Valance panel structure which includes: - a first shell which extends in a longitudinal direction and comprises: - a flat bottom wall which includes two opposite longitudinal edges (12a, 12b), - a flat side wall (14a, 14b) which is connected to each longitudinal edge (12a, 12b) of the bottom wall, said side walls (14a, 14b) being parallel or substantially parallel and extending in a perpendicular or a substantially perpendicular manner with respect to the bottom wall, - two joining rims (16a, 16b) which are parallel or substantially parallel to the bottom wall and are each connected to a longitudinal edge (15a, 15b) of a side wall, said edge being remote from said bottom wall, - a second shell which extends in the same longitudinal direction as the first shell and has a substantially U-shaped section, said second shell including two joining rims (22a, 22b) which are applied against the joining rims (16a, 16b) of the first shell and are fixed to the same so that the first and second shells form a hollow body, - an inner reinforcement which extends in said longitudinal direction and is situated inside said hollow body, characterized in that said inner reinforcement includes three flat walls (32, 34, 36) which are arranged in pairs at right angles or substantially at right angles so as to form a section with a form similar to a Z, a longitudinal free edge of a first flat wall being inserted between two assembled joining rims (16a, 22a) of the first and second shells, at least over part of the length of said joining rims, a third flat wall which is parallel or substantially parallel to the first wall being applied against the bottom wall of the first shell and fixed to said first shell. - a first shell which extends in a longitudinal direction and comprises: - a flat bottom wall which includes two opposite longitudinal edges (12a, 12b), - a flat side wall (14a, 14b) which is connected to each longitudinal edge (12a, 12b) of the bottom wall, said side walls (14a, 14b) being parallel or substantially parallel and extending in a perpendicular or a substantially perpendicular manner with respect to the bottom wall, - two joining rims (16a, 16b) which are parallel or substantially parallel to the bottom wall and are each connected to a longitudinal edge (15a, 15b) of a side wall, said edge being remote from said bottom wall, - a flat bottom wall which includes two opposite longitudinal edges (12a, 12b), - a flat side wall (14a, 14b) which is connected to each longitudinal edge (12a, 12b) of the bottom wall, said side walls (14a, 14b) being parallel or substantially parallel and extending in a perpendicular or a substantially perpendicular manner with respect to the bottom wall, - two joining rims (16a, 16b) which are parallel or substantially parallel to the bottom wall and are each connected to a longitudinal edge (15a, 15b) of a side wall, said edge being remote from said bottom wall, - a second shell which extends in the same longitudinal direction as the first shell and has a substantially U-shaped section, said second shell including two joining rims (22a, 22b) which are applied against the joining rims (16a, 16b) of the first shell and are fixed to the same so that the first and second shells form a hollow body, - an inner reinforcement which extends in said longitudinal direction and is situated inside said hollow body,