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ACI 318-19 Special Moment Frame Column — Confinement

§18.7.5 rectilinear hoops: Table 18.7.5.4 with the high-axial-load expression (c) = 0.2·k_f·k_n·P_u/(f_yt·A_ch), spacing limits including the Grade 80 limit of 5d_b (ACI 318-19), s_o, the h_x limits (14 in / 8 in) and the length ℓ_o. Turkish counterpart: TBDY confinement tool.

Column

b in

h in

Cover to outside of hoop in

f′c psi

f_yt (hoops) psi

P_u (max compression) kip

Smallest longitudinal bar

Longitudinal grade

n_l — perimeter bars laterally supported

h_x — max spacing of supported bars in

Hoop bar

Legs perpendicular to b_c along b (running in the h direction)

Legs perpendicular to b_c along h (running in the b direction)

Clear height ℓ_u in

Results

P_u > 0.3A_gf′c or f′c > 10 ksi?—

A_sh/(s·b_c): (a) · (b) · (c) Table 18.7.5.4—

k_f · k_n—

s required by A_sh (b / h) in—

s limits: dim/4 · 6d_b|5d_b · s_o §18.7.5.3—

Hoop spacing within ℓ_o—

ℓ_o = max(h_max, ℓ_u/6, 18 in) §18.7.5.1—

Table 18.7.5.4(a) 0.3(A_g/A_ch − 1)f′c/f_yt; (b) 0.09f′c/f_yt; (c) 0.2k_fk_nP_u/(f_ytA_ch) only when P_u > 0.3A_gf′c or f′c > 10,000 psi; k_f = f′c/25,000 + 0.6 ≥ 1.0; k_n = n_l/(n_l − 2)

§18.7.5.3s ≤ least of: smallest column dimension/4; 6d_b (Grade 60) or 5d_b (Grade 80) of the smallest longitudinal bar; s_o = 4 + (14 − h_x)/3, 4 ≤ s_o ≤ 6 in

§18.7.5.2h_x ≤ 14 in; with high axial load or f′c > 10,000 psi, h_x ≤ 8 in and every perimeter bar supported by a hoop corner or seismic hook

DirectionA_sh is the steel perpendicular to the core dimension b_c: for b_c measured along b, count the legs that run in the h direction.

VerifyHand calculation (regression test): 24×24 in, f′c 5 ksi, #4 × 4 legs, b_c = 21 in — P_u 600 kip: (a) = 0.00765 governs, s = 4.98 in; P_u 1000 kip: k_n = 1.2, (c) = 0.00907 governs, s = 4.20 in. Sources: S. K. Ghosh Associates, STRUCTURE, ideCAD, Tekla.

ScopeRectilinear hoops only (spiral ρ_s not included); spacing outside ℓ_o (§18.7.5.5), shear design (§18.7.6) and strong-column/weak-beam (§18.7.3) 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).