Abstract: This invention relates to a pre-cast, prestressed hollow core beam unit for use in a two-way building system and is adapted to be supported in continuous spanning relationship on two or more building supports. The beam unit has floor panel supports along opposite edges thereof, and comprises an elongate, continuous body of pre-cast, prestressed concrete having a plurality of hollow cores extending longitudinally throughout its length, with the hollow cores being selectively openable through the top surface of the beam unit to vary the design characteristics of the beam unit. In use, at least one of the hollow cores is opened through the top surface of the beam unit along at least a portion of its length, preferably at a point of high stress where the beam rests on a support, for example.