The manner in which the central fold 18 provides stiffness to floor slab 10 is illustrated by way of reference to FIGS. 1 and 3 in combination. Floor slab 18 has a truncated V-section which includes a depressed planar portion 44 and a pair of inclined portions 45, 46. The upper extremities 47, 47' of the inclined portions 45, 46 of fold 18 are joined to major planar portions 15, 16 so that the fold provides an upwardly opening concavity.