ACI 318-25 / ASCE 7 Structural Design Guide
Quick-reference for concrete material properties, ductility classification, section sizing, load combinations, and reinforcement design per ACI 318-25 and ASCE 7-22.
1.1 Concrete (f'c)
| Parameter | US (psi) | SI (MPa) | Reference |
|---|---|---|---|
| Minimum f'c | 2,500 psi | 17 MPa | ACI 318-25 §19.2.1.1 |
| Maximum f'c (normal provisions) | 10,000 psi | 70 MPa | ACI 318-25 §19.2.1.3 |
| Typical structural use | 3,000–6,000 psi | 21–42 MPa | — |
Higher f'c values permitted with special provisions per §19.2.1.3.
Unit weight (normalweight concrete): US: 150 pcf | SI: 24 kN/m³ (ACI 318-25 §19.2.2)
1.2 Reinforcing Steel (fy)
| Parameter | US (ksi) | SI (MPa) | Reference |
|---|---|---|---|
| Minimum fy | 40 ksi | 280 MPa | ACI 318-25 §20.2.2.4 |
| Maximum fy (general) | 80 ksi | 550 MPa | ACI 318-25 §20.2.2.4 |
| Common grades | Grade 60 (60 ksi) | Grade 420 (420 MPa) | — |
Modulus of elasticity (rebar): US: 29,000 ksi | SI: 200,000 MPa (ACI 318-25 §20.2.2.2)
→ Rebar Properties & Bar Size Reference
1.3 Modulus of Elasticity (Concrete)
US: Ec = 57,000√f'c (f'c in psi)
SI: Ec = 4,700√f'c (f'c in MPa)
ACI 318-25 §19.2.2.1
See Concrete Properties Reference for full unit weight, strength class, and elastic modulus tables.
1.4 Lightweight Concrete — Modification Factor (λ)
ACI 318-25 §19.2.4.2
| Concrete Type | λ |
|---|---|
| Normalweight | 1.00 |
| Sand-lightweight (sanded) | 0.85 |
| All-lightweight (unsanded) | 0.75 |
Linear interpolation permitted between values based on volumetric fraction of normalweight sand replacement.
Alternative (test-based) method — §19.2.4.3:
US: λ = fct / (6.7√f'c), capped at λ ≤ 1.0
SI: λ = fct / (0.56√f'c), capped at λ ≤ 1.0
where fct = average splitting tensile strength (measured)
2.1 Seismic Design Category (SDC)
ASCE 7-22 §11.6 — SDC is determined from Risk Category and spectral response parameters (SDS, SD1).
Table 11.6-1 (based on SDS):
| SDS Value | Risk Category I, II, III | Risk Category IV |
|---|---|---|
| SDS < 0.167g | A | A |
| 0.167g ≤ SDS < 0.33g | B | C |
| 0.33g ≤ SDS < 0.50g | C | D |
| 0.50g ≤ SDS | D | D |
Table 11.6-2 (based on SD1): Similar tiered structure based on SD1 values.
Final SDC = the more critical (higher) result from the two tables.
Site-specific seismic parameters (SDS, SD1) require a geotechnical/seismic hazard report.
2.2 Moment Frame Ductility Classes
ACI 318-25 §18.2.1.3 — permitted frame types by SDC:
| SDC | Permitted Frame Type | ACI 318-25 Reference |
|---|---|---|
| A, B | Ordinary Moment Frame (OMF) | Ch. 1–17 (no Ch. 18 special detailing required) |
| C | Intermediate Moment Frame (IMF) minimum | §18.4 |
| D, E, F | Special Moment Frame (SMF) required | §18.6–18.10 |
2.3 Response Modification Factor (R) — System Limitations
ASCE 7-22 Table 12.2-1 — R factor is tied to specific system limitations (height, SDC permissibility, and force-sharing rules). Selecting R without checking these triggers a code violation.
