Australian Composite Beam — AS 2327 Composite Steel-Concrete Design Guide

Complete reference for AS 2327:2017 Composite Steel-Concrete Beams — the Australian standard for steel-concrete composite beam design. Covers shear stud connectors (welded through-deck to AS 1554.6), effective flange width (Clause 3.3), moment capacity in sagging and hogging bending (Clause 4.3), vertical shear capacity (Clause 4.4), deflection control including shrinkage and creep effects (Clause 5.2), and construction-stage checks. Includes a worked example for a 530UB92.4 composite floor beam.

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AS 2327 Composite Beam Design — Overview

Composite steel-concrete beams use shear connectors to join a steel beam to a concrete slab, creating a T-section that acts as a single structural element. The concrete slab resists compression in sagging bending, while the steel beam carries tension — each material used where it performs best. This results in 30-50% greater strength and 40-60% greater stiffness compared to a non-composite steel beam of the same size.

AS 2327:2017 governs composite beam design in Australia, working in conjunction with:

Shear Stud Connectors (AS 2327 Clause 3.2)

Stud Types

Shear stud connectors are welded to the top flange of the steel beam through the steel deck (decking) using a drawn-arc welding process per AS 1554.6:

Stud Diameter (mm) Height (mm) Ultimate Strength f_uc (MPa) Characteristic Capacity (kN)
13 65 450 42
16 75 450 63
19 100 450 90
22 125 450 119
25 150 450 150

Stud steel is typically Grade 450 (f_uc = 450 MPa) per AS 1554.6.

Design Shear Capacity of Studs (Clause 3.2.3)

The design shear capacity of a single stud connector in a solid slab is:

phi-Vsc = phi × 0.65 × (d_s² × √(f_c × E_c)) / (1.3 × f_uc × A_s)

where:
  phi    = 0.80 (capacity factor for connectors)
  d_s    = stud shank diameter (mm)
  f_c    = concrete compressive strength (MPa)
  E_c    = concrete elastic modulus = 0.043 × ρ^1.5 × √(f_c) (MPa)
  A_s    = cross-sectional area of stud = pi × d_s² / 4 (mm²)
  f_uc   = ultimate tensile strength of stud steel (MPa)

The 0.65 factor accounts for the ductility requirement (studs must be ductile enough to redistribute load). The formula gives the minimum of:

For a 19 mm stud in 32 MPa concrete: f_c = 32 MPa, E_c = 0.043 × 2400^1.5 × √32 = 0.043 × 117,576 × 5.657 = 28,600 MPa phi-Vsc = 0.80 × 0.65 × (19² × √(32 × 28,600)) / 1000 = 0.80 × 0.65 × (361 × √915,200) / 1000 = 0.80 × 0.65 × (361 × 956.7) / 1000 = 0.80 × 0.65 × 345,368 / 1000 = 179.6 kN per stud

Reduction for Deck Profiles

When studs are welded through steel decking, the capacity is reduced by:

phi-Vsc_reduced = phi-Vsc × k_deck
k_deck = 0.85 × (b_r / h_s) × (h_sc / h_s - 1.0) ≤ 1.0

where:
  b_r  = average width of deck rib (mm)
  h_s  = height of deck rib (mm)
  h_sc = stud height after welding (mm)

For trapezoidal deck with 60 mm rib, 140 mm average rib width, 19 mm studs (100 mm height): k_deck = 0.85 × (140/60) × (100/60 - 1.0) = 0.85 × 2.33 × (1.667 - 1.0) = 0.85 × 2.33 × 0.667 = 1.32 But capped at 1.0, so k_deck = 1.0 (no reduction for this deck profile with adequate stud length).

