Cantilever

A cantilever is a beam or slab that is held at one end only, with the other end projecting freely into space. Balconies, overhanging upper floors, eaves and entrance canopies usually work this way. For the architect it is a way to push a building beyond its footprint and give it a sense of lightness; for the engineer it means a large bending moment concentrated at the support.

How a cantilever works

A beam resting on two supports shares its load between them. A cantilever sends all of it back to a single fixed connection. That is why the section gets deeper near the root, reinforcement is concentrated there, and the deflection at the tip is checked carefully. In a concrete frame, a cantilevered slab needs an internal span or beam behind it to balance it; in steel, the beam is often extended back into the building or tied with a hanger. In seismic regions, long and heavy cantilevers need extra care, and permitted lengths are set by the structural design and the applicable codes.

Cantilevers in design and in renders

A well-resolved cantilever has a clean soffit and a slim edge. Getting there means thinking early about tapering the slab towards the tip, adding a drip detail and lighting the underside. In a render, the shadow under a cantilever is one of the strongest cues of depth on a facade, so choosing a sun angle that makes that shadow readable pays off.