Fatigue Strength Of Overlapping Tubular N-Joints

ABSTRACT The main objective of this study is to obtain the quantitative guidance for the fatigue design of saddle type tubular joints with overlapping braces on the chord surface. Systematically planned sixty-one specimens were totally tested under the completely reversed sine wave axial load (R =-1). All of them were isolated N-joints with a chord of 318.5mm in outer diameter. The specimens were classified into 13 groups in accordance with the parameters D/T, d/D, T/t and g/d, where D and T are outer diameter and wall thickness of the chord, d and t are outer diameter and wall thickness of the braces and g is gap or lap length of the braces on the chord surface, and another group of joints with in-plane gusset plate reinforcement. Materials used were mild steel with the specified ultimate strength of ?u ?402 N/mm2 and high tensile strength steel with ?u ? 784 N/mm2 (equivalent of ASTM A514 Grade F steel). The cycle range repeated for each group was N = 103 ~ 2 × 106. Characteristics of the test results were :The joints with a gusset plate reinforcement are inferior to those with a small positive gap length in the fatigue strength.By overlapping the braces on the chord surface, a fatigue life to failure of the joints is increased about 10 times as long as that of conventional gap-joints or joints with a gusset plate.The use of a high tensile strength steel in overlapping-joints has adequate effects on longevities of the joint fatigue life. INTRODUCTION Steel tubular structures in offshore are generally characterized by the direct welding of brace members upon the chord wall. In such connections, the stress concentration is so severe that the static and dynamic strength of the joints is much decreased. The problem is mainly caused by the matter that the axial forces of the braces directly act on the chord wall in the out-of-plane direction. In general, because the compressive and tensional forces act from the braces upon the chord wall and the sum of the vertical component forces to the chord is not so large, the joint strength is expected to be remarkably improved by direct cancellation of the vertical component forces among the braces, without passing through the chord wall. Overlapping of the braces is one of the methods for improving the joint strength by the above effect. It is known that those joints, that is, overlapping-joints give their connections sufficient efficiencies under the statical load. The empirical formulae for the prediction of static strength of these joints have already been established on the basis of numbers of previous test results.1,2 While it is also expected that the use of these joints may increase the fatigue strength of tubular structures, enough fatigue tests for the joint have not been conducted. The researches on the fatigue strength of these joints have been deferred by the technical difficulty of the edge preparation of the braces for their complicated cutting lines.