Wall Stiffness Modifier

Method & Equations

AS 3600:2018 Cl 6.2.4.2 specifies the moment of inertia to use for walls in elastic analysis of lateral force-resisting systems at the ultimate limit state. The clause distinguishes between uncracked and cracked behaviour:

• Cl 6.2.4.2(a) — Uncracked sections: use the full gross uncracked section (I_eff = I_g, i.e. modifier = 1.0).
• Cl 6.2.4.2(b) — Cracked sections: apply the modifier from Table 6.2.4, which varies with the axial load ratio N*/(A_g·f'_c).

Cracking criterion — Cl 3.1.1.3

A wall is taken as cracked when the peak in-plane tensile membrane stress S22 (from elastic shell analysis) reaches the characteristic uniaxial tensile strength of concrete:

Direct (uniaxial) tensile strength is used rather than flexural f'_ct.f = 0.6·√f'_c because S22 from a shell element represents a membrane stress with no strain gradient through the wall thickness. The flexural value would non-conservatively delay the cracked classification.

Table 6.2.4 — wall modifiers (cracked)

For intermediate values of the axial ratio, linear interpolation is permitted (Table 6.2.4 footnote). Net tension (N* < 0) is outside the table's range and is conservatively clamped at the ratio = 0 row (I_eff = 0.25·I_g); engineers expecting significant net tension in a wall should consider a separate detailed assessment.

Limitations & Assumptions

• For ULS elastic analysis only (lateral force distribution, period, inter-storey drift). Serviceability deflection of slabs/beams uses Cl 8.5/9.5 Branson-based I_ef — a different method not implemented here.
• S22 is taken as the peak in-plane membrane stress from shell-element output. The engineer must judge where to extract S22 (panel average vs. edge peak vs. specific load combo) — the tool does not extract this automatically.
• Cracking criterion uses direct tensile strength f'_ct per Cl 3.1.1.3, on the basis that S22 from a shell is a membrane stress.
• Net tension (N* < 0) is clamped to ratio = 0; this is outside the explicit range of Table 6.2.4 and engineers should consider whether a more detailed assessment is warranted.
• Does not address shear-stiffness modifiers, P-delta amplification, or non-linear cracked behaviour — refer to AS 3600 Section 6 and seismic guidance (e.g. AS 1170.4) for those effects.
• Single wall element only. Coupled walls, walls with openings, and walls connected to slabs above/below may require separate consideration of effective stiffness at the connection.