Composite Slab System Overview

A composite slab consists of a cold-formed profiled steel deck (trapezoidal or re-entrant) acting as permanent formwork and tensile reinforcement for an in-situ concrete slab. The steel deck and concrete act compositely through:

  1. Mechanical interlock — embossments or indentations on the deck profile that lock into the concrete
  2. Frictional bond — chemical adhesion between the steel deck and concrete
  3. End anchorage — shear studs welded through the deck to the supporting beam at the slab edge

The composite action means the steel deck provides all or part of the tensile reinforcement, reducing or eliminating the need for conventional rebar in the slab span direction.

Deck Type Profile Shape Span Range Typical Depth Common Use
Trapezoidal (open) Open trapezoidal ribs, wide top flange 2.5-4.5 m 50-75 mm Steel-framed buildings, carparks
Re-entrant Narrow dovetail ribs, flat soffit 2.0-3.6 m 50-60 mm Architectural (flat ceiling)
Deep deck Deep trapezoidal rib, long spanning 4.0-6.5 m 80-120 mm Carparks, long spans, heavy load

AS 2327:2017 Design Framework

AS 2327:2017 is the Australian/New Zealand standard for composite slabs, harmonised with Eurocode 4 (EN 1994-1-1) but adapted for Australian practice. The standard covers:

Key Design Principles

  1. Construction stage (non-composite) — the steel deck alone supports the wet concrete weight and construction loads. Deflection is checked for the bare deck with wet concrete.
  2. Composite stage — the hardened concrete slab and steel deck act together. The slab is checked for flexure, longitudinal shear, and vertical shear.

Construction Stage Design — Steel Deck as Formwork

During construction, the steel deck must resist:

Load Combination for Construction Stage

Per AS 1170.0 and AS 2327:2017: w*_construction = 1.2 x (g1 + g2) + 1.5 x q_construction

Bending Capacity of Bare Deck

The steel deck section modulus Z_eff (effective) is determined from the manufacturer's technical data sheet, considering the profile geometry and local buckling effects. The nominal moment capacity:

M_s = Z_eff x fy

Where fy is the yield strength of the deck material (typically 550 MPa for G550 steel).

Capacity reduction factor: phi = 0.80 for the construction stage (reduced from 0.90 to account for thin-gauge cold-formed behaviour).

Deflection Limit

Per AS 2327 Clause 4.4, the deck deflection under wet concrete weight alone (without live load) should not exceed:

delta_max = span / 180 (for manual application of concrete) delta_max = span / 250 (for pumped concrete, stricter due to ponding effects)

If deflections exceed these limits, temporary propping (shoring) is required during the pour. Propping can add 25-35% to the erection cost, so serviceability often controls deck selection.

Composite Stage Design — Flexural Capacity

Effective Slab Width

For simply supported slabs, the full width is effective. For continuous slabs, the effective width per support is:

b_eff = b0 + L/8 (per side of the support)

Where b0 is the width of the steel beam flange plus the stud projection, and L is the span between points of zero moment.

Plastic Moment Resistance — Full Shear Connection

When sufficient shear connection is provided, the plastic neutral axis lies within the concrete slab (common for deep decks) or within the steel deck (for shallow slabs with high deck yield strength). The moment capacity is:

PNA in concrete (common case): x = N_cf / (0.85 x b x f'c)

M_pl_Rd = N_cf x (d_p - x/2)

Where:

Partial safety factor: phi = 0.80 for composite slabs in bending (reduced from the AS 4100 value of 0.90 due to the less ductile failure mode).

Partial Shear Connection

When fewer shear studs are provided than required for full shear connection, the longitudinal shear resistance is less than the full deck tensile capacity. The degree of shear connection (eta) is:

eta = N_c / N_cf (where N_c is the provided longitudinal shear resistance)

For ductile longitudinal shear behaviour (where the deck has adequate slip capacity — typically > 0.1 mm characteristic slip), partial shear connection down to eta = 0.4 is permitted. The reduced moment capacity is determined by linear interpolation between the bare steel deck moment and the full composite moment:

M_pl_eta = M_deck + eta x (M_pl_Rd - M_deck)

For non-ductile shear connectors (most embossed decks), partial shear connection is not recommended — the m-k method should be used instead.

