Cantilevered awnings and balconies

A cantilever is a structure supported at one end only. A balcony projecting from a building, an awning over a footpath, a carport roof with posts on one side.

They are elegant and they are unforgiving, because there is no second support to take over if the first one deteriorates.

The mechanics in one paragraph

Load on the free end of a cantilever produces a large bending moment at the fixed end, and it tries to rotate the whole thing.

Resisting that requires two things at the support: a connection strong enough to take the moment, and enough weight or hold-down behind it to stop the fixed end lifting.

In a concrete balcony, the top reinforcement carries the tension and the slab inside the building holds it down. In a steel awning, bolts and a back-span do the same job. Get either wrong and the cantilever rotates.

Concrete balconies: the top steel

In a cantilevered concrete slab, the tension is at the top. That is the opposite of a normal simply supported slab, where the tension is at the bottom.

Which means the top reinforcement is the critical steel, and it sits close to the top surface, under whatever finish is applied, in the part of the slab most exposed to water.

Two failure paths follow from that.

Corrosion. Water penetrating the top surface reaches the steel. Corroding steel expands, cracks and spalls the concrete, and loses section. Because it is the tension steel in a cantilever, losing it is losing the structure.

Top steel in the wrong place. If the reinforcement was placed too low during construction, the effective depth is reduced and the slab is weaker than designed. This is a construction defect that only reveals itself under load or investigation.

Every cantilevered balcony therefore depends on its waterproofing. Waterproofing on a balcony is not a finish, it is structural protection.

Steel and timber awnings

The failure points are different and equally predictable.

The fixing into the building. Bolts into masonry that has weathered, into mortar joints, or into a wall never designed for the load. Corroding fixings in a coastal environment.

Uplift. An awning is a large flat surface in wind, and the uplift case frequently governs over the downward load. Fixings adequate for weight are not necessarily adequate for suction.

Back-span hold-down. Where the cantilever is balanced by a back-span, the far end has to be held down. That connection is often the weakest and least inspected part.

Corrosion at the connection. Water tracks along a member and sits at the fixing, which is where the section matters most.

The load nobody designed for

Cantilevers are commonly loaded with things added later.

Planters full of wet soil on a balcony. Air conditioning condensers. Screening that turns an open balustrade into a solid wind-catching panel. Tiling and a screed added over the original finish, which adds permanent weight and raises levels so water no longer drains.

Each is modest on its own. On a cantilever, where the moment at the support is the load multiplied by its distance out, load at the free end counts for far more than the same load near a wall.

Warning signs worth acting on

Cracking on the underside near the wall, or on the top surface at the support line.

Rust staining, spalling concrete or exposed reinforcement on the soffit.

A balustrade that has become loose, which often indicates movement in the slab it is fixed to.

Water staining under the slab, which means the waterproofing has failed and the steel is being exposed.

Any visible deflection or a change in the line of the edge.

What an assessment involves

Establishing what the structure is, which usually requires investigation rather than assumption on an older building.

Locating and assessing the condition of the tension reinforcement or the fixings.

Checking the current load, including everything added since construction.

Assessing the connection and hold-down at the support.

Specifying the repair or strengthening, and the waterproofing that stops the same thing happening again.

Why this one matters

Cantilever failures are among the more serious structural failures because they occur without redundancy and usually in an occupied space.

We assess balconies, awnings and cantilevered structures under structural engineering, including remediation design and the certification a strata scheme or a council needs.

If a balcony in a building you own or manage has staining, cracking or a loose balustrade, it is worth looking at before it is worth reporting.