Cantilever

A cantilever is a structural beam fixed at one end and free at the other, projecting outward from its support without an anchor at the free end. The internal forces in a cantilever are concentrated at the fixed end — maximum bending moment at the fixed end, decreasing linearly to zero at the free tip; maximum shear at the fixed end, decreasing to zero at the tip; maximum deflection at the free tip. Per IS 456:2000 Cl. 22.2 + 23.2.1, cantilever design follows the same flexural and shear principles as simply-supported beams, with adjusted span/depth ratios and deflection limits.

Maximum bending moment for a cantilever of length L carrying UDL w: M = wL²/2 — twice the simply-supported moment for the same total load. For a point load P at the free tip: M = PL — four times the simply-supported moment for the same total point load. Deflection at the tip for UDL: δ = wL⁴/(8EI) — eight times the simply-supported deflection for the same total load. These large moments and deflections mean cantilevers are conservatively designed with span/depth ratio ≤ 7 per IS 456 Cl. 23.2.1 (compared to 20 for simply-supported, 26 for continuous).

Common Indian cantilever applications: (1) Balcony slabs and beams projecting from the building edge — typical 0.9-1.5 m projection; (2) Sunshade / chajja slabs above windows — typical 0.6-1.2 m projection; (3) Cantilever columns in water towers, transmission towers; (4) Cantilever trusses in long-span industrial buildings; (5) Highway sign gantries spanning travel lanes. The most-overlooked design detail is the top reinforcement — in cantilever beams, the top steel is the tension steel (negative moment); cover, anchorage past the support, and continuity into the back-span are all critical. The top steel must extend Ld + the back-span half-length past the support per IS 456 Cl. 26.2.3.1, otherwise pull-out failure occurs at the support.