A structural beam which comprises a first beam and a reinforcement to be attached to it. The first beam has a substantially U-shaped cross-section along at least a first portion of its length and comprises a bottom wall and two sidewalls wherein the bottom wall comprises a substantially flat first joining region. The reinforcement of the structural beam has a substantially flat second joining region which may be welded to a first joining region. Both parts of the structure are made of hardened UHSS. Methods for manufacturing such beams are also provided.