Effective Flange Width (T-Beam)

Effective flange width (bf) is the limited portion of a monolithic slab that can be assumed to act together with the beam web as the compression flange of a T- or L-beam in flexure. The whole slab does not act fully because of 'shear lag' — compressive stress is highest over the web and falls off across the slab — so the code prescribes a conservative effective width that, used at uniform stress, gives a safe equivalent flexural capacity.

IS 456 Cl. 23.1.2 gives bf for a T-beam as the least of (l0/6 + bw + 6Df), the actual available width (centre-to-centre of adjacent beams), and (bw + clear span between beams)-type limits, with separate (smaller) expressions for L-beams and isolated beams, where l0 is the distance between points of zero moment. Because the flange greatly increases the compression area, T-beam action usually keeps the neutral axis within the flange, producing economical, shallow, ductile beams — ignoring it (designing as rectangular) is overly conservative and uneconomical.