SUMMARY It is assumed that a semi-rigid joints behaviour allows to reduce the beam size respect the pinned joints because they have some rotational stiffness, so the sagging moment decrease. In the other hand, leaving out the stiffeners, reduces the complexity of joints reluting in significant savings in the cost of fabrication. Although the economic benefits of semi-rigid joints are worthwhile, they are not really used in practice. Tradicionaly because previous standards have just provided methods for calculate the stress of the joint, and recently because, although the Annex J to Eurocode allows to calculate joint stiffness and resistance knowing the joint geometry, it doesn't include a design method for semi-rigid steel structures. Trying to establish a design strategy, and considerating that in general, minimun weigth solutions are obtained when the joints in the frame are rigid, this research work intends to analyse the real applicability of not-stiffened joints with similar behaviour than rigid ones (welded joints and bolted end-plate connections) In this way, they are analysed more than 1500 steel edification structures designed with this kind of joints where its real stiffness is considerated (calculated with the Eurocode component method), modelling the joints behaviour according the different possible ways provided by this code, analysing the structural frame response and checking its resistence considerating diferent-bay-storey frames and various beams and columns length, comparing the results to pinned and rigid alternatives, trying to provide a design guideline and a criteria about when it is advisable use this type of joints.