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ACI 318-19 Special Structural Wall — Boundary Elements

§18.10.6: displacement-based trigger (a) c ≥ ℓ_w/(600·1.5δ_u/h_wcs), the new ACI 318-19 drift-capacity check (b) through its published simplified constraint b ≥ √(0.025·ℓ_w·c), the stress-based method (0.2f′c / 0.15f′c) and the boundary-element extents. Turkish counterpart: TBDY wall end-zone (perde uç bölgesi) checks.

Wall

ℓ_w in

b (thickness) in

h_wcs (above critical section) in

f′c psi

f_y

Neutral axis depth c

c entered directly in

A_s at each end in²

End steel from edge in

Web ρ_l

Demand

P_u kip

M_u kip-ft

V_u kip

Design displacement δ_u at top in

Results

h_wcs/ℓ_w · displacement method applies?—

c · source—

δ_u/h_wcs (≥ 0.005) · c limit—

Special boundary element — displacement method §18.10.6.2(a)—

Drift-capacity check (b), simplified: b ≥ √(0.025ℓ_wc)—

Stress method: σ / f′c §18.10.6.3—

Extent: horizontal · vertical §18.10.6.4(a)—

§18.10.6.2Walls with h_wcs/ℓ_w ≥ 2.0, one critical section, continuous. (a) Special boundary elements where c ≥ ℓ_w/(600(1.5δ_u/h_wcs)), δ_u/h_wcs ≥ 0.005. (b) New in 318-19: 1.5δ_u/h_wcs ≤ δ_c/h_wcs — drift capacity from the Abdullah & Wallace (2019) database of 164 tests.

(b) simplifiedWith V_e/(8A_cv√f′c) = 1 and δ_u/h_wcs = 0.015 the check reduces to b ≥ √(0.025·ℓ_w·c) (Ghosh & Taylor, Concrete International 2021). The full Eq. 18.10.6.2b is not computed here — if the simplified constraint fails, check it directly. Note: the form “√(c·ℓ_w)/40” sometimes quoted is wrong (40 belongs inside the root).

§18.10.6.3 · §18.10.6.4Stress method: σ = P_u/A_g + M_u(ℓ_w/2)/I_g > 0.2f′c; may stop where < 0.15f′c. Length ≥ max(c − 0.1ℓ_w, c/2); height ≥ max(ℓ_w, M_u/4V_u) above and below the critical section.

VerifyHand calculation (regression test): ℓ_w 240, h_wcs 960, δ_u 9.6 in → c limit 26.67 in; c = 40 → required; √(0.025·240·40) = 15.49 in > b = 14; σ = 773.8 psi = 0.155f′c. c from reinforcement uses the strain-compatibility function verified against spColumn.

ScopeRectangular walls (no flanges). Boundary transverse reinforcement (Table 18.10.6.4(g)), the shear amplification V_e = Ω_vω_vV_u and coupled walls are separate checks.

Preliminary calculation tool. Final design requires the full text of ACI 318 and review by a licensed PE. This tool follows ACI 318-19, the edition referenced by the 2021 and 2024 IBC; ACI CODE-318-25 was published in January 2025 and is expected in the 2027 IBC (changes include development length equations and seismic provisions).