Results of the Jaco project: Effect of in-plane and out-of-plane loading on the cracking behaviour of tubular joints during large-scale tests

Tubular joints are common in offshore structures, among others for foundations for wind energy convertors. In the JaCo project, large-scale tests were performed in-plane and out-of-plane on 45° joints. A consistent different cracking behaviour was observed between joints loaded in-plane compared to joints loaded out-of-plane. The in-plane loaded joints fail by cracking through the brace, after crack initiation at the crown heel and a limited crack growth in the width. The stiffness of the joint is also almost constant until failure. Cracks initiate at the saddle points in out-of-plane loaded joints and propagate along the weld toes, reaching a very large width. A significant decrease of the joint stiffness is observed at failure, allowing for a redistribution of the load during the test. Although the fatigue life in design rules is described by the same S-N curve, it was found that the fatigue life for in-plane loading is shorter than for out-of-plane loading, highlighting a limitation of the current hot spot method.