Seismic detailing

Seismic ductility checks under AS 3600 Cl 14.4.6 — the k_uo limit and compression-face capacity fraction — and advisory tie-spacing requirements for beams and columns.

Overview

When Design Code is set to AS 3600 and Seismic is enabled in the General tab, ACS performs two additional checks under Section 14:

1. Ductility check (Cl 14.4.6) — limits the $k_{uo}$ value and compression-face capacity fraction to ensure the section can sustain the inelastic deformations required for seismic energy dissipation.
2. Tie-spacing advisory (Cl 14.5.2.2(d) for beams, Cl 14.5.4 for columns) — reports the maximum permissible seismic tie spacing. This is advisory-only and carries no pass/fail verdict.

The ductility check applies only to a member declared as a beam in the General tab — Cl 14.4.6 is a beam provision, and a declared column, wall, or slab does not trigger it. The tie-spacing advisory shows the beam formula, the column formula, or both, depending on the declared member type (see Classification and ambiguity below). ACS also checks the declared type against the axial load ratio relative to $0.10 f'_c A_g$ — the AS 3600 Cl 10.3.2 beam/column boundary — and surfaces a mismatch as an advisory warning, but the declared type is always what selects which check runs.

:::caution[Reinforcement above 500 MPa: the ductility check is withheld] Where the longitudinal reinforcement declares $f_{sy} > 500$ MPa, ACS does not evaluate Cl 14.4.6 and reports the check as not evaluated rather than passing or failing it. AS 3600 Cl 1.1.2(d) admits higher grades to the Standard generally, but the Section 14 plastic-hinge provisions — and the Cl 10.7.4.3(b)(iii) bar-restraint detailing they depend on — are calibrated to Class N 500 MPa steel, and no clause extends them. Computing the ratios anyway would publish a seismic verdict on an assumption the Standard does not make.

A withheld check never coexists with a green ULS badge: the Design Summary shows the row with no utilisation and the reason beside it, and the verdict is withheld. Use 500 MPa reinforcement for the seismic load cases, or carry out the Cl 14.4.6 assessment outside the tool. :::

Ductility check (Cl 14.4.6)

AS 3600 Cl 14.4.6 imposes two independent requirements on a member declared as a beam. ACS evaluates both for each ULS load combination and reports a combined pass/fail result.

Part b — $k_{uo}$ limit

The neutral-axis parameter $k_{uo}$ — the ratio of the neutral-axis depth to the effective depth at pure bending ($N^* = 0$) — is limited to:

This is tighter than the non-seismic limit of 0.36. A lower $k_{uo}$ means the section is more heavily tension-controlled, ensuring the reinforcement yields before the concrete crushes and providing greater ductility and warning before the ultimate limit state is reached.

The check is evaluated for both the sagging and hogging senses, because seismic loading is cyclic and the section must have adequate ductility in both directions.

Part a — compression-face capacity fraction

The total tensile capacity on the tension face must not exceed one-third of the compressive capacity on the compression face:

Where:

• $C_{tension}$ = total capacity of all bars and tendons on the tension face
• $C_{compression}$ = total capacity of bars only (not tendons) on the compression face

Tendons are excluded from the compression-face calculation because they carry net tension under the prestress and would overstate the available compressive resistance if included.

The compression face is the face closest to the extreme compressive fibre for the current bending sense. Since both sagging and hogging senses are evaluated, the compression and tension faces swap between the two cases.

T-beam and L-beam sections

For T-beams and L-beams with a cast-in-place flange, AS 3600 Cl 14.4.6 applies the standard effective flange rules. ACS applies the clause’s flange contribution provisions for both Part a and Part b.

ACS assumes the flange is cast in place (monolithic with the web). Precast or composite flanges are not distinguished from cast-in-place in the current model; if your section has a non-monolithic flange, verify the applicability of the flange rules manually.

When the check is skipped

The ductility check returns no result (shown as N/A) when any of the following conditions apply:

Tie-spacing advisory (Cl 14.5)

ACS reports the maximum seismic tie spacing as an advisory quantity — a guidance value with no pass/fail verdict. The spacing shown is the smallest value from the applicable clause limits. Verify that your stirrup spacing satisfies the advisory before finalising the design.

Beam tie spacing (Cl 14.5.2.2(d))

For a member declared as a beam (or where the classification is ambiguous — see below), the maximum seismic tie spacing is the least of:

Where:

• $d_o$ = overall depth of the member (mm)
• $d_b$ = diameter of the smallest longitudinal bar enclosed by the tie (mm)
• $d_f$ = diameter of the fitment (tie) bar (mm)

Column tie spacing (Cl 14.5.4)

For a member declared as a column (or where the classification is ambiguous — see below), the maximum seismic tie spacing for bar restraint is the least of:

Where:

• $b_{min}$ = the smaller cross-section dimension (mm)
• Other terms as for the beam formula above

ACS triggers the column tie-spacing advisory when either condition is met:

• $P^* > \phi \cdot 0.3 \cdot A_g \cdot f'_c$ with $\phi = 0.6$ — compression load is large relative to the squash capacity
• $f'_c > 65\text{ MPa}$ — high-strength concrete, which is more susceptible to sudden cover spalling

Classification and ambiguity

ACS selects the tie-spacing formula from the declared member type in the General tab: a declared “Beam” shows the Cl 14.5.2.2(d) formula, and a declared “Column” shows the Cl 14.5.4 formula. A declared “Wall” is inherently ambiguous — it is neither a Cl 14.5.2.2 beam nor a Cl 14.5.4 column — so both formulas are always shown. ACS also cross-checks the declared type against the axial load ratio relative to $0.10 f'_c A_g$ (the AS 3600 Cl 10.3.2 beam/column boundary); when the declared type disagrees with that axial regime — for example, a member declared “Beam” whose worst-case design axial force reaches column levels — both formulas are shown and the disagreement is noted as an advisory. The declared type, not the axial regime, is always the source of truth for which check applies.

Terms not available

When a term required by a formula is not defined — for example, there are no longitudinal bars so $d_b$ is unknown — ACS excludes that term from the minimum rather than substituting zero. A missing bar diameter is unknown, not zero; treating it as zero would produce an artificially conservative result of 0 mm and would not reflect the actual section configuration.

Results panel

Seismic results appear in the Seismic section of the right-side panel when seismic is enabled:

Tips

• Enable Seismic in the General tab before running the ULS analysis — the ductility check only runs when the seismic flag is on.
• If the $k_{uo}$ check fails, reduce the tension-face reinforcement, increase the section effective depth, or both. Adding compression-face bars can also lower $k_{uo}$ by shifting the neutral axis toward the compression face.
• The 1/3 compression-face capacity fraction is sensitive to the balance between tension and compression reinforcement. Symmetric reinforcement (equal bars on both faces) is the most direct path to satisfying Part a.
• The tie-spacing advisory has no pass/fail — use it as a target when specifying stirrup spacing in the Stirrups panel.
• For sections near the beam/column threshold ($N^*$ close to $0.10 f'_c A_g$), review both beam and column tie-spacing formulas and apply the more conservative.

Related pages

• Section analysis — ULS analysis and design checks
• Reinforcement — placing bars and stirrups
• Design standards — seismic design code references