Although High Strength Steel (HSS) has been primarily employed in mechanical applications, its use is becoming more common in the construction industry too, owing to its excellent structural properties. HSS is particularly advantageous for columns when the horizontal displacement of the building is not a limiting condition or when columns are stocky. Circular columns are suitable in structures subjected to loading in multiple directions, like welded steel frames, because of uniform geometrical characteristics along all cross-sectional directions. Therefore, HSS can be conveniently employed in tubular columns of moment resisting frames located in moderate seismic-prone regions. In this respect, the Research Fund for Coal and Steel (RFCS) project called ATTEL was funded in order to promote the use of HSS members with circular sections in buildings by investigating both seismic and fire behaviour with the aim to show possible benefits with respect to mild steel. In detail, the present work focuses on the experimental campaign of beam-to-column joints and of column base joints designed in order to withstand seismic actions. The joints are part of a moment resisting frame designed according to the capacity design where beams are made of mild steel whereas columns are made of HSS and they are filled with concrete. Numerical simulations performed with the aim of calibrating the seismic behaviour of joints against experimental evidence are also presented along with the analysis of the frame used to determine the behaviour factor and the overstrength factor of such structural typology.