Design standards

Comparison of AS 3600, ACI 318, and EN 1992-1-1 for concrete section design including key differences in approach, factors, and limits.

Introduction

ACS supports three major concrete design standards simultaneously. While all three share the same fundamental mechanics (equilibrium, strain compatibility, material constitutive laws), they differ in safety factors, ductility requirements, stress block parameters, and serviceability provisions. This page summarises the key differences to help you understand how code selection affects your results.

Supported standard revisions

Standard	Supported revision	Notes
AS 3600	AS 3600:2018 Amendment 2:2021	Shear, torsion, crack-width, and capacity reduction ( $\phi$ / $k_\phi$ ) modules reconciled to the amendment. Covers Cl. 8.1–8.6 (flexure, shear, torsion, crack width).
ACI 318	ACI 318-19	—
EN 1992-1-1	EN 1992-1-1:2004 (with EN 1992-1-1:2023 crack-width provisions)	—

If you are working to the pre-amendment AS 3600:2018 document, the results should be identical for most provisions — Amendment 2 primarily clarified the $k_\phi$ table and tightened a small number of shear provisions. See Section analysis for the specific amendment changes.

Safety format

The standards use different approaches to achieving structural safety:

Aspect	AS 3600	ACI 318	EN 1992
Safety approach	$\phi$ factors on capacity	$\phi$ factors on capacity	$\gamma$ factors on materials
$\phi$ (flexure, tension-controlled)	0.85	0.90	1.0 (uses $\gamma_c = 1.5$ , $\gamma_s = 1.15$ )
$\phi$ (flexure, compression-controlled)	0.65 × $k_\phi$	0.65	1.0
$\phi$ (shear)	0.75	0.75	1.0
Design strength	$f'_c$ (characteristic)	$f'_c$ (specified)	$f_{cd} = f_{ck} / \gamma_c$

In EN 1992, the material partial factors are applied to the material strengths before computing capacity, so the reduction factor on the capacity itself is 1.0. The net effect on design capacity is broadly comparable across codes.

AS 3600 Amendment 2:2021 — $k_\phi$ capacity reduction modifier

AS 3600:2018 Amendment 2:2021 introduced a per-load-combination $k_\phi$ class that modifies the compression-controlled floor (Table 2.2.2). ACS exposes this as a toggle on each ULS combination:

$k_\phi$ class	Factor	$\phi_o$ (compression-controlled floor)	Condition
Full	1.0	0.65	Short column (Cl. 10.3) and $Q/G \geq 0.25$
Reduced	12/13 ≈ 0.923	0.60	All other cases

The $\phi_o$ floor applies when the section is compression-controlled; the tension-controlled value of 0.85 is unaffected by $k_\phi$ . Most gravity-dominated combinations should use Reduced; use Full only when the design is explicitly for a short column under predominantly live load.

Rectangular stress block

All three codes use a simplified rectangular stress block for routine design, but with different parameters:

Parameter	AS 3600	ACI 318	EN 1992
Stress intensity ( $\alpha_2$ )	$1.0 - 0.003f'_c$ ( $\geq 0.67$ )	$0.85$	$1.0$ (on $f_{cd}$ )
Depth factor ( $\gamma$ )	$1.05 - 0.007f'_c$ ( $\geq 0.67$ )	$\beta_1 = 0.85 - 0.05(f'_c - 28)/7$ ( $\geq 0.65$ )	$\lambda = 0.8$ (for $f_{ck} \leq 50$ MPa)
Ultimate strain ( $\varepsilon_{cu}$ )	0.003	0.003	0.0035

These differences mean that the same section with the same materials will produce slightly different capacities depending on the selected code.

Ductility requirements

Ductility ensures that the section fails in a ductile manner (steel yielding before concrete crushing), providing warning before collapse.

Code	Ductility parameter	Limit	Reference
AS 3600	$k_u = c/d$	$\leq 0.36$ (without compression steel)	Cl. 8.1.5
ACI 318	$\varepsilon_t$ (net tensile strain)	$\geq 0.005$ for tension-controlled	Cl. 21.2.2
EN 1992	$x/d$	$\leq 0.45$ (for $f_{ck} \leq 50$ MPa)	Cl. 5.5

These limits are conceptually equivalent — they all ensure that the neutral axis is not too deep, so the tension steel yields before the concrete crushes.

