ASCE 7-22 Seismic Design Category Guide — SDC A Through F
The Seismic Design Category (SDC) is the single most important classification in US seismic design. It determines which structural systems are permitted, what height limits apply, which analysis procedures are required, and what level of connection detailing is mandated. Per ASCE 7-22 Section 11.6, every building must be assigned an SDC before any lateral design work begins.
Related pages: Seismic Force-Resisting System Guide | ASCE 7 ELF Worked Example | Seismic Load Calculator
The SDC Determination Pipeline
Determining the SDC is a six-step sequential process per ASCE 7-22 Chapter 11. Each step feeds the next; an error at any step propagates through the entire lateral design.
Step 1 — Determine the Risk Category (Table 1.5-1)
The Risk Category classifies buildings by the consequence of structural failure. It is an INPUT to SDC, not the SDC itself.
| Risk Category | Description | Examples | Ie |
|---|---|---|---|
| I | Low hazard to human life | Agricultural buildings, minor storage | 1.00 |
| II | Standard occupancy | Houses, apartments, offices, retail | 1.00 |
| III | Substantial hazard | Schools, 300+ occupancy halls, power stations | 1.25 |
| IV | Essential facilities | Hospitals, fire stations, emergency centers | 1.50 |
RC III and IV trigger higher SDCs at lower spectral accelerations, and Seismic Importance Factor Ie directly scales the design base shear.
Step 2 — Obtain Mapped MCER Accelerations Ss and S1
Ss (0.2 s short-period) and S1 (1.0 s long-period) come from the USGS Seismic Design Geodatabase. They represent Risk-Targeted Maximum Considered Earthquake (MCER) spectral response acceleration at 5% damping on Site Class BC, adjusted for a uniform collapse risk (1% probability in 50 years).
| Location | Ss | S1 | Hazard Source |
|---|---|---|---|
| San Francisco, CA | 1.50 | 0.60 | San Andreas / Hayward faults |
| Los Angeles, CA | 1.50–2.50 | 0.60–1.00 | Puente Hills / San Andreas |
| Seattle, WA | 1.20–1.80 | 0.40–0.60 | Cascadia Subduction Zone |
| New York, NY | 0.25 | 0.06 | Intraplate, low |
| Charleston, SC | 1.00–1.50 | 0.30–0.50 | Intraplate, moderate |
| Miami, FL | 0.05–0.10 | 0.02 | Very low |
Step 3 — Classify the Site (Chapter 20)
The Site Class describes soil stiffness from surface to rock, determined by v_s30 (average shear wave velocity upper 100 ft), N_bar (SPT blow count), or s_u (undrained shear strength).
| Site Class | v_s30 (ft/s) | Typical Material |
|---|---|---|
| A — Hard rock | > 5,000 | Massive granite, limestone |
| B — Rock | 2,500–5,000 | Competent sedimentary rock |
| C — Dense soil / soft rock | 1,200–2,500 | Dense till, weathered rock |
| D — Stiff soil (default) | 600–1,200 | Stiff clays, dense sands |
| E — Soft clay soil | < 600 | Bay mud, soft alluvium |
| F — Site-specific required | — | Liquefiable, peat, H > 3 |
Site Class D is the default when geotechnical data is insufficient (Section 20.1).
Step 4 — Site Coefficients Fa and Fv (Tables 11.4-1 and 11.4-2)
Site coefficients amplify bedrock motion for soil effects. Fa applies to short-period; Fv to long-period.
Fa — Short-Period (Table 11.4-1):
| Site Class | Ss <= 0.25 | Ss = 0.50 | Ss = 0.75 | Ss = 1.00 | Ss >= 1.25 |
|---|---|---|---|---|---|
| A (hard rock) | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| B (rock) | 0.9 | 0.9 | 0.9 | 0.9 | 0.9 |
| C (dense) | 1.3 | 1.3 | 1.2 | 1.2 | 1.2 |
| D (stiff) | 1.6 | 1.4 | 1.2 | 1.1 | 1.0 |
| E (soft) | 2.4 | 1.7 | 1.3 | 0.9 | 0.9 |
The decreasing Fa at high Ss on soft soil reflects nonlinear soil response: very strong shaking limits amplification.
Fv — Long-Period (Table 11.4-2):
| Site Class | S1 <= 0.10 | S1 = 0.20 | S1 = 0.30 | S1 = 0.40 | S1 >= 0.50 |
|---|---|---|---|---|---|
| A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| B | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| C | 1.5 | 1.5 | 1.5 | 1.5 | 1.4 |
| D | 2.4 | 2.2 | 2.0 | 1.9 | 1.7 |
| E | 4.2 | 3.3 | 2.8 | 2.4 | 2.0 |
Fv is much larger than Fa for soft soils because long-period waves amplify more in deep basins — the mechanism behind Mexico City (1985) and Marina District (1989) damage.
Step 5 — Compute Design Spectral Accelerations (Section 11.4.5)
The 2/3 factor converts MCER (collapse-prevention) to design-basis (life-safety):
SMS = Fa * Ss
SM1 = Fv * S1
SDS = (2/3) * SMS
SD1 = (2/3) * SM1
Step 6 — Determine SDC (Tables 11.6-1 and 11.6-2)
Two separate determinations; the MORE SEVERE governs.
