using this unified method, the fatigue life of the weld toes on both the pipe and sphere surfaces was estimated. The results indicated that the logarithmic ratios between the predicted fatigue life and experimental results were typically 0.93–1.08 for weld toes on pipe surfaces and 0.97–1.13 for weld toes on spherical surfaces, confirming the accuracy of the method. This unified method is applicable to predict the fatigue life of PSJs of various sizes and involves concise mathematical calculations independent of finite element analysis, thereby facilitating engineering applications. The DRF method can be used as a reference for fitting SCFs to specific structures. Furthermore, this prediction method enables the identification of the failure modes in PSJs. As the size of the steel pipe increased, the fatigue failure gradually shifted from the pipe surface to the sphere surface.