2.3.1 Moment Frame Systems (selected values)
| System | R | SDC B | SDC C | SDC D | SDC E | SDC F |
|---|---|---|---|---|---|---|
| Ordinary RC Moment Frame | 3 | NL | NP | NP | NP | NP |
| Intermediate RC Moment Frame | 5 | NL | NL | NP | NP | NP |
| Special RC Moment Frame | 8 | NL | NL | NL | NL | NL |
NL = Not Limited (height) · NP = Not Permitted
Note: ACI 318-25 §18.2.1.3 — Ordinary moment frame is permitted in lieu of intermediate for SDC B/C only.
2.3.2 Shear Wall Systems — Bearing Wall (no frame backup)
| System | R | SDC B | SDC C | SDC D | SDC E | SDC F |
|---|---|---|---|---|---|---|
| Ordinary RC Shear Wall | 4 | NL | NL | NP | NP | NP |
| Special RC Shear Wall | 5 | NL | NL | 160 ft (48.8 m) | 160 ft (48.8 m) | 100 ft (30.5 m) |
Bearing wall = shear walls support both gravity AND lateral load, no separate moment frame for redundancy.
2.3.3 Shear Wall Systems — Building Frame (gravity frame separate)
| System | R | SDC D/E | SDC F |
|---|---|---|---|
| Ordinary RC Shear Wall | 5 | NP | NP |
| Special RC Shear Wall | 6 | 160 ft | 100 ft |
Building frame system = gravity frame is separate; shear walls resist 100% of lateral load (gravity frame is not designed to share lateral resistance).
2.3.4 Dual Systems — Moment Frame + Shear Wall
ASCE 7-22 §12.2.5.1 — In a dual system, the moment frame must be capable of resisting at least 25% of the design seismic forces, independent of the shear wall contribution. The total lateral resistance is distributed between the moment frame and shear walls in proportion to their relative rigidities.
| System | R (SMF + Special Wall) | Frame min. share |
|---|---|---|
| Dual System — Special Moment Frame + Special RC Shear Wall | 7–8 (varies by SDC) | ≥ 25% of base shear |
If the moment frame does not achieve this 25% minimum, the system cannot be classified as "dual" — it must be redesigned as a bearing wall or building frame system with the corresponding (lower) R value.
2.3.5 Shear Wall–Frame Interactive System (Ordinary, SDC A/B only)
ASCE 7-22 §12.2.5.8 — A distinct combination available only in SDC A and B:
- Shear walls must resist at least 75% of the design story shear at any level
- Moment frames must independently resist at least 25% of the design story shear
R = 4.5 (typical, verify against current Table 12.2-1 edition)
2.3.6 Practical Note
R selection is not a standalone choice — it locks in: (1) height limit applicability, (2) SDC permissibility, (3) minimum force-sharing ratios for combined systems, and (4) the detailing chapter required (Section 8). Always verify against the current edition of ASCE 7-22 Table 12.2-1 and §12.2.5, as values are subject to periodic revision.
2.4 Practical Implication
Once SDC is determined, the required frame type is established. This governs which subsection of Section 8 (Reinforcement Detailing) applies.
3.1 Minimum Dimensions (Non-Seismic)
| Element | US | SI | Reference |
|---|---|---|---|
| Beam width (practical min) | 10 in | 250 mm | — |
| Column dimension (practical min) | 12 in | 300 mm | — |
| Slab thickness (min, non-deflection govern) | 4 in | 100 mm | §7.3.1.1 |
3.2 Span-to-Depth Ratios (Deflection Control)
ACI 318-25 Table 7.3.1.1 / 9.3.1.1 — minimum thickness without deflection computation:
| Support Condition | Minimum h |
|---|---|
| Simply supported | L/20 |
| One end continuous | L/24 |
| Both ends continuous | L/28 |
| Cantilever | L/10 |
Values for non-prestressed beams/one-way slabs, normalweight concrete, fy = 60,000 psi (420 MPa). See §7.3.1.1 for adjustment factors.
