The invention relates to a rigid screwed assembly system for metal structures, which is designed to fix secondary sections (3) to a H-shaped receiving section (2) or similar. According to the invention, accessories (1) are provided at the assembly points, said accessories being fitted into the receiving section (2), and the secondary sections are fixed using screwed attachments (4) which pass through the corresponding assembly parts of the sections (2 and 3) as well as the corresponding accessories (1).