T-Beam

A T-beam is a reinforced concrete beam cast monolithically with a slab, where the slab acts as the top flange of the T-shaped cross-section. Per IS 456:2000 Cl. 23.1.2, the slab participating in the T-beam flexural action — called the effective flange width — is computed from rules accounting for slab span, beam spacing, beam depth, and rib spacing. For interior T-beams: bf = (lo/6) + bw + 6Df, where lo is the distance between points of zero moment, bw is rib width, and Df is flange thickness. For end T-beams: half the interior value plus bw.

T-beams are highly efficient in flexure when the slab is in compression (positive moment regions of continuous beams). The effective flange acts as a wide compression block, allowing the neutral axis to remain near the slab and most of the cross-section to be in tension where steel is placed. For a typical 230×500 RCC beam supporting a 125 mm slab spanning 6 m: as a T-beam the moment capacity ≈ 280 kNm (with 4-T16 tension steel); as a rectangular beam ignoring the flange ≈ 165 kNm. The T-beam contribution roughly increases capacity by 70% with no additional steel — it is essentially free capacity from the existing slab.

Design requirements: bf must satisfy IS 456 Cl. 23.1.2 limits; transverse reinforcement in slab must extend over the flange to prevent flange-web separation under load (Cl. 26.5.1.7); compressive stress in flange ≤ 0.45 fck per IS 456 Cl. 38.1; for hogging moment regions (negative moment, top of beam in compression), the rectangular section governs because the slab is in tension. The most-overlooked design clause is the requirement for distribution reinforcement perpendicular to the main slab steel running across the T-beam — this provides the 'flange' restraint and prevents transverse cracking under concentrated loads near the rib.