SDC based on SDS (Table 11.6-1):
| SDS Range | RC I, II, III | RC IV |
|---|---|---|
| SDS < 0.167 | A | A |
| 0.167 <= SDS < 0.33 | B | C |
| 0.33 <= SDS < 0.50 | C | D |
| SDS >= 0.50 | D | D |
SDC based on SD1 (Table 11.6-2):
| SD1 Range | RC I, II, III | RC IV |
|---|---|---|
| SD1 < 0.067 | A | A |
| 0.067 <= SD1 < 0.133 | B | C |
| 0.133 <= SD1 < 0.20 | C | D |
| SD1 >= 0.20 | D | D |
Additional mandatory assignments (Section 11.6):
- SDC E: RC I, II, III with S1 >= 0.75
- SDC F: RC IV with S1 >= 0.75
What Each SDC Means for Design
SDC A
No seismic design required. Lateral system designed for wind only. Basic structural integrity per ASCE 7 Section 1.4.
SDC B
Basic seismic detailing. ELF procedure permitted. AISC 360 design only; AISC 341 not triggered. Continuity plates at moment connections may be required but full connection prequalification is not.
SDC C
Transition zone. ELF permitted for all heights. Most standard systems permitted. AISC 341 OMF and OCBF provisions apply. Brace slenderness KL/r <= 200 for OCBF.
SDC D
The predominant category for western US design. Full seismic ductility requirements:
- Limited system selection with height limits (OMF <= 35 ft in SDC D)
- Redundancy factor rho = 1.3 unless two-bays-per-side criterion met
- AISC 341 SMF / SCBF default systems
- AISC 358 connection prequalification required
- Protected zones mandatory at beam plastic hinge regions
- Demand-critical welds with notch-tough filler metal
- QA/QC per AISC 341 Chapter J: continuous inspection, UT of CJP welds
SDC E and F
Reserved for buildings near major active faults (within ~10 km) with S1 >= 0.75. Near-fault pulse effects dominate. Site-specific ground motion hazard analysis required per Section 21.2. AISC 341 special seismic provisions apply in full.
Worked Example: 4-Story Steel Office, Seattle WA
Given
- 4-story office, 54 ft height, Risk Category II (Ie = 1.0)
- Ss = 1.50, S1 = 0.60 from USGS tool at project coordinates
- Site Class D (stiff soil, default)
Calculation
Fa and Fv: Fa = 1.0 (Table 11.4-1, Ss >= 1.25, Site Class D) Fv = 1.7 (Table 11.4-2, S1 >= 0.50, Site Class D — note: ASCE 7-22 §11.4.8 requires site-specific analysis since S1 >= 0.20 on Site Class D)
Spectral accelerations: SMS = 1.0 _ 1.50 = 1.50 SM1 = 1.7 _ 0.60 = 1.02 SDS = (2/3) _ 1.50 = 1.00 SD1 = (2/3) _ 1.02 = 0.68
SDC determination: Table 11.6-1: SDS = 1.00 >= 0.50, RC II => SDC D Table 11.6-2: SD1 = 0.68 >= 0.20, RC II => SDC D Governing SDC: D
Implications
- SMF or SCBF permitted (unlimited height). OMF limited to 35 ft — does NOT apply because 54 ft > 35 ft.
- Drift limit: 0.020 * hsx per Table 12.12-1.
- Design base shear per ELF: V = Cs _ W. Cs = SDS / (R/Ie) = 1.00 / (8/1.0) = 0.125 with lower bound Cs = 0.044 _ SDS _ Ie = 0.044 _ 1.0 _ 1.0 = 0.044, upper bound Cs = SD1 / (T _ R/Ie). Governs at Cs = 0.125.
- Connections: AISC 341 special provisions, protected zones, demand-critical welds.
Variation: Same Building, Site Class C
If geotechnical investigation reclassifies to Site Class C: Fa = 1.2, Fv = 1.4. SMS = 1.80, SM1 = 0.84. SDS = 1.20, SD1 = 0.56. Still SDC D — the high Ss is driving the classification regardless of the site improvement.
Variation: 4-Story Office, Memphis TN
Ss = 0.80, S1 = 0.20, Site Class D: Fa = 1.2, Fv = 2.2. SMS = 0.96, SM1 = 0.44. SDS = 0.64, SD1 = 0.293. Table 11.6-1 => SDC D (SDS = 0.64 >= 0.50). Table 11.6-2 => SDC D (SD1 = 0.293 >= 0.20). Also SDC D — the long-period amplification Fv = 2.2 on soft New Madrid basin soil pushes SD1 above the threshold.
Practical Tips
Always get a geotechnical report. Assuming Site Class D is permitted only when data is insufficient, but it can misclassify the structure. Site Class E can jump SDC from B to D.
Check §11.4.8 triggers. When S1 >= 0.20 on Site Class D/E/F, a site-specific ground motion analysis is mandatory. This may reduce (never increase) the general-procedure values, with a floor of 80% of the general values.
Beware of S1 >= 0.75. This threshold overrides the tables and forces SDC E (RC I-III) or SDC F (RC IV) — severely restricting structural system options.
Check the independent SDC assignments first. Sections 11.6-1.1 and 11.6-2.1 assign SDC C, D, or E for Site Class E with Ss >= 1.0 BEFORE consulting the tables.
Document everything. The Basis of Design must record: coordinates, USGS tool version, Site Class rationale, Fa/Fv table values, SDS/SD1 calculations, and governing SDC. This is the first document reviewed during peer review and plan check.