Beam Design (RCC/Steel)

Beam design is the structural sizing and reinforcement detailing of a horizontal flexural member that carries gravity loads from slabs and walls and transfers them to columns or walls. Per IS 456:2000 Cl. 22 and 26, an RCC beam is designed for ultimate moment (Mu), shear (Vu), and torsion (Tu) at the limit state with serviceability checks for deflection and crack width. The depth-to-span ratio rule of thumb (Cl. 23.2.1) gives basic spans/effective depth: 7 (cantilever), 20 (simply supported), 26 (continuous) — modified by reinforcement percentage and tension-flange contribution.

Main reinforcement is determined from Mu = 0.87 × fy × Ast × (d − 0.42 × xu), with xu/d ≤ xu,max/d (= 0.46 for Fe500) for under-reinforced design (the IS 456 default for ductile failure). Minimum tension steel per Cl. 26.5.1.1 = 0.205 × fck × b × d / fy (≈ 0.12% for M25 + Fe500). Maximum 4%. Shear reinforcement (vertical stirrups) is provided when Vu > τc × b × d (where τc is from Table 19), with spacing ≤ 0.75d or 300 mm (Cl. 26.5.1.5). For seismic frames (IS 13920 Cl. 6), tighter stirrup spacing is required at plastic hinge zones — typically d/4 or 100 mm, whichever less, with 135° hooks.

Practical Indian beam dimensions: residential 230 × 450 to 300 × 600, commercial 300 × 600 to 400 × 750, transfer beams 600 × 900 to 1000 × 1500. The single most-violated rule is the 'curtailment' clause (Cl. 26.2.3) — bars are often terminated wherever convenient rather than at the point of contraflexure plus development length. Modern detailing software (ETABS, STAAD) generates curtailment lengths automatically; site engineers must verify the BBS reflects them. Inadequate curtailment causes premature shear failure at the cut-off section, a common cause of distress in 1980s-1990s buildings.