Scope and limitations

Capabilities, assumptions, and known limitations of the Advanced Concrete Section tool.

Scope

The Advanced Concrete Section tool is designed for cross-section level analysis and design of reinforced, prestressed, and post-tensioned concrete members. It computes section capacities and checks them against applied actions provided by the user.

ACS supports:

Arbitrary polygonal cross-sections with voids
Reinforced concrete with multiple bar sizes and grades
Prestressed concrete with bonded and unbonded tendons
Combined axial force, biaxial bending, shear, and torsion actions
Ultimate and serviceability limit state checks
Fire resistance assessment with 2D heat transfer
Nonlinear moment-curvature analysis
Time-dependent effects (creep and shrinkage)
Three design codes: AS 3600, ACI 318, EN 1992-1-1

Assumptions

The following assumptions apply to all analyses:

Assumption	Impact	Standard reference
Plane sections remain plane	Linear strain distribution across the section	Euler-Bernoulli beam theory
Perfect bond between steel and concrete	No bond-slip at the steel-concrete interface	All codes assume this for design
Uniaxial stress state	Concrete stress is function of uniaxial strain only; no biaxial or triaxial effects (unless Mander model selected)	Simplified constitutive model
Monotonic loading	No cyclic or reversed loading; no hysteretic behaviour	Not applicable for seismic cyclic analysis
Small deformations	No geometric nonlinearity at the section level	Section-level analysis only
Concrete tension ignored after cracking	Concrete carries no tensile stress after cracking (conservative for ULS; tension stiffening available for SLS)	AS 3600 Cl. 8.1, ACI 318 Ch. 22

Known limitations

Section-level analysis only

ACS analyses the cross-section in isolation. It does not account for:

Member-level effects: slenderness, moment magnification ( $\delta$ factors), P- $\Delta$ effects. You must compute magnified moments externally and input them as the design actions.
System-level effects: load redistribution, continuity moments, lateral stability. ACS assumes you have determined the design actions from a separate structural analysis.
Detailing: anchorage, lap splices, development length, bar curtailment. ACS checks section capacity but not reinforcement detailing.

Shear and torsion

Shear capacity is computed using the simplified truss model with a single critical section. Strut-and-tie models for disturbed regions (D-regions) are not supported.
Torsion capacity is accepted as an input but the torsion design check is not yet implemented. The shear check does not account for torsion-shear interaction.

Fire design

No spalling modelling. Explosive spalling of high-strength concrete cover is not captured. For $f'_c > 55$ MPa, the fire analysis may be unconservative if spalling occurs.
Thermal properties assume normal-weight siliceous aggregate concrete. Calcareous and lightweight aggregate have different thermal properties that are not currently selectable.
Fire exposure is assumed uniform along the member length (2D section analysis).

Prestressing

Friction losses assume a simplified linear model. Complex tendon profiles with reverse curvature are not supported.
Unbonded tendon stress increase at ultimate uses the simplified code formula, not a full member-level analysis.
Post-tensioning anchorage zone design (bursting and spalling reinforcement) is not included.

Geometry

Self-intersecting polygons are not supported. The outline must be a simple (non-crossing) polygon.
Circular sections are approximated as polygons (typically 36 or more sides). This introduces negligible error for practical sizes.

Material models

Concrete tension stiffening is not available for all analysis types.
The Mander confined concrete model requires the user to select it explicitly; ACS does not automatically detect confinement from stirrup configuration.
Time-dependent effects (AEMM) assume a single loading age. Multiple loading events at different ages are not supported.

Valid input ranges

Parameter	Minimum	Maximum	Units	Notes
$f'_c$	20	100	MPa	Standard grades per code
$f_y$	250	600	MPa	Standard grades per code
Section width	50	5000	mm	Practical range
Section depth	50	5000	mm	Practical range
Cover	15	100	mm	Per code minimum tables
Bar diameter	6	40	mm	Standard sizes
Number of bars	1	500	—	Performance limit
Fire duration	0	360	min	Standard fire curve range
Interaction diagram points	10	200	—	More points = slower but smoother
M- $\kappa$ fibres	20	200	—	More fibres = more accurate

Accuracy and validation

ACS has been validated against hand calculations and published benchmark problems:

Benchmark	Source	Expected	Calculated	Difference
Rectangular beam, pure bending	AS 3600 worked example	$\phi M_u = 272$ kN.m	$\phi M_u = 271$ kN.m	< 1%
Square column, uniaxial	Park & Paulay Example 4.3	$N_b = 1850$ kN	$N_b = 1843$ kN	< 1%
Biaxial column, Bresler	Wight & MacGregor Example 11.2	$\phi N_u = 3200$ kN	$\phi N_u = 3180$ kN	< 1%
M- $\kappa$ curve, rectangular	Hognestad (1955) benchmark	Ultimate $M_u = 285$ kN.m	$M_u = 283$ kN.m	< 1%

Differences of less than 1% are typical and arise from iteration convergence tolerances and the finite number of integration points.

Features not yet implemented

Feature	Status	Notes
Torsion design check	Planned	Torsion input accepted but no capacity check
Strut-and-tie analysis	Planned	For D-regions and deep beams
Confined concrete auto-detection	Planned	Currently requires manual Mander model selection
Multiple loading ages (AEMM)	Planned	Currently single loading age only
Cyclic M- $\kappa$ analysis	Under consideration	For seismic detailing
Lightweight aggregate thermal properties	Planned	Currently siliceous only for fire

Section analysis — full analysis documentation
Design standards — code comparison