الفهرس 
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Abstract
Circular hollow sections are most often found in heavy offshore structures subjected to oscillating wave loads. The joints between hollow sections are simple in form, but not simple when stress distribution and design calculations are considered. Extensive studies have been carried out on unstiffened joints, as a result of which their behaviour under various types of loading have been well understood. Methods of assessing their strengths have also been very well developed. But little effort has been made to study in detail the behavior of stiffened hollow section joints in intact conditions and the development of a methodology to assess their strengths. Information on the strength and behavior of these joints in their intact conditions are seldom available. They are useful in the development of guidelines for the design of such joints in offshore structures and also in the evaluation of the reduction in their strengths in case they are damaged due to various causes.
The behaviour and strength of both stiffened and unstiffened hollow section joints subjected to both axial compression force and in plane bending moment have been studied theoretically in this work. A nonlinear finite element model which allows for geometric non-linearities is constructed for the study using the nonlinear finite element program ABAQUS. A pre-study is performed in order to verify the developed numerical finite element model and its capability to investigate the behaviour of the joints.
Large numbers of joints with different chord and branch dimensions are selected, in order to draw complete Load-displacement curves and to study the different failure modes. Different strengthening techniques are used which are: a) Strengthening using external gusset plates, b) Strengthening using internal ring stiffeners, c) Strengthening using collar plates and d) Strengthening using knife plates. Strength equations are proposed for the case of using internal ring stiffeners.
Finally, comparison between different strengthening methods is made to find the most applicable strengthening technique.