The composite structures studied in this thesis involve concrete slabs with profiled steel sheeting supported by steel beams. The composite interaction is realized by providing shear connectors welded to the top flange of the steel beams. The columns are normally noncomposite. The steel beam-to column connections are either designed as pinned or designed as rigid. Recent research has shown that semi-rigid joint behaviour may be achieved in composite joints by considering only a small amount of reinforcement over the connections. However due to the lack of design provisions and a simple and effective analytical method, semi-rigid frame design is rarely adopted by engineers. In this thesis the behaviour of semi-rigid composite connections, composite beams, and composite frames is investigated. Simple and effective finite element analysis models for composite joints, composite beams and composite frames are proposed. The models are validated against published tests on composite joints and beams. Three composite frames are analyzed and the results are compared with different proposals. Satisfactory agreements are achieved. Design recommendations are promoted for semi-rigid composite frames. Seismic analysis of composite frames is performed using the proposed frame model. The influence of semi-rigid joints on the overall performance of composite frame is investigated. The proposed method is so simple and straightforward that it provides an effective tool for both static and dynamic analysis of composite semi-rigid structures.