A method for forming and assembling H-beams, the method comprising the steps of: - providing a first and a second mandrel , 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 including a planar face configured to form a surface of a web of a C-section , and two opposite side faces configured to form two respective surfaces of flanges of the C-section, each mandrel providing a distributed perforation on at least one of said outer faces ; 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 , the passages establishing fluid communication between the distributed perforation and said inlet/outlet connection ; - applying the respective C-section of fiber-reinforced polymerizable thermosetting composite material on three consecutive outer faces 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 adhering to the mandrel being rotated, - arranging the webs of the C-sections carried by the first and second mandrel facing one another; - bringing the two mandrels close to one another so as to bring the webs of the two C-profiles into contact with one another, thus obtaining an H-beam , - transferring the mandrels 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 , so as to facilitate the removal of the compacted H-beam from the mandrel . - providing a first and a second mandrel , 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 including a planar face configured to form a surface of a web of a C-section , and two opposite side faces configured to form two respective surfaces of flanges of the C-section, each mandrel providing - applying the respective C-section of fiber-reinforced polymerizable thermosetting composite material on three consecutive outer faces 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 adhering to the mandrel being rotated, - arranging the webs of the C-sections carried by the first and second mandrel facing one another; - bringing the two mandrels close to one another so as to bring the webs of the two C-profiles into contact with one another, thus obtaining an H-beam , - transferring the mandrels 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 , so as to facilitate the removal of the compacted H-beam from the mandrel . <EOS>