Punching Shear In Flat Slabs

A flat slab supported directly on columns with no beams is efficient, fast to build and gives a clean soffit. It also concentrates the entire load of a floor panel into a small area around each column.

The failure mode there is punching shear, and it is one of the few concrete failures that arrives without useful warning.

What the failure looks like

The column pushes a roughly conical plug of concrete through the slab. The crack starts at the column face on the tension side and propagates outward and downward at an angle through the slab depth.

It is a brittle failure. Unlike a bending failure, which deflects and cracks visibly over a long period before it gives up, punching shear can go from apparently sound to collapsed with very little deflection.

Worse, it propagates. A slab that punches at one column drops, which transfers its load to the neighbouring columns, which can then punch in turn. That progressive mechanism is why punching shear has caused a disproportionate share of the serious flat-slab collapses on record, several of them during construction.

What governs it

The slab depth, which is the single largest factor. Shear capacity scales with the effective depth, and a flat plate that is too thin around its columns cannot be fixed by adding bending reinforcement.

The column size and shape, because the critical perimeter is measured around the column. A larger column has a longer perimeter and more capacity.

Concrete strength, which contributes, though less than depth.

Unbalanced moment, meaning the moment transferred from slab to column. This is the factor most often underestimated. At an edge or corner column, or anywhere spans are unequal, moment transfer adds shear stress on one side of the perimeter and the effective capacity drops well below the concentric value.

Openings near the column, which cut the critical perimeter. A service penetration placed near a column by a trade on site can remove a meaningful proportion of the shear capacity, which is why [penetrations in existing slabs](/blog/core-drilling-and-penetrations-in-existing-slabs) are never a site decision.

How capacity is increased

In rough order of preference.

Thicken the slab, which is the most reliable answer and the one with programme and height consequences.

A drop panel, a local thickening around the column. Effective and it complicates the formwork.

A column capital or flared head, which enlarges the perimeter.

Enlarge the column, which helps the slab and may not suit the architecture.

Shear reinforcement, meaning closed ligatures, stud rails or shear heads within the critical zone. Standard practice and detail-sensitive: the reinforcement only works if it is anchored correctly and placed in the right zone, and it is one of the densest, most congested areas of reinforcement in a building, which connects straight to [reinforcement detailing](/blog/reinforcement-detailing-and-congestion).

Higher strength concrete, which gives the smallest return per dollar of the options here.

Where it goes wrong in practice

Edge and corner columns, where the perimeter is already reduced and the moment transfer is highest. More punching problems originate at edges than at internal columns.

A penetration cut late, for a duct, a pipe or a cable tray, inside the critical zone.

Stud rails or ligatures placed wrongly, at the wrong spacing, or stopping short of where the critical perimeter actually sits.

Loads that changed, meaning a slab designed for one use and later loaded for another. A plant platform, a planter box, a storage area, or anything in the [change of use](/blog/change-of-use-structural-assessment) category.

Construction loading, where stacked materials or a propped slab above put more load on a young slab than the finished structure ever will. This is where several recorded collapses happened, and it is why [backpropping](/blog/backpropping-and-temporary-works) is a designed system rather than a site habit.

Assessing it in an existing building

It is checkable, with the original drawings, a confirmed slab thickness, confirmed reinforcement and a confirmed concrete strength. Without drawings it means investigation: cover meter, scanning, and in some cases local exposure of the reinforcement, which is the ordinary process for [assessing a building with no drawings](/blog/assessing-an-existing-building-with-no-drawings).

The outcome matters, because the remedy for insufficient punching capacity on an existing slab is usually a steel collar, a bonded plate or a new column, and all of them are more disruptive than getting the thickness right on the drawing.

ACSES provides structural engineering and structural assessment across Sydney. Talk to us about a project.