Two-Way Slab

A two-way slab is supported on all four edges and bends in both directions when the longer-to-shorter span ratio Ly/Lx ≤ 2. Load is shared between the two spans roughly in proportion to the fourth power of the inverse span lengths, so the shorter span carries the larger share and needs more steel.

IS 456 Annex D tabulates bending-moment coefficients (αx, αy) for nine edge conditions (interior, one/two/three/four edges discontinuous etc.) giving Mx = αx·w·Lx² and My = αy·w·Lx². Restrained (monolithic) slabs additionally require torsion reinforcement at the corners (Cl. D-1.8). Two-way action gives thinner, more economical slabs than one-way for square-ish panels, which is why grid + flat-slab framing is popular for large floor plates. Deflection is again controlled by the IS 456 Cl. 23.2 span/depth rule using the shorter span.