Abstract: A metal beam includes a top flange; a bottom flange; and an intermediate web joining together the top flange and the bottom flange. The bottom flange includes an upper deck and a lower deck joined at their inner sides to the intermediate web in parallel spaced relation to each other to form a double-deck structure. The space between the upper and lower decks is occupied by tensioning elements embedded in concrete for prestressing the beam. The top flange is of smaller width but of substantially greater thickness than the bottom flange lower deck. In one described embodiment, the intermediate web is of a single wall construction; and in a second described embodiment, it is of a double-wall construction and also receives tensioning elements and concrete for prestressing the beam. Also described is a building structure in which the metal beams are used for supporting horizontal floor panels.