Ring stiffeners are usually used to enhance the strengths of welded hollow section joints, which may change the magnitude and distribution of hot spot stress around intersections related to the fatigue life of the joints. Up to date, there is no relevant research on the hot spot stress and its calculation method for welded CHS X-joints with internal ring stiffeners in combined load cases, causing a lack of confidence in the fatigue design process. Experimental tests including two types of load cases are firstly carried out to provide a reference for the finite element (FE) modeling. FE analysis is conducted to investigate the influence of internal ring stiffeners on hot spot stress, covering a variety of numbers, thicknesses and spacing of internal ring stiffeners. The hot spot stress calculation method is developed. A fatigue test is subsequently conducted. It is shown that the hot spot stress near the intersection region of the ring stiffeners and tubes is relatively large in addition to those at the crowns and saddles. The locations of interest include both traditional locations and intersection locations of ring stiffener contours and tube contours. Setting ring stiffeners can averagely reduce the hot spot stress ranges by 23.6% in chord and 27.8% in brace, respectively. The extended hot spot stress calculation method is verified for the fatigue design of welded CHS X-joints with ring stiffeners. In the subsequent fatigue test, fatigue failure is found at the intersection locations. A dynamic effect is also observed which is related to the change in load eccentricity. The fatigue strength curve specified in CIDECT design guide No.8 could also be applicable to predict the fatigue life of the welded CHS X-joint with ring stiffeners.
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