Abstract: Disclosed are modular, multi-floor buildings wherein each floor comprises a plurality of semi-boxes which are cast from concrete as independent, rigid units and which are basically rectangular in shape, comprising mutually perpendicular walls but no floor or ceiling. The semi-boxes are arranged between horizontal concrete slabs, which function as both floors and ceilings, and a plurality of vertical posttensioning tendons pass through the semi-boxes and the slabs to bind the building together.