Effective Flange Width (AS 2327 Clause 3.3)

The effective width of the concrete flange in composite action is:

b_eff = b_ei + b_e2 + b_e3

where:
  b_ei = minimum of L_e / 8, S_i / 2, and (D_c × 10 + t_s)

  L_e  = distance between points of contraflexure (mm)
  S_i  = distance to adjacent beam centreline on side i (mm)
  D_c  = depth of composite section (mm)
  t_s  = slab thickness (mm)

For internal beams, b_eff is the sum of half the effective width on each side. For edge beams, the external effective width is limited to b_e1 + b_e2 where b_e2 is the overhang.

For a 12 m span internal beam with beams at 3.0 m spacing: b_ei = min(12000/8, 3000/2) = min(1500, 1500) = 1500 each side b_eff = 1500 + 1500 = 3000 mm

For an edge beam with 1.5 m overhang: b_ei = min(1500, 750) = 750 external, 1500 internal b_eff = 750 + 1500 = 2250 mm

Moment Capacity — Sagging Bending (AS 2327 Clause 4.3)

In sagging (positive) bending, the concrete slab is in compression and the steel beam is primarily in tension. The design moment capacity depends on the degree of shear connection and the location of the plastic neutral axis (PNA).

Full Shear Connection

With full shear connection (sufficient studs to develop the full plastic moment):

Case 1 — PNA in slab:
  C_c = 0.85 × f_c × b_eff × t_c
  T_s = A_s × f_y
  If C_c ≥ T_s: PNA in slab, M_s = T_s × (d/2 + t_c - a/2)

Case 2 — PNA in steel flange:
  If C_c < T_s: PNA in steel section
  C_f = (T_s - C_c) / 2  (flange compression required for equilibrium)

where:
  C_c  = concrete compression force (N)
  T_s  = steel tension force = A_s × f_y (N)
  t_c  = depth of concrete slab above deck ribs (mm)
  a    = depth of stress block = T_s / (0.85 × f_c × b_eff) (mm)

For a 530UB92.4 (As = 11,800 mm², fy = 300 MPa, d = 528 mm) with 32 MPa concrete, 3000 mm effective width, 120 mm slab above deck:

T_s = 11,800 × 300 = 3,540 kN C_c = 0.85 × 32 × 3000 × 120 = 9,792 kN Since C_c >> T_s → PNA is in the slab.

a = 3,540,000 / (0.85 × 32 × 3000) = 3,540,000 / 81,600 = 43.4 mm M_s = 3,540,000 × (528/2 + 120 - 43.4/2) = 3,540,000 × (264 + 120 - 21.7) = 3,540,000 × 362.3 = 1,283 × 10⁶ N·mm = 1,283 kN·m

Without composite action, the bare steel beam capacity: phi-Ms = 0.90 × 300 × 2,370 × 10³ / 10⁶ = 640 kN·m

Composite action increases moment capacity by 2.0× (1,283 vs 640 kN·m).

Partial Shear Connection

When fewer studs are used (partial shear connection), the moment capacity is reduced:

N_sc = number of studs provided between supports
N_scf = number of studs required for full shear connection
eta_c = N_sc / N_scf (degree of shear connection, 0.4 ≤ eta_c ≤ 1.0)

M_sp = M_s_bare + eta_c × (M_s_full - M_s_bare)

AS 2327 Clause 4.3.3 requires that the degree of shear connection eta_c be at least 0.4 for beams with span ≤ 20 m. For longer spans, min eta_c = 0.5. This minimum ensures acceptable ductility and prevents brittle connector failure.

Vertical Shear Capacity (AS 2327 Clause 4.4)

The vertical shear capacity of a composite beam is based on the steel web alone:

phi-Vv = phi × 0.60 × f_y × A_w × k_v

where:
  phi  = 0.90 (same as AS 4100)
  A_w  = web area = d × t_w (mm²)
  t_w  = web thickness (mm)
  k_v  = web slenderness reduction factor per AS 4100 Clause 5.11

AS 2327 does not attribute shear capacity to the concrete slab — the steel section must resist all vertical shear. For the 530UB92.4: A_w = 528 × 10.2 = 5,386 mm² phi-Vv = 0.90 × 0.60 × 300 × 5,386 / 1000 = 872 kN

