Semi-rigid connections between I-section beams and tubular columns can be used economically for buildings and offshore structures. The lack of stiffening plates allows the fabrication of these connections in a cost effective way. Furthermore, by taking into account the connection strength and stiffness, the connections are also economically in terms of material use. By filling the tubular column with reinforced concrete, the connection strength and stiffness are increased. With this concrete filling sufficient fire resistance can be achieved without the need of external fire protection. The strength and stiffness of the connection can be further increased in case of a composite steel-concrete floor. Current design codes and recommendations lack sufficient information on the strength of I-beam to tubular column connections to design these connections efficiently. The research consists of an experimental and a numerical investigation on the static strength and behaviour of multiplanar connections between I-section beams or plates and circular hollow section columns. The influence of a reinforced concrete filling of the columns and the effect of a composite floor or a steel floor on the behaviour and strength of the connections is also included in the research. The results of the experimental work are used to calibrate numerical models. Generally, a good agreement is found between the numerical and experimental results. With the calibrated models an extensive parameter investigation has been carried out. The main geometrical parameters are varied as well as the loading conditions. Analytical models have been derived based upon Togo's Ring model. These models, in combination with the finite element results form the basis of newly developed strength formulae for the investigated connection types. The strength formulae derived can form the basis for future design recommendations.