Balustrades and handrails, and the loads they are designed for

Balustrades and handrails, and the loads they are designed for

Balustrades get specified by the architect, priced by the fabricator and installed near the end of a job. They are structural elements carrying a life safety load, and the number of them I inspect that cannot take that load is higher than it should be.

Where a barrier is required

The National Construction Code requires a barrier wherever there is a fall of more than one metre from a trafficable surface. That includes balconies, landings, stairs, ramps, roof terraces and the edge of a raised floor.

The required height differs by location. A balcony or landing barrier is measured from the finished floor. A stair barrier is measured from the line of the nosings, which is lower, and the two are frequently drawn as the same height and then fail on site once the treads are in.

Openings and climbability

Openings in a barrier are limited so a small child cannot pass through. That limit governs the spacing of vertical members and the gap under the bottom rail, and it applies to the gap at the finished floor rather than at the structural slab, so a change in floor build-up can put a compliant design out.

In buildings where children are likely to be present, horizontal or near-horizontal elements within a defined band above the floor are restricted because they can be climbed. That is why a design with a horizontal mid-rail passes in one building class and fails in another.

The load it carries

AS/NZS 1170.1 sets a horizontal load applied at the top of the barrier, and it varies with occupancy. A domestic balcony is one figure. A public assembly area where a crowd can press against a barrier is several times higher.

That single number drives everything. A post spacing that works at the domestic load will not work at the assembly load with the same section and the same fixing.

There is also an inward and outward case, and an infill load applied to the panels themselves rather than to the posts.

Glass balustrades

Glass barriers are covered by AS 1288, and the two viable arrangements are different in principle.

Laminated glass keeps working after one pane breaks, because the interlayer holds the fragments and retains some capacity.

Monolithic toughened glass does not. If it breaks, it goes entirely, which is why that arrangement generally requires an interlinking handrail capable of holding the balustrade in place with a panel missing.

The other issue is the base. A glass balustrade fixed into a channel is transferring a substantial moment into whatever the channel is bolted to, and that concentrated load has to be checked against the slab edge, not assumed.

Where they actually fail

Almost never in the balustrade itself. Almost always in the fixing.

Anchors into the slab edge where the concrete cover is too shallow. Bolts into a topping slab rather than into structure. Fixings into a masonry parapet that was never designed for a lateral load. Chemical anchors installed at the wrong embedment or into a hole that was not cleaned.

On existing buildings, the additional problem is corrosion. A stainless post on a coastal balcony fixed with the wrong grade of bolt will look sound and have lost most of its capacity.

What to ask for

A design that states the occupancy and the load case it was designed to. A fixing detail that names the anchor, the embedment and the substrate. And a check against the structure receiving the load, which is the part that sits outside the balustrade contractor's scope and is the reason we get called after the fact.

We cover this within structural engineering, on new work and on remedial assessments of existing barriers in strata buildings.