The 1994 Northridge Earthquake (USA) has shown the vulnerability of welded moment connections in steel moment resisting frames (MRFs) subject to severe earthquake ground motions (EQGMs). Since then, as an alternative connection type, bolted connections, often called semi-rigid connections, are considered for the retrofit arid new design of steel MRFs in high seismicity regions. This paper investigates the seismic design and performance of a hybrid system, consisting of welded moment frames and bolted semi-rigid frames. For this purpose, an analytical study is carried out on two steel buildings with interior semi-rigid steel frames having beam-to-column connections with different strength and stiffness ratios. The two buildings, of 5 and 10 stories, are designed in compliance with the current seismic design codes for two different cases. In the first ease (Case 1), the interior frames are assumed to be simply connected. In the second case (Case 2), the interior frames are assumed to be semi-rigidly connected. The buildings are subjected to three representative earthquakes. The evaluation of the results indicates that making the interior frames semi-rigid can lead to less story shear and lower column and connection moments and will increase the lateral load capacity of the building.