Longitudinal Shear Capacity — The m-k Method

The most common method in Australian practice for determining the longitudinal shear resistance of composite slabs is the empirical m-k method, derived from full-scale bending tests per AS 2327 Appendix A.

m-k Equation

V_l_Rd = b x d_p / gamma_V x (m x A_deck / (b x L_s) + k)

Where:

Typical m-k values for Australian decks:

These are indicative values only — always use the manufacturer's latest published test data as referenced in the product technical manual.

Vertical Shear Capacity

Per AS 2327 Clause 4.5.2.2, the vertical shear resistance per unit width is:

V_v_Rd = 0.035 x k_v^(3/2) x sqrt(f'c) x b x d_p (European formula, adopted by AS 2327)

Where k_v = 1 + sqrt(200/d_p) <= 2.0 and d_p is in millimetres.

For typical 120 mm composite slabs with 50 mm deck: V_v_Rd = approximately 45-55 kN/m width for f'c = 32 MPa. Vertical shear rarely governs for uniformly loaded slabs but should be checked near point loads and at supports.

Worked Example — 3.6 m Span Composite Slab

Problem: Design a composite slab for an office floor in a steel-framed building. Span = 3.6 m (simply supported). Imposed load = 3.0 kPa (general office). Concrete: f'c = 32 MPa. Deck: Bondek II (0.75 mm BMT, G550 steel, fy = 550 MPa).

Deck properties (Bondek II, 0.75 mm BMT per Lysaght): Deck depth = 54 mm, sheet mass = 10.3 kg/m^2 A_deck = 1,370 mm^2/m width Z_eff = 18,600 mm^3/m (construction stage)

Step 1 — Construction stage check (bare deck + wet concrete): Total slab depth = 120 mm (54 mm deck + 66 mm topping above ribs). Concrete volume = (54 x 0.62 + 66) / 1000 = 0.0995 m^3/m^2 (accounting for rib void fraction ~38%). Wet concrete weight = 0.0995 x 25 = 2.49 kPa. g1 (deck) = 0.103 kPa. g2 (wet concrete) = 2.49 kPa. q_construction = 1.5 kPa.

w*_construction = 1.2 x (0.103 + 2.49) + 1.5 x 1.5 = 3.11 + 2.25 = 5.36 kPa/m width.

M*_construction = 5.36 x 3.6^2 / 8 = 8.68 kN.m/m width.

phi x M_s = 0.80 x 18,600 x 550 x 10^-6 = 8.18 kN.m/m < 8.68 kN.m/m. NOT OK without propping.

The deck alone is slightly overstressed in construction. Options: (a) Specify one row of temporary props at mid-span — reduces effective span to 1.8 m, delta decreases by factor of ~16, M* reduces to 2.17 kN.m/m. Easily OK. (b) Increase deck thickness to 1.0 mm BMT — Z_eff increases to approximately 24,500 mm^3/m. phi_Ms = 0.80 x 24,500 x 550 x 10^-6 = 10.78 kN.m/m > 8.68. OK without props.

Choose option (b): 1.0 mm BMT Bondek II, no propping required.

Step 2 — Composite stage flexural check: Effective depth d_p = 120 - 54/2 = 93 mm (deck centroid at half rib depth). N_cf = 1,830 x 550 / 1000 = 1,006 kN/m width (A_deck for 1.0 mm = 1,830 mm^2/m).

PNA depth in concrete: x = 1,006,000 / (0.85 x 1000 x 32) = 37.0 mm (within the 66 mm topping — PNA in concrete).

M_pl_Rd = 1,006 x (93 - 37.0/2) / 1000 = 1,006 x 74.5 / 1000 = 75.0 kN.m/m.

phi_M = 0.80 x 75.0 = 60.0 kN.m/m.

Step 3 — Applied moment at ULS: Total dead load = 0.103 (deck) + 0.0995 x 25 (concrete) = 2.59 kPa. Additional finishes = 1.0 kPa (ceilings, services). Total g = 3.59 kPa. q = 3.0 kPa.

w* = 1.2 x 3.59 + 1.5 x 3.0 = 4.31 + 4.50 = 8.81 kPa/m width. M* = 8.81 x 3.6^2 / 8 = 14.27 kN.m/m < 60.0 kN.m/m. OK — large reserve.

The slab is heavily over-designed in the composite stage, confirming that construction stage (not composite flexure) governs the deck selection.

Step 4 — Longitudinal shear (m-k check): For Bondek II: m = 184.5, k = 0.0732 (design values including gamma_V). L_s = 3600/4 = 900 mm (shear span for uniformly loaded simply supported slab).

