Abstract: An improved method and apparatus for structures comprising a plurality of substantially identical structural cells, each comprising a cube having concrete walls of selected or calculated thickness. The walls enclose a hollow interior so that each cube is floatable. The walls are constructed so that tendons can be passed inside the walls in orthogonal directions. The tendons are tensioned after the structure has been assembled so as to prestress all walls along at least two dimensions so as to provide a three dimensional posttensioning effect on the concrete in the structure. Means are also provided which communicate with each of the cells for selectively filling or emptying each cell with liquid. The cells can be joined together at a drydock on land or at sea to form the structure.