A projecting structural member that is fixed (restrained against rotation and translation) at one end and free at the other, carrying applied loads along its length or at the free end by developing bending moments and shear transmitted to the fixed support.

Principle

A fixed support provides reaction shear and a resisting moment equal to the internal bending demand at the support; maximum bending moment for a uniformly loaded cantilever occurs at the fixed end and deflection increases rapidly with unsupported length.

Demonstration

Illustrative scenario → Situation: A balcony slab extends beyond a building façade as a cantilever. Recognition: Designer treats the overhang as a cantilever. Action: Reinforcement and section and support detail are sized for large negative moment at the support and for serviceability deflection limits. Consequence: Loads at the free edge induce high support moments and shear; inadequate reinforcement or insufficient support stiffness causes cracking, excessive deflection, or failure at the support.

Misapplication

Mistaken interpretation: Labeling any projecting element as a cantilever when it actually has a secondary support or continuity (e.g., a continuous beam with an overhang). Semantic error: Failing to recognize support restraint conditions leads to wrong moment and deflection estimates and improper reinforcement detailing.

Consequence

Correct recognition leads to concentrated design effort at the fixed support (moment transfer, reinforcement, anchorage) and serviceability checks for deflection and cracking; incorrect assumptions about restraint produce under‑designed supports and possible local failure or serviceability problems.

Reversal

If the 'fixed' end is actually partially restrained (semi‑rigid) or supported by a counterweight or tie, the simple cantilever moment distribution changes—support moment is reduced and load path can convert to a continuous or propped condition; long‑term effects like creep or settlement may also alter restraint and behaviour.

Boundary

Clearly within: an element rigidly fixed at one end with the opposite end free. Boundary case: a continuous beam with a large overhang—behaviour near the overhang approximates a cantilever but global continuity alters moments. Clearly outside: simply supported beams, propped cantilevers with an additional support that changes the internal moment diagram.

Semantic Tension

Support restraint versus serviceability — designing for the large support moments of a cantilever gives structural safety but can conflict with control of deflection, cracking and architectural slenderness.

Synthesis

A cantilever’s behaviour is governed by the degree of rotational restraint at its support; correct classification of support conditions is essential because small changes in restraint dramatically change internal moment distribution and serviceability demands.