Abstract: A load-bearing building system employing a plurality of standard steel columns and a plurality of precast reinforced concrete slabs of certain standard room sizes. A plurality of such slabs are mounted at every floor elevation in side by side relationship. Concrete is then poured between adjacent slabs to provide, together with ribs of the slabs, a load-resisting space frame and floor decks serving simultaneously as horizontal diaphragms and room ceilings. The slabs are two-way flat plates having perimeter ribs reinforced with bars and light gauge corrugated steel which serve as shear reinforcement and as non-reuseable framework for the ribs. The slabs are each supported at four points. The space frame's composite girders comprises steel reinforcement together with poured-in-place and prefabricated concrete acting integrally. This construction has special moment connections, providing positive bending in the girders, regardless of the horizontal load direction.