Shear design

Aspect	AS 3600	ACI 318	EN 1992
Model	Modified truss model	Truss model	Variable-angle truss model
Concrete contribution	$V_{uc}$ (always present)	$V_c$ (simplified or detailed)	$V_{Rd,c}$ (members without shear reinforcement)
Strut angle	Variable ( $25^\circ$ — $50^\circ$ )	Fixed at $45^\circ$	Variable ( $21.8^\circ$ — $45^\circ$ )
Min. shear reinforcement	$A_{sv,min} = 0.06\sqrt{f'_c} \cdot b_w \cdot s / f_{sy}$	Similar formula	$\rho_{w,min} = 0.08\sqrt{f_{ck}} / f_{yk}$

Serviceability

Crack width

Aspect	AS 3600	ACI 318	EN 1992
Method	Direct crack width calculation (Cl. 8.6.2.3)	Simplified (max bar spacing)	Direct crack width (Cl. 7.3.4)
Limit	0.3 mm (exposure B1)	Indirect (spacing rules)	0.3 mm (exposure XC1)
Formula	$w_k = s_{r,max} \cdot (\varepsilon_{sm} - \varepsilon_{cm})$ with the published Cl. 8.6.2.3(2) strain term: fixed 0.6 coefficient, mean axial $f_{ct} = 1.4 \times 0.36\sqrt{f'_c}$ , $n_e = (1+\varphi_{cc})E_s/E_c$ , plus the shrinkage strain $\varepsilon_{cs}$ on long-term combinations	—	$w_k = s_{r,max} \cdot (\varepsilon_{sm} - \varepsilon_{cm})$ with the $k_t$ -parametrised strain term (no shrinkage term)

The two code families deliberately diverge on the strain term: AS 3600 adds the final design shrinkage strain $\varepsilon_{cs}$ for long-term crack widths (following Gilbert’s research, the clause’s basis), while EN 1992-1-1 — in both the 2004 edition and the 2023 revision — retains the $k_t$ form with no shrinkage term. Long-term AS crack widths are therefore substantially larger than the corresponding short-term widths; this is the published clause’s intent.

Deflection

Aspect	AS 3600	ACI 318	EN 1992
$I_{ef}$ formula	Modified Branson	Branson	EN approach (interpolation)
Creep treatment	$k_{cs}$ factor	ACI 209 or explicit	Creep coefficient $\varphi$
Span/depth limits	Table 8.5.4	ACI Table 7.3.1.1	Cl. 7.4.2

Prestressed concrete

Aspect	AS 3600	ACI 318	EN 1992
Loss calculation	Cl. 3.4	Ch. 27	Cl. 5.10.6
Transfer stress limits	Compression: $0.6 f_{ci}$ ; Tension: $0.5\sqrt{f_{ci}}$	Similar limits	Cl. 5.10.2.2
Service stress limits	Compression: $0.45 f'_c$ ; Tension: code-dependent	Compression: $0.45 f'_c$	$0.6 f_{ck}$

Fire design

Aspect	AS 3600	ACI 216.1	EN 1992-1-2
Standard fire curve	ISO 834	ASTM E119	ISO 834
Methods	Tabulated + advanced	Tabulated + rational	Tabulated + simplified + advanced
Cover to axis	Minimum tabulated values	Minimum tabulated values	Minimum tabulated values
Advanced analysis	References EN 1992-1-2	Rational analysis per ASCE	Full thermal + mechanical FE

Practical guidance

When choosing a design code in ACS:

Use AS 3600 for Australian projects and projects referencing the Building Code of Australia (BCA / NCC). ACS implements AS 3600:2018 Amendment 2:2021 — the most current revision. This amendment updated the capacity reduction table (Table 2.2.2, $k_\phi$ ), refined several shear and torsion provisions, and tightened the crack-width formulation.
Use ACI 318 for US projects and projects referencing IBC or ASCE 7.
Use EN 1992 for European projects, UK projects (via National Annex), and many international projects that adopt Eurocodes.

If you need to compare designs across codes (e.g., for an international project with multiple jurisdictions), run the same section with each code selected and compare the utilisation ratios side by side.

Materials — setting the design code and material grades
Section analysis — running design checks
M-N interaction theory — how code differences affect interaction diagrams