Composite Steel And Concrete Floors

A steel beam and a concrete slab sitting on top of each other are two members. Connect them so they cannot slip relative to one another and they become a single, far deeper member.

That is composite action, and the gain is substantial: a composite beam can carry considerably more than the same beam acting alone, or carry the same load at a much shallower depth.

Why it works

In bending, the top of a member is in compression and the bottom in tension.

Concrete is good in compression and poor in tension. Steel is excellent in both. Put the concrete on top where the compression is and the steel underneath where the tension is, and each material is doing what it is best at.

The catch is that the gain only exists if the two cannot slide against each other at their interface. Without connection, the slab bends about its own axis and the beam about its own, and the capacity is just the sum of two shallow members.

The shear studs are the whole system

Headed studs welded to the top flange of the beam and embedded in the slab are what transfer the horizontal shear at the interface.

Which makes them the critical component. Three things about them matter.

The number and spacing determine how much composite action is achieved. Full composite action requires enough studs to transfer the full force; fewer studs give partial composite action, which is a legitimate and commonly used design with reduced capacity.

The weld quality, because a stud that is not properly fused transfers nothing. Stud welding is done through the deck in most cases, and it depends on clean, dry, unpainted steel and correct equipment settings. Wet deck or a painted flange produces studs that look identical and fail under load.

Position relative to the deck ribs, because a stud in an unfavourable position within a profiled deck rib has reduced capacity. This is a detailing item with a real capacity consequence.

Studs are testable by bend test on a sample, and on a project of any size they are.

Profiled steel deck

The deck does two jobs and they are often confused.

During construction it is the formwork, spanning between beams and carrying the wet concrete and the construction load without propping, or with propping where the span requires it. That is a temporary works check and it governs the deck selection as much as the finished condition.

In service it acts as the bottom reinforcement of the slab, which is why the slab needs far less conventional reinforcement than a solid slab would.

Which means the deck has to be continuous, correctly lapped, and fixed down, and any penetration cut through it is cutting reinforcement. That connects straight to [core drilling and penetrations in existing slabs](/blog/core-drilling-and-penetrations-in-existing-slabs) when the floor is later modified.

Propped or unpropped

The decision that changes the design more than any other.

Unpropped, meaning the steel beam carries the wet concrete alone and composite action only exists for the loads applied afterwards. Faster to build, no props below, and the beam has to be sized for the construction condition, which is frequently the governing case.

Propped, meaning the wet concrete is carried by props and the composite section carries everything once the concrete has gained strength. A lighter beam, and a slower build with props in the way, plus the deflection consequences of loading a young slab, which run into [formwork and propping cycles](/blog/formwork-and-propping-cycles).

The two produce genuinely different beams, so the construction method is a design input stated on the drawings rather than a site choice.

Where it goes wrong

Studs welded through a wet or painted flange, the most consequential and least visible error.

Fewer studs than drawn, because a bundle ran short and nobody counted.

Penetrations cut late, removing deck and interrupting the composite zone near a support.

Deflection under the wet pour, on an unpropped beam, which ponds the concrete, which increases the load, which increases the deflection. Precamber exists to manage it and it has to be specified.

Vibration, because composite floors are efficient, which means long spans and relatively low mass, which is exactly the recipe for a floor that satisfies deflection and feels lively. That is the subject of [floor vibration and footfall](/blog/floor-vibration-and-footfall-in-apartments) and it is the most common complaint on long-span composite floors.

Fire, because the steel beam is exposed below and needs protection, while the slab above has inherent resistance. The protection system is part of the design rather than an addition, which ties into [fire engineering integration](/blog/fire-engineering-integration-structural-design).

Assessing an existing composite floor

The question is almost always whether composite action can be relied on.

Which means establishing whether studs exist, at what spacing, and whether the deck is still continuous after years of penetrations. Scanning, local exposure and the original shop drawings are the usual route, and where studs cannot be confirmed, the conservative assessment treats the beam and slab as non-composite, which is a large reduction.

That is why the shop drawings matter long after construction, and it is part of the general problem in [assessing an existing building with no drawings](/blog/assessing-an-existing-building-with-no-drawings).

ACSES provides structural engineering for commercial and residential projects across Sydney. Talk to us about a project.