Behaviour of in-service CHS gap K-joints strengthened with external stiffeners

This paper evaluated the behaviour of CHS gap K-joints strengthened under service load by welding in-plane external stiffeners,with a special focus on the combined effects of service load and welding process. Two identical joints were strengthened under service load but with different methods,one with stiffeners only while the other with additional cover plates,and were subsequently loaded to failure. External stiffeners significantly enhanced the joint stiffness and strength by enlarging the bearing area of the chord face and also helping the tension and compression forces in the braces to be counterbalanced above the chord face. The addition of cover plates to the chord and braces was able to enhance the strengthening effect by avoiding local ovalisation of the CHS members caused by force transmission through the stiffeners. The strengthening process by welding resulted in residual deformation reflected by a plateau on the load-deformation curve,which was unbeneficial to the ultimate resistance of strengthened joints with a maximum reduction of up to 10%. Complementary coupled thermal-mechanical FE models allowing for the effects of welding under service load,were developed and validated against the test results. A mechanical model was proposed to predict the ultimate resistance of strengthened joints,with the effects of geometric properties,service load,and welding process taken into account. The proposed model was validated against the results from comprehensive parametric FE simulations including a total of 976 models,and proved accurate for in-service CHS gap K-joint strengthened with external stiffeners.