Concrete over Metal Deck

Concrete over Metal Deck (Composite Floor Systems)

The composite floor system (a concrete slab cast over corrugated metal deck, connected to supporting beams by shear studs) is the dominant floor construction method in steel-framed commercial, office, and multi-family buildings. It combines the rapid erection speed and long-span capability of steel framing with the stiffness, fire resistance, and acoustic mass of a concrete slab.

The result is a floor system that is structurally efficient, fast to build, and familiar to every steel fabricator and concrete subcontractor in the country.

How Composite Action Works

Shear studs (headed steel anchors) are welded to the top flange of the supporting beam through the metal deck after erection. When concrete is poured over the deck and cures around the studs, the slab and beam are locked together mechanically. Under load, the slab acts in compression and the beam acts in tension: the ideal use of each material’s strength.

Partial composite design (typically 25–75% composite) is common: enough studs to meaningfully stiffen and strengthen the beam without the cost of full composite. The degree of composite action is tuned to the required stiffness for deflection and vibration control.

Slab Thickness and Deck Profile

Total slab thickness is typically 3½" to 4½" above the top of the deck ribs for standard applications. The deck profile (usually 1½" or 2" deep for composite applications, 3" for longer spans) determines the minimum slab thickness and the usable concrete area for fire ratings.

Normal-weight concrete (150 pcf) is common for floors requiring acoustic mass or fire rating. Lightweight concrete (110 pcf, typically with expanded shale aggregate) reduces dead load and improves fire rating performance, which can reduce the number of shear studs required.

Fire Resistance

Composite deck-and-slab assemblies achieve 1- to 3-hour fire ratings without spray-applied fireproofing on the deck itself, depending on slab thickness and deck depth. UL-listed assemblies define the exact combination of deck gauge, concrete thickness, reinforcing, and support conditions required to achieve a given rating.

Engineering Considerations

Design involves confirming the deck’s ability to carry wet concrete during the pour, the composite beam’s strength and stiffness in service, and the slab’s capacity to act as the building diaphragm. Concrete cracking over supports, re-entrant corners at openings, and shrinkage control reinforcing are details we address early to avoid costly corrections in the field.

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Frontier Structural Engineering brings 20 years of commercial and residential design experience to projects across Colorado and California. Whether you're in schematic design or already in the field, we're available.