Flat slabs

A flat slab is a concrete slab supported directly on columns, without beams. It gives a flat soffit, lower storey heights and fast, simple formwork, which is why it is common in offices, car parks and hospitals. The price is that the slab must carry the load to the columns on its own, and the most critical region is the small area around each column.

Bending in a flat slab is two-way. For design, each panel is divided into column strips over the column lines and middle strips between them. IS 456 allows the direct design method for fairly regular layouts, which distributes a total static moment between negative and positive regions and between strips, and the equivalent frame method for more general cases. Drop panels, local thickenings around columns, and column heads, flared tops, increase the depth where moments and shear are highest.

Punching shear is the critical check. The column tries to push through the slab along a truncated cone. IS 456 checks shear stress on a critical perimeter at half the effective depth from the column face, against an allowable value that grows with the square root of concrete strength and is reduced for elongated columns. When the stress is too high, the designer thickens the slab, adds a drop panel, enlarges the column, or provides shear reinforcement such as studs or links.

Flat slabs are flexible, so deflection and long-term creep must be checked, and openings near columns cut the punching perimeter. In earthquake zones a flat slab is not relied upon as the lateral system; walls or frames carry the seismic load, and the slab-column connections must still tolerate the drift.