This work focused on the development of an efficient and economic composite building system. This was accomplished by the conception of two new composite major axis connection details for external and intermediate columns. The external column connection detail motivated the creation of a new connection element: the T-rib connector. The main purpose of this element is to transmit the reinforcement bars forces directly to the external columns in negative moment regions The present study also comprehended an nvestigation of two semi-rigid connections design models:the simplified analytical model of Kishi-Chen, [CHEN93], and the Eurocode 3 mechanical model,[EURO98], leading to further developments in both structural models. The first proposal extends the Kishi-Chen model, [CHEN93], for the external and intermediate columns connections investigated in this work. The second proposal incorporated the contributions of the T-rib connector, the web angles and the reinforcement bars on the EUROCODE3 model, since they were not originally taken into account. Both model were calibrated against the experimental results produced in the present investigation. An alternative solution for composite beams shear connectors was also developed. This solution consisted of an adaptation of the perfobond rib shear connector, generally used in composite bridges, to residential or commercial building construction. This investigation resulted in the development of a new test layout for the required experiments. These experiments enabled the determination of the individual contribution of the concrete dowels and reinforcement bars used in the perfobond rib connector holes. As far as it is known, these tests enable the calibration and evaluation V of each individual term of the perfobond rib resistance formula proposed by Oguejiofor and Hosain, [OGUE94]. A set of six tests was also made to characterise the T-Rib connector ultimate limit states. This procedure also required the development of a new pullout test procedure and layout especially designed for this purpose. To evaluate the global performance of the proposed building system a full-scale composite semi-rigid portal frame was built and tested up to collapse. The experimental programme provided information on real structural behaviour where the influence of connections, beam as beam column behaviour ads second order efectswere full investigated. This experimental research enabled the determination of: load versus rotation curves, an estimation of the maximum connection s bending forces, etc.