Shear-Moment Interaction (Clause 4.4.3)

When the design shear V* exceeds 0.6 × phi-Vv, the moment capacity must be reduced to account for shear-moment interaction (same principle as AS 4100 Clause 5.12):

M_s_reduced = M_s × (1 - (V* / phi-Vv - 0.6)² / (0.4 - 0.6)²)

For V* = 600 kN (69% of phi-Vv): V*/phi-Vv = 0.69 > 0.6 → shear-moment interaction applies. M_s_reduced = 1,283 × (1 - (0.69 - 0.6)² / 0.16) = 1,283 × (1 - 0.0081/0.16) = 1,283 × (1 - 0.0506) = 1,218 kN·m Reduction: 5% — minor for this section and loading.

Deflection — Shrinkage and Creep Effects (AS 2327 Clause 5.2)

Composite beam deflections must account for:

  1. Construction-stage deflection — steel beam self-weight + wet concrete (non-composite)
  2. Long-term deflection — superimposed dead load + live load (composite, with shrinkage/creep)

Shrinkage Curvature

Differential shrinkage between the concrete slab and steel beam induces curvature:

kappa_sh = epsilon_sh / (d/2 + t_c - y_bar_comp) × alpha_e

where:
  epsilon_sh = design shrinkage strain (typically 400-800 microstrain)
  y_bar_comp = neutral axis depth of composite section
  alpha_e    = modular ratio adjustment for creep

For standard Australian concrete (32 MPa, normal weight):

Creep Effect

Creep increases the long-term deflection of composite beams:

delta_lt = delta_elastic + delta_creep + delta_shrinkage

delta_creep = delta_elastic × phi_cc × (G_seff / G_total)
delta_shrinkage = kappa_sh × L² / 8

For a 12 m beam with delta_elastic = 15 mm, phi_cc = 2.5, and sustained load ratio of 0.6: delta_creep = 15 × 2.5 × 0.6 = 22.5 mm delta_elastic + delta_creep = 37.5 mm

AS 2327 Clause 5.2 limits total deflection to L/300 (40 mm for 12 m span) and incremental deflection (after construction) to L/500 (24 mm). For this example, long-term deflection of 37.5 mm exceeds L/500, so the beam requires either more pre-camber or increased section size.

Construction Stage Checks

Before the concrete has cured, the steel beam alone must support:

  1. Steel beam self-weight
  2. Wet concrete weight (includes the slab + deck profile weight)
  3. Construction live load (typically 0.5 kPa)
  4. Steel decking weight

Construction Stage Design Check

For the 530UB92.4 with 3.0 m spacing, 12 m span, 130 mm total slab depth (120 mm above deck + 60 mm rib, average 150 mm concrete thickness):

Concrete weight: 0.150 × 25 = 3.75 kPa × 3.0 m = 11.25 kN/m Steel: 0.92 kN/m Construction live: 0.5 kPa × 3.0 m = 1.5 kN/m

Total construction load: 11.25 + 0.92 + 1.5 = 13.67 kN/m

M_construction = 13.67 × 12² / 8 = 246 kN·m phi-Ms_bare = 640 kN·m → OK (38% utilisation)

Delta_construction = 5 × 11.25 × 12000⁴ / (384 × 200,000 × 460 × 10⁶) = 5 × 11.25 × 2.07 × 10¹⁶ / (384 × 200,000 × 460 × 10⁶) = 1.164 × 10¹⁸ / (3.53 × 10¹⁶) = 33 mm

L/300 = 40 mm → 33 mm OK for construction. Pre-camber of 25 mm recommended for long-term control.

Worked Example — Composite Floor Beam Design

Problem: Design a composite floor beam for a 12 m × 9 m bay in an office building.

