Seismic Design Categories — IBC and ASCE 7 Reference
Seismic Design Categories (SDC) are letter designations — A through F — assigned to buildings and other structures under IBC 2021 and ASCE 7-22. The SDC determines which lateral force-resisting systems are permitted, what detailing requirements apply, whether special inspections are mandatory, and what height limits constrain a given structural system. Every project in a jurisdiction that adopts IBC must establish the SDC before the structural system can be selected; using an inappropriate system for the assigned SDC is a code violation regardless of how the analysis turns out numerically.
The SDC is a function of two things: the site-adjusted design spectral accelerations at the building location and the Risk Category (RC) of the building based on its occupancy and importance. Higher occupancy and higher ground-motion demand both push the SDC upward.
Spectral Acceleration Parameters
Mapped Ground-Motion Parameters
ASCE 7-22 defines two reference spectral accelerations taken from national hazard maps:
- S_S — Maximum Considered Earthquake (MCER) spectral acceleration at the 0.2-second (short) period, per ASCE 7-22 Figure 22-1. This parameter controls short-period (stiff) structures.
- S_1 — MCER spectral acceleration at the 1.0-second (long) period, per ASCE 7-22 Figure 22-2. This parameter controls flexible structures and governs the SDC E/F thresholds.
These values are unitless (expressed as a fraction of g) and are looked up using the building's geographic coordinates via the USGS seismic hazard tool or equivalent.
Site Classes
Site class is assigned based on the averaged shear wave velocity in the upper 30 m of soil (V_s30), standard penetration resistance, or undrained shear strength. ASCE 7-22 Table 20.3-1 defines six classes:
| Site Class | Description |
|---|---|
| A | Hard rock (V_s30 > 1500 m/s) |
| B | Rock (760–1500 m/s) |
| C | Very dense soil or soft rock (360–760 m/s) |
| D | Stiff soil (180–360 m/s) — default when data unavailable |
| E | Soft clay (< 180 m/s) |
| F | Liquefiable, quick-sensitive clays, or peats (site-specific study required) |
When subsurface data are not available, Site Class D must be assumed unless the authority having jurisdiction approves Site Class B or C based on known geology.
Site-Modified Spectral Accelerations
Site amplification factors F_a and F_v adjust the mapped values for local soil conditions:
SMS = Fa × SS (short-period, site-modified MCER)
SM1 = Fv × S1 (1-second, site-modified MCER)
Design spectral accelerations are then taken at two-thirds of the MCER values:
SDS = (2/3) × SMS
SD1 = (2/3) × SM1
F_a Values — Short-Period Site Coefficient (ASCE 7-22 Table 11.4-1)
| Site Class | S_S = 0.25 | S_S = 0.50 | S_S = 0.75 | S_S = 1.00 | S_S = 1.25 |
|---|---|---|---|---|---|
| A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| B | 0.9 | 0.9 | 0.9 | 0.9 | 0.9 |
| C | 1.3 | 1.3 | 1.2 | 1.2 | 1.2 |
| D | 1.6 | 1.4 | 1.2 | 1.1 | 1.0 |
| E | 2.4 | 1.7 | 1.3 | 1.1 | 0.9 |
| F | Site-specific analysis required |
F_v Values — Long-Period Site Coefficient (ASCE 7-22 Table 11.4-2)
| Site Class | S_1 = 0.1 | S_1 = 0.2 | S_1 = 0.3 | S_1 = 0.4 | S_1 = 0.5 |
|---|---|---|---|---|---|
| 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 | 1.5 |
| D | 2.4 | 2.2 | 2.0 | 1.9 | 1.8 |
| E | 4.2 | 3.3 | 2.8 | 2.4 | 2.2 |
| F | Site-specific analysis required |
Values between tabulated S_S or S_1 points are obtained by linear interpolation.
SDC Determination — ASCE 7-22 Tables 11.6-1 and 11.6-2
The SDC is determined separately from S_DS and S_D1, and the more severe of the two results governs.
Risk Categories (IBC Table 1604.5)
- Risk Category I — low-hazard structures (agricultural, minor storage)
- Risk Category II — ordinary buildings (most commercial, residential)
- Risk Category III — substantial hazard (schools > 250 occupants, assembly > 300, utilities)
- Risk Category IV — essential facilities (hospitals, fire stations, emergency operations)
Table 1: SDC from S_DS (ASCE 7-22 Table 11.6-1)
| S_DS Value | Risk Cat. I/II | Risk Cat. III | Risk Cat. IV |
|---|---|---|---|
| S_DS < 0.167 | A | A | A |
| 0.167 ≤ S_DS < 0.33 | B | B | C |
| 0.33 ≤ S_DS < 0.50 | C | C | D |
| S_DS ≥ 0.50 | D | D | D |
Table 2: SDC from S_D1 (ASCE 7-22 Table 11.6-2)
| S_D1 Value | Risk Cat. I/II | Risk Cat. III | Risk Cat. IV |
|---|---|---|---|
| S_D1 < 0.067 | A | A | A |
| 0.067 ≤ S_D1 < 0.133 | B | B | C |
| 0.133 ≤ S_D1 < 0.20 | C | C | D |
| S_D1 ≥ 0.20 | D | D | D |
SDC E and F — S_1-Triggered Categories
Two additional categories override the table results when mapped S_1 is very high:
- SDC E: Risk Category I or II structures at a site where S_1 ≥ 0.75
- SDC F: Risk Category III or IV structures at a site where S_1 ≥ 0.75
These are assigned regardless of S_DS and S_D1 values and represent near-fault or high-hazard coastal/western US sites. SDC E and F trigger the most stringent detailing and system restrictions in the code.
Short-Period Building Exception
ASCE 7-22 Section 11.6 permits a building to be assigned a lower SDC (from Table 11.6-1 only) when all of the following apply: T < 0.8 T_S, T_S = S_D1/S_DS, and the building meets all requirements of the SDC determined from S_DS alone. This exception allows short, stiff structures on sites where S_D1 would otherwise govern a higher SDC.
SDC Implications
The assigned SDC controls lateral system selection, height limits, and special inspection requirements directly. The following table summarizes the practical consequences for each category.
| SDC | Typical Location | Lateral System Requirement | Height Limits | Special Inspection |
|---|---|---|---|---|
| A | Very low seismic (e.g., parts of central/eastern US far from faults) | No seismic lateral system required; wind-governed | None specific to seismic | Not required |
| B | Minor seismic | Ordinary systems permitted; minimal detailing | Generally unrestricted | Limited requirements |
| C | Moderate seismic (e.g., parts of Pacific Northwest inland, central US) | Intermediate and some Special systems required; Ordinary MF restricted | Some height limits begin | Required for lateral systems |
| D | High seismic (e.g., coastal CA, Pacific NW, AK) | Special systems required for most occupancies; Intermediate restricted | Significant height limits (e.g., 35 ft for Ordinary MF) | Required; expanded scope |
| E | Very high seismic (S_1 ≥ 0.75), Risk Cat. I/II | Special systems only; most Intermediate prohibited | Strict limits; some systems prohibited entirely | Required; full special inspection program |
| F | Very high seismic (S_1 ≥ 0.75), Risk Cat. III/IV | Special systems only; heightened peer review often required | Most restrictive limits in ASCE 7 | Mandatory; enhanced program |
Common Structural Systems by SDC
AISC 341-22 (Seismic Provisions) and ASCE 7-22 Table 12.2-1 define which structural systems are permitted in each SDC. The key systems for steel construction are summarized below.
Moment Frames
| System | Permitted SDC |