Cantilever
A member fixed at one end and free at the other, like a diving board.
Quick Answer
A cantilever is a beam, slab, truss or other structural member that is rigidly supported at only one end, with the other end projecting freely. Balconies, canopies and roof overhangs are common examples. The fixed end must resist both shear and a large bending moment, so the anchorage is the critical design and construction detail.
The Full Picture
A cantilever works like a diving board: one end is held firmly and the other extends into space. The load on the free part creates shear at the support and a bending moment that is largest right at the fixed end. For a given load, the moment grows quickly as the projection gets longer, so cantilevers are usually shallower at the tip and deeper at the support, and the engineer limits how far they can reach.
In buildings, cantilevers show up as balconies, entry canopies, building overhangs, signage supports and exterior walkways. They are made in cast-in-place concrete, steel and timber. The important detail is the support: the member has to be anchored back into the structure with a back-span or counterweight, with top reinforcement in a concrete slab or a rigid connection in steel. This anchorage works by resisting rotation, so it needs more than simply resting on a wall.
Cantilevers also bring practical building issues. They deflect more than simply supported members of similar size, so the engineer checks the tip deflection and sometimes builds in camber. Where a concrete slab or beam crosses the building envelope, the continuous structure can act as a thermal bridge, and designers often use special thermal break connections at balconies. Waterproofing and drainage at the edge, and the guard attachment, are further details that tend to be coordinated late.
For the contractor, a cantilever is a place where shoring and sequencing matter, since a cantilevered slab or beam cannot be considered self-supporting until its anchorage is complete and concrete has reached the strength the engineer requires. Top reinforcement in a cantilevered slab must stay at the correct elevation, and chairs or supports are carried in the rebar scope so the bars do not sink during the pour.
Real Examples
Common Misconceptions
People assume: A cantilever is just a beam that sticks out of a wall.
Actually: The wall alone does not make it work. The member has to be anchored so it resists both shear and rotation at the support, usually by extending back into the structure, with the reinforcement or connection to match.
People assume: You can extend a balcony slab further without changing anything.
Actually: A longer projection raises the bending moment at the support and the deflection at the tip. Extending an existing cantilever generally needs a new design from the structural engineer.
People assume: Cantilevers can be formed and stripped like ordinary slabs.
Actually: Stripping or removing shoring too early can overload the member. Shoring removal for cantilevers is governed by the engineer's required concrete strength or sequence.
Frequently Asked Questions
What is an example of a cantilever in a building?
Balconies, entrance canopies, roof overhangs and projecting floors are common. In each, one end is anchored to the structure and the other extends freely beyond the supporting wall or column.
Why do cantilevers need extra reinforcement?
The load on the free end creates a bending moment that is largest at the support and tends to put the top of the member in tension. Concrete cantilevers therefore have reinforcement near the top and anchorage extending back into the supporting structure.
Do cantilevers cost more?
Often a cantilever adds cost for reinforcement, connections, formwork or shoring, and edge details. How much depends on the projection, the material and the design, so there is no standard premium.
What problems occur at balcony cantilevers?
Typical issues include thermal bridging where the slab passes through the wall, water entering at the edge or the guard attachment, and deflection. Coordination with the envelope and waterproofing trades helps avoid them.