→ Beam Flexural & Shear Design Calculator · Slab Design Calculator
3.3 SDC-Dependent Minimum Dimensions (Seismic)
| Element | SMF Requirement | Reference |
|---|---|---|
| Beam width | ≥ 10 in (250 mm), bw/h ≥ 0.3 | §18.6.2.1 |
| Column min dimension | ≥ 12 in (300 mm) | §18.7.2.1 |
| Column aspect ratio | shortest/longest ≥ 0.4 | §18.7.2.1 |
See Section 2 for SDC → frame type mapping.
4.1 Dead Loads
Actual self-weight of materials and fixed equipment. ASCE 7-22 §3.1. Typical material unit weights: see Concrete Properties Reference and Steel Properties Reference pages.
4.2 Live Loads
Occupancy-based minimum uniform/concentrated loads — ASCE 7-22 Table 4.3-1 (selected):
| Occupancy | US (psf) | SI (kPa) |
|---|---|---|
| Office (general) | 50 psf | 2.4 kPa |
| Residential (private) | 40 psf | 1.9 kPa |
| Assembly (fixed seats) | 60 psf | 2.9 kPa |
| Parking garage (passenger vehicles) | 40 psf | 1.9 kPa |
Full table: ASCE 7-22 Table 4.3-1. Live load reduction permitted per §4.7.
4.3 Wind Loads (Overview)
ASCE 7-22 Ch. 26-30 — Directional Procedure or Envelope Procedure for MWFRS.
Basic wind pressure: q = 0.00256·Kz·Kzt·Kd·Ke·V² (US, psf) | q = 0.613·Kz·Kzt·Kd·Ke·V² (SI, Pa, V in m/s)
Kz = velocity pressure exposure coefficient (§26.10), V = basic wind speed (Ch. 26 maps), Kzt = topographic factor, Kd = directionality, Ke = ground elevation factor.
Full procedure requires site-specific risk category, exposure category, and topographic data — see ASCE 7-22 Ch. 26-30.
4.4 Snow Loads (Overview)
ASCE 7-22 Ch. 7 — Flat roof snow load:
pf = 0.7·Ce·Ct·Is·pg (US & SI, consistent units)
pg = ground snow load (site-specific, Ch. 7 maps), Ce = exposure factor, Ct = thermal factor, Is = importance factor.
5.1 Site Class
ASCE 7-22 Table 20.3-1 — based on soil shear wave velocity / SPT / undrained shear strength:
| Site Class | Description |
|---|---|
| A | Hard rock |
| B | Rock |
| C | Very dense soil / soft rock |
| D | Stiff soil (default if unknown, §11.4.3) |
| E | Soft soil |
| F | Special soils requiring site-specific analysis |
5.2 Equivalent Lateral Force Procedure
ASCE 7-22 §12.8 — Base shear:
V = Cs · W
Cs = SDS / (R / Ie) [upper bound, §12.8.1.1]
where W = effective seismic weight, R = response modification factor (Section 2.3), Ie = importance factor.
Minimum and long-period Cs limits apply — see §12.8.1.1 Eq. 12.8-3 through 12.8-6.
5.3 Vertical Distribution of Seismic Force
ASCE 7-22 §12.8.3:
Fx = Cvx · V, where Cvx = (wx·hxk) / Σ(wi·hik)
k = distribution exponent: 1.0 for T ≤ 0.5 s, 2.0 for T ≥ 2.5 s, linear interpolation between.
6.1 ACI 318-25 Strength Design (§5.3.1)
| Combination | Equation |
|---|---|
| U1 | 1.4D |
| U2 | 1.2D + 1.6L + 0.5(Lr or S or R) |
| U3 | 1.2D + 1.6(Lr or S or R) + 1.0L (or 0.5W) |
| U4 | 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R) |
| U5 | 1.2D + 1.0E + 1.0L + 0.2S |
| U6 | 0.9D + 1.0W |
| U7 | 0.9D + 1.0E |
Identical in US and SI (dimensionless load factors). Reference: ACI 318-25 §5.3.1, ASCE 7-22 §2.3.6.
