Key takeaways
- Composite deck and non-composite (form) deck have completely different structural roles and different tables apply.
- Construction stage load — wet concrete plus live load — often governs on short spans; composite stage governs on longer ones.
- Propping at mid-span during the pour allows lighter gauge or longer spans at the cost of added site complexity.
- Construction deflection (span/180) must be checked to prevent concrete ponding during the pour.
- Profile depth, gauge and propping arrangement are the three variables that can be traded against each other to optimise cost.
What floor deck load-span tables show
A floor deck load-span table shows the maximum uniformly distributed load (UDL) that a given deck section can carry over a given span under specified conditions. The table is the primary engineering tool for selecting deck depth, gauge and prop arrangement for a composite or non-composite slab.
Three loading conditions appear in most published tables: construction stage (wet concrete plus construction live load, before the concrete has cured), composite stage (permanent load after the concrete has gained strength, with or without propping during construction), and fire condition (reduced load at elevated temperature). The governing condition depends on the span and the structural design.
Composite versus non-composite deck
In a composite floor system, the cured concrete and the steel deck act together as a single structural element. The deck profile — specifically the embossed ribs on the deck surface — provides mechanical interlock with the concrete, allowing the composite section to carry much higher loads than the deck alone. Composite design is used in the great majority of commercial and industrial multi-storey construction.
Non-composite deck (also called form deck) acts only as permanent formwork during construction and carries no structural load in service — all service loads are carried by the concrete slab and the primary structure below. Non-composite deck is thinner and cheaper but requires more concrete thickness to achieve the same structural depth.
Specifying composite or non-composite affects which table applies, which deck profiles are acceptable, and whether shear studs must be welded through the deck to the beam flanges below.
| Parameter | Composite deck | Non-composite (form) deck |
|---|---|---|
| Structural role | Deck + concrete act together | Deck is formwork only |
| Embossment required | Yes — for concrete interlock | No |
| Shear studs | Usually required | Not required |
| Typical depth | 51–76 mm (2–3 in) | 19–38 mm (0.75–1.5 in) |
| Slab depth | Shallower possible | Greater concrete needed |
How to read a span table row
A typical span table row reads: Profile 3WH, 20 gauge (0.91 mm), single span, construction load 1.44 kPa, allowable span 2.6 m. This means: using a 3WH profile in 20 gauge steel, acting as an unpropped single-span deck, a construction live load of 1.44 kPa (approximately 30 psf), the maximum clear span between supports is 2.6 m.
The governing load condition changes with span. On short spans (under about 2.5 m), the construction stage typically governs because the deck is thin and the wet concrete is heavy. On longer spans, the composite stage or deflection under service load governs. Published tables already account for this — you do not need to check both conditions manually if the table is for the deck you are using.
The effect of propping during construction
Propping the deck at mid-span during the pour allows the use of longer spans or lighter gauge than an unpropped design. The prop transfers load to the ground during construction; once the concrete cures and the prop is removed, the composite section carries all loads without it.
Propped construction increases site complexity and labour cost. The decision to prop or not is usually made by comparing the cost saving from using lighter gauge deck or longer spans against the cost of installing and removing props. On spans above roughly 3 m (10 ft), propping often enables a one-gauge reduction in the deck that more than pays for the prop cost.
The table entry for a propped condition will show a longer allowable span or a higher allowable load than the corresponding unpropped entry for the same deck. Always verify which condition your table is based on — the heading or footnote on published span tables will state this.
Deflection limits in floor deck design
Deflection limits for floor deck are set in two stages. During construction, the limit is typically span/180 under the weight of wet concrete alone, to prevent ponding — a situation where concrete deflects the deck and additional concrete flows to the low point, increasing load and deflection further until the deck overloads.
Under service loads, the composite slab deflection limit is usually span/360 under live load, or an absolute limit of L/480, depending on the standard. Floor deck itself rarely governs the service deflection — once the composite slab is formed, the deflection is controlled by the overall slab stiffness. The construction stage deflection check is the critical one for the deck selection.
Selecting deck gauge and profile for your span
The practical selection sequence is: (1) establish the structural bay dimensions and the design loads; (2) determine whether composite or non-composite design is required; (3) select the deck depth that fits within the available floor zone (total slab depth minus deck rib height equals the concrete cover above the top of the ribs); (4) use the span table to find the minimum gauge that satisfies both the construction and composite stage checks at your span; (5) check that the selected deck is available in the market you are building in, since regional standards and stocking patterns vary.