V_l_Rd = 1000 x 93 / 1.25 x (184.5 x 1830 / (1000 x 900) + 0.0732) = 1000 x 93 / 1.25 x (184.5 x 0.002033 + 0.0732) = 1000 x 93 / 1.25 x (0.375 + 0.0732) = 74,400 x 0.448 / 1.25 = 26.7 kN/m width.

Applied shear at support: V* = 8.81 x 3.6 / 2 = 15.86 kN/m < 26.7 kN/m. OK.

Step 5 — Deflection (serviceability): Service load (short-term): g + psi_s x q = 3.59 + 0.7 x 3.0 = 5.69 kPa. For composite section I_eff (average of cracked and uncracked, typically 70% of gross for partial interaction): I_eff = 0.70 x I_gross.

Typical I_gross for 120 mm slab = 1000 x 120^3/12 + n x A_deck x (d_p - x)^2 (where n = E_steel/E_cm). E_cm = 30,100 MPa (f'c = 32 MPa). n = 205,000 / 30,100 = 6.81. I_gross = approximately 165 x 10^6 mm^4/m. I_eff = 0.70 x 165 x 10^6 = 115.5 x 10^6 mm^4/m.

delta = 5 x 5.69 x 3600^4 / (384 x 205,000 x 115.5 x 10^6) = 5 x 5.69 x 168 x 10^12 / (384 x 205,000 x 115.5 x 10^6) = 4780 x 10^12 / (9,095 x 10^6) = 0.53 mm.

Allowable = span / 250 = 3600/250 = 14.4 mm >> 0.53 mm. OK.

Final specification: Bondek II 1.0 mm BMT, G550 steel, 120 mm total slab depth, f'c = 32 MPa, no propping required. One row of shear studs at 300 mm centres along supporting beams for end anchorage. Conforms to AS 2327:2017.

Fire Resistance of Composite Slabs

AS 2327 references AS 4100 Clause 12 for fire design. Composite slabs have inherent fire resistance advantages over bare steel:

For higher FRL (120, 180, or 240 minutes), additional reinforcement (SL92 mesh or additional bars) is placed in the topping, and the deck is treated as sacrificial (no strength contribution) for fire design.

Acoustic Performance

Composite slabs provide excellent acoustic separation between floors due to their mass (approximately 2.4-3.0 kPa for a 120 mm slab). For office and residential applications, a typical composite slab with carpet and ceiling achieves:

Frequently Asked Questions

When is the m-k method required instead of the partial shear connection method? The m-k method is the default in Australian practice because it is based on full-scale testing of the specific deck profile. The partial shear connection method (EN 1994-1-1 Annex B) requires the deck to demonstrate ductile end-slip behaviour (characteristic slip > 0.1 mm), which most embossed (non-re-entrant) decks do not achieve. Always check the manufacturer's published m and k values — these are legally required for compliance with AS 2327:2017.

What minimum concrete cover is required over the top of the steel deck? Per AS 2327 Clause 4.5.3, the minimum concrete cover above the top of the steel deck ribs is 40 mm for slabs with mesh reinforcement and 50 mm without mesh. In practice, typical composite slab depths are 120-150 mm total, giving 66 mm to 96 mm topping above a 54 mm deck — well above the minimum.

How are composite slabs detailed around openings? Openings up to 300 mm square can be cut through the slab without additional reinforcement if they are not within the shear-critical zone (within d from the support). Larger openings require trimming steel (angles or channels) bolted or welded to the deck around the perimeter, and additional reinforcement in the concrete topping. All openings must be coordinated with the structural engineer and are best positioned near mid-span where shear is low.

What are the propping (shoring) requirements for composite slabs during construction? Propping is required when: (a) the bare deck deflection under wet concrete exceeds span/250, (b) the construction moment exceeds the bare deck bending capacity, or (c) the slab is continuous and negative moment over the support governs the deck design. Single-span propping (one row at mid-span) is the most common arrangement. For spans exceeding 4.5 m, two rows of props (at third-points) may be required. Props must remain in place until the concrete reaches 75% of the specified 28-day strength (typically 3-7 days for f'c = 32 MPa at 20 degrees C).

This page is for educational reference. Composite slab design per AS 2327:2017 and AS 4100:2020. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer registered with Engineers Australia or the relevant state registration board. Results are PRELIMINARY — NOT FOR CONSTRUCTION.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.