Influence of semi-rigid connections on behaviour of steel frames is analyzed in this thesis. The entire thesis is based on a steel frame, part of a fictitious multi-storey building. Firstly the beams and columns are designed, which is followed by the first chapter concerning the connections, in which the moment capacity of the connections is calculated using the component method. The joints are designed as bolted extended end plate connections. In the following part the initial rotational stiffness of the connection Sj,ini is calculated, also using the component method. Using moment capacity and stiffness, the moment-rotation diagram, which defines the behaviour of the connection, is defined. A geometrically nonlinear numerical analysis is then performed in SCIA Engineer 2013, where joints are modelled as nonlinear springs. From the results, the critical load coefficient acr, horizontal displacement of the upper storey and the rotation of joints are considered. Furthermore the thickness of the end plate is then altered and the effect on the results listed before is considered.