A flat slab is a two-way slab generally of uniform thickness, but it may be thickened¬†or otherwise strengthened in the region of columns by a drop panel, while the top¬†of the column below the slab may be enlarged by a capital (round) or bracket¬†(prismatic). If a drop panel is used to increase depth for negative reinforcement,¬†the minimum side dimensions of this panel are L3 /3 and L2 /3, where L1 and L2 are¬†the center-to-center spans in perpendicular directions. Minimum depth of a drop¬†panel is 1.25h, where h is the slab thickness elsewhere.
A waffle flat slab or waffle flat plate consists of a thin, two-way top slab and¬†a grid of joists in perpendicular directions, cast on square dome forms. For strengthening¬†around columns, the domes are omitted in the drop panel areas, to form a¬†solid head, which also may be made deeper than the joists. Other variations of¬†waffle patterns include various arrangements with solid beams on column centerlines¬†both ways. Standard sizes of two-way joist forms are given in Table 9.21.
The drop panel increases shear capacity. Hence, a solid flat slab can ordinarily¬†be designed for concrete for lower strength than for a flat plate. Also, deflection of¬†a flat slab is reduced by the added stiffness that drop panels provide.
The depth of drop panels can be increased beyond 1.25h to reduce negativemoment¬†reinforcement and to increase shear capacity when smaller columns are¬†desired. If this adjustment is made, shear in the slab at the edge of the drop panel¬†may become critical. In that case, shear capacity can be increased by making the¬†drop panel larger, up to about 40% of the span. See Fig. 9.31 for bar details (column¬†strip).
Waffle flat plates behave like solid flat slabs with drop panels. Somewhat higherstrength¬†concrete, to avoid the need of stirrups in the joists immediately around the¬†solid head, is usually desirable. If required, however, such stirrups can be made in¬†one piece as a longitudinal assembly, to extend the width of one dome between the¬†drop head and the first transverse joist. For exceptional cases, such stirrups can be¬†used between the second row of domes also. See Fig. 9.32 for reinforcement details.