Design Parameters:

Loads: Dead loads: Slab (0.150 × 25) = 3.75 kPa, steel deck = 0.12 kPa, services/ceiling = 0.50 kPa, steel beam (per m) = 0.92 kN/m Live loads: Office = 3.0 kPa (AS 1170.1 Table 3.1), partitions = 0.5 kPa

Ultimate Load (ULS — 1.2G + 1.5Q): w = 1.2 × (3.75 + 0.12 + 0.50) + 1.5 × (3.0 + 0.5) = 1.2 × 4.37 + 1.5 × 3.5 = 5.24 + 5.25 = 10.49 kPa Beam load: w_beam = 10.49 × 3.0 + 1.2 × 0.92 = 31.47 + 1.10 = 32.57 kN/m

M* = 32.57 × 12² / 8 = 586.3 kN·m V* = 32.57 × 12 / 2 = 195.4 kN

Composite Moment Capacity: T_s = 11,800 × 300 = 3,540 kN C_c = 0.85 × 32 × 3000 × 120 = 9,792 kN a = 3,540,000 / (0.85 × 32 × 3000) = 43.4 mm (PNA in slab) M_s = 3,540 × (0.264 + 0.120 - 0.0217) = 3,540 × 0.3623 = 1,283 kN·m phi-Ms = 0.90 × 1,283 = 1,155 kN·m 586.3 ≤ 1,155 → OK (51% utilisation)

Shear Check: phi-Vv = 0.90 × 0.60 × 300 × 528 × 10.2 / 1000 = 872 kN 195.4 ≤ 872 → OK (22% utilisation)

Shear Stud Design: Full shear connection — calculate required studs to develop T_s = 3,540 kN. phi-Vsc per 19 mm stud = 0.80 × 0.65 × (361 × √(32 × 28,600)) / 1000 = 179.6 kN (in solid slab) Through-deck reduction: k_deck = 0.85 (AISC-recommended for 60 mm rib, 19 mm stud) phi-Vsc_reduced = 0.85 × 179.6 = 152.7 kN per stud

N_scf (each side of midspan) = 3,540 / 152.7 = 23.2 → 24 studs per side = 48 studs total Stud spacing: 12,000 mm span / 24 = 500 mm centres Minimum: 6 stud diameters = 114 mm, maximum: 900 mm (Clause 3.2.4)

Partial shear check: N_sc = 20 per side (eta_c = 0.83 > 0.4 minimum): M_sp = 640 + 0.83 × (1,283 - 640) = 640 + 534 = 1,174 kN·m phi-Msp = 0.90 × 1,174 = 1,057 kN·m → OK (55% utilisation)

Deflection Check: Construction stage: delta_0 = 33 mm (above). Pre-camber 25 mm recommended. Superimposed dead: 0.50 kPa × 3.0 m + 0.5 kPa partitions × 3.0 m = 1.5 + 1.5 = 3.0 kN/m (sustained) Live load: 3.0 kPa × 3.0 m = 9.0 kN/m (transient)

Short-term composite I: I_c ≈ 1,800 × 10⁶ mm⁴ (approximate for 530UB + 3000 mm slab) Delta_live = 5 × 9.0 × 12000⁴ / (384 × 200000 × 1800 × 10⁶) = 8.1 mm L/500 = 24 mm → OK

Long-term delta (including creep, phi_cc = 2.5): Delta_lt = 33 (construction) + 1.3 (steel DL) + 5.2 (superimposed) + 8.1 (live) = 47.6 mm L/300 = 40 mm → Requires pre-camber of 15 mm to satisfy.

Result: 530UB92.4 composite beam with 20 studs per side (19 mm × 100 mm), partial shear connection (eta_c = 0.83), 15 mm pre-camber, satisfies AS 2327. Beam capacity: phi-Ms = 1,057 kN·m (conservative, using partial shear).

Educational reference only. Verify against AS 4100 and relevant standards. Results are PRELIMINARY — NOT FOR CONSTRUCTION.