رفتار استاتیکی اتصالات X شکل سخت‌شده با حلقه بیرونی تحت بار محوری کششی

Tubular members, due to their convenient equipment installation and high-strength performance, are widely applied in the support system of offshore platforms such as jack-ups and jackets. In most steel tubular structures; the circular hollow section (CHS) members are mainly joined using welding. Commonly, one or more braces are welded directly onto the surface of a chord member to form that so named welded connection. So far, some techniques to improve the performance of tubular connections have been proposed. Most of these methods (e.g., internal ring, doubler plate) can only be used for structures during the design, but there are only a few techniques (e.g., outer ring, FRP) which can be applied during both fabrication and service. This paper studies the static strength of CHS X-joints reinforced with external ring subjected to axially tensile load. The SOLID186 in ANSYS version 21 was used to establish the finite element (FE) models of the tubular X-joints. Validation of the FE model with experimental data showed that the present FE model can accurately predict the static behavior of the external-ring stiffened and un-stiffened tubular X-joints under tension. Afterwards, 143 FE models were generated and analyzed to investigate the effect of the joint geometry and the external ring size on the ultimate strength, failure mechanisms, and initial stiffness through a parametric study. In these models, both geometric and material non-linearity were considered. Moreover, the welds joining the chord and brace members were modeled. Results indicated that the ultimate strength of the ring stiffened X-joints under brace tension can be up to 289% that of the ultimate strength of the corresponding un-stiffened joint. Also, the increase of the β (the ratio of the brace diameter to chord diameter) results in the increase of the ultimate strength and initial stiffens (in a fixed chord diameter). Because, the increase of the β leads to the increase of the brace diameter. The increase of this member results in the increase of the joints stiffness. In addition, the decrease of the γ (the ratio of the chord radius to chord thickness) leads to the remarkable increase of the ultimate strength. Also, the increase of the τ (the ratio of the brace to chord thickness) leads to the increase of the ultimate strength (in a fixed chord thickness). However, it is not remarkable. Moreover, the comparison between failure modes of reinforced and un-reinforced joints showed that the ring can significantly improve the failure mechanisms. Also, the ring can remarkably increase the initial stiffness. Despite this significant difference between the ultimate strength, failure mode, and initial stiffness of unreinforced and ring reinforced X-joints under brace tension, the investigations on this type of the reinforced joints have been limited to only three X-joint tests. Also, no design equation is available to determine the ultimate strength of X-joints reinforced with the external ring. Therefore, the geometrically parametric study was followed by the nonlinear regression analysis to develop an ultimate strength parametric formula for the static design of ring stiffened X-joints subjected to brace tension. The proposed formula was evaluated based on the UK DoE acceptance standard.