The preferred embodiment of the floor slab construction of the present invention is illustrated by way of reference to FIG. 1 wherein one half of a single floor slab 10 is the remaining half being symmetrically and repeatively formed to the broken section at the right of the drawing. Floor slab 10 comprises a unitary slab of poured reinforced concrete material. The preferred embodiment is described in relation to a floor slab 10 which is formed at the base of a core tower such as 24 and thereafter hoisted to a preselected elevated position and fastened to the core tower, as described in more detail hereinafter. However, it will be understood that the present invention can be applied to other construction techniques as well.