7.1 Flexural Design
Rn = Mu / (φ·b·d²)
ρ = (0.85f'c/fy)·[1 − √(1 − 2Rn/0.85f'c)]
As = ρ·b·d
ρmin = max(3√f'c/fy, 200/fy) [US] | max(0.25√f'c/fy, 1.4/fy) [SI] — §9.6.1.2
φ = 0.90 (tension-controlled) — Table 21.2.2
→ Beam Flexural Design Calculator
7.2 Shear Design
φVc = φ·2λ√f'c·bw·d [US, psi] | φ·0.17λ√f'c·bw·d [SI, MPa] — §22.5.5.1
Vs = Av·fy·d/s (stirrup contribution) — §22.5.10.5.3
φ = 0.75 (shear) — Table 21.2.1
→ Beam Shear Design Calculator
7.3 Shear-Friction
Vn = Avf·fy·μ — §22.9.4.2
| Interface Condition | μ |
|---|---|
| Monolithic concrete | 1.4λ |
| Roughened hardened concrete | 1.0λ |
| Non-roughened hardened concrete | 0.6λ |
| Concrete to steel (anchored) | 0.7λ |
Applies to: corbels, brackets, cold joints, composite member interfaces. Vn capped per §22.9.4.4.
7.4 Column Design (Axial + Flexure)
PMM interaction — combined axial load (Pn) and biaxial moment (Mnx, Mny) checked against interaction diagram. See Column PMM Design Calculator for interactive diagram generation.
φ = 0.65 (compression-controlled) to 0.90 (tension-controlled), transition per Table 21.2.2.
7.5 Torsion
Threshold (torsion may be neglected below):
Tth = 0.083λ√f'c·(Acp²/pcp) [SI] | λ√f'c·(Acp²/pcp) [US] — §22.7.4.1
Above threshold, full torsion design per §22.7 required (closed stirrups + longitudinal bars).
8.1 Non-Seismic Detailing (SDC A/B, Ch. 1-17, 20-25)
Standard cover (§20.5.1), standard development length (§25.4), standard stirrup/tie spacing (§25.7). No special ductile detailing required.
→ Development Length Calculator (ACI 318)
8.2 Ordinary Moment Frame (OMF) — SDC A/B (§18.3)
Minimal additional requirements beyond Ch. 1-17. Beam-column joints follow standard provisions.
8.3 Intermediate Moment Frame (IMF) — SDC C (§18.4)
Hoop spacing at beam/column ends: so ≤ min(8db, 24·dtie, 0.5h or b, 12 in / 300 mm) — §18.4.2 / 18.4.3
Stirrups required over length 2h from member face.
8.4 Special Moment Frame (SMF) — SDC D/E/F (§18.6-18.10)
Hoop spacing (boundary regions): so ≤ min(6db, 6 in / 150 mm) — §18.6.4.4
Boundary element requirements (shear walls) — §18.10.6: special transverse reinforcement where compression strain demand exceeds threshold.
Strong column-weak beam requirement: ΣMnc ≥ (6/5)ΣMnb — §18.7.3.2
9.1 Diaphragm Design (Overview)
ACI 318-25 Ch. 12 / ASCE 7-22 §12.10 — in-plane shear and chord force transfer for floor/roof systems acting as horizontal diaphragms.
9.2 Drift Limits
ASCE 7-22 Table 12.12-1 — Allowable story drift (Δa) as fraction of story height (hsx):
| Risk Category | Δa Limit |
|---|---|
| I, II | 0.025·hsx (most structures) |
| III | 0.020·hsx |
| IV | 0.015·hsx |