A method for forming and assembling H-beams, the method comprising the steps of: - providing a first and a second mandrel (22, 22', 50, 50'), each mandrel having a rigid body with an elongated shape along a given axis (A, A') and a substantially rectangular or trapezium-like cross-section, the mandrel providing three consecutive outer faces (24, 26, 27, 54a, 55a, 56a) including a planar face (24, 56a) configured to form a surface (14b, 20b) of a web (14, 20) of a C-section (10, 16), and two opposite side faces (26, 27, 54a, 55a) configured to form two respective surfaces (12a, 13a, 18a, 19a) of flanges (12, 13, 18, 19) of the C-section, each mandrel providing a distributed perforation (34, 58) on at least one of said outer faces (24, 26, 27, 54a, 55a, 56a); at least one inlet/outlet connection for connecting the mandrel to at least one external source of vacuum and/or pressurized air; and one or more passages within the mandrel (22, 50), the passages establishing fluid communication between the distributed perforation (34, 58) and said inlet/outlet connection; - applying the respective C-section (10, 16) of fiber-reinforced polymerizable thermosetting composite material on three consecutive outer faces (24, 26, 27, 24', 26', 27', 54a, 55a, 56a, 54a', 55a', 56a') of each mandrel, - rotating at least one of said first and second mandrel about an axis parallel to said given axis (A, A'), while applying an at least partial vacuum to the passages within the mandrel being rotated, so as to retain the C-section (10, 16) adhering to the mandrel (22, 22', 50, 50') being rotated, - arranging the webs (14, 20) of the C-sections (10, 16) carried by the first and second mandrel facing one another; - bringing the two mandrels close to one another so as to bring the webs (14, 20) of the two C-profiles (10, 16) into contact with one another, thus obtaining an H-beam, - transferring the mandrels (22, 22', 50, 50') and the H-beam to a compacting workstation; - compacting the H-beam by hot forming; - injecting pressurized air in the inner passages of at least one mandrel (22, 22', 50, 50'), so as to facilitate the removal of the compacted H-beam from the mandrel (22, 22', 50, 50'). - providing a first and a second mandrel (22, 22', 50, 50'), each mandrel having a rigid body with an elongated shape along a given axis (A, A') and a substantially rectangular or trapezium-like cross-section, the mandrel providing three consecutive outer faces (24, 26, 27, 54a, 55a, 56a) including a planar face (24, 56a) configured to form a surface (14b, 20b) of a web (14, 20) of a C-section (10, 16), and two opposite side faces (26, 27, 54a, 55a) configured to form two respective surfaces (12a, 13a, 18a, 19a) of flanges (12, 13, 18, 19) of the C-section, each mandrel providing - applying the respective C-section (10, 16) of fiber-reinforced polymerizable thermosetting composite material on three consecutive outer faces (24, 26, 27, 24', 26', 27', 54a, 55a, 56a, 54a', 55a', 56a') of each mandrel, - rotating at least one of said first and second mandrel about an axis parallel to said given axis (A, A'), while applying an at least partial vacuum to the passages within the mandrel being rotated, so as to retain the C-section (10, 16) adhering to the mandrel (22, 22', 50, 50') being rotated, - arranging the webs (14, 20) of the C-sections (10, 16) carried by the first and second mandrel facing one another; - bringing the two mandrels close to one another so as to bring the webs (14, 20) of the two C-profiles (10, 16) into contact with one another, thus obtaining an H-beam, - transferring the mandrels (22, 22', 50, 50') and the H-beam to a compacting workstation; - compacting the H-beam by hot forming; - injecting pressurized air in the inner passages of at least one mandrel (22, 22', 50, 50'), so as to facilitate the removal of the compacted H-beam from the mandrel (22, 22', 50, 50').