Doctoral Thesis / Dissertation from the year 2022 in the subject Engineering - Civil Engineering, grade: Summa cum laude, , language: English, abstract: Steel structures, which are made of hollow section profiles are mainly used for construction structures. There are many fields of applications for structures made of hollow section profiles. Hollow section profiles are lightweight and have got a high resistance to compression, which is an advantage for column structures. The most common profile types have squared or circular hollow cross sections. Connecting two or more pipes by welding, the joints have got different shapes. These joints are part of trusses or columns with a tree-shaped optic. The standards have got restrictions regarding the geometry, material or load cases. By choosing a node, which is excluded in the standards, designing engineers have to create their own models. This thesis is about the designing of a Y-shaped steel joint, full overlapped on top connection, which is not defined in current standards up to the time the thesis is created. In this thesis only uniplanar joints are investigated. A special focus is on the welding line, which has got a three-dimensional shape. The aim is to figure out the influence on the inclination angle between the two members on the resistance of the joint. Beside this, the profile shape and profile thickness vary to see the differences in their resistance. Square and circular hollow section profiles are in focus. To analyse the behaviour of the different steel joints, a numerical and experimental investigation is executed. Design charts are created as a medium for designing engineers to calculate the ultimate resistance in the elastic and plastic state, depending on the steel profile, thickness and inclination angle. The structure of the thesis is split into four main parts. Firstly, there is an explanation of the general topic with its difficultness. Secondly, there is the state-of-the-art presentation of literature and standards in this range. Thirdly, there is a numerical analysis of the joints. The basement of the numerical studies is detailed in pre-studies, which explain and compare different mesh types or geometrical variations. Fourthly, there is an experimental verification of the numerical tests. It is figured out, if the inclination angle increase, the resistance of the steel joint will decrease significantly. This effect is non-linear. Beside this, it is found, that the full overlapped joint with the squared profile has got a higher resistance than a comparable joint with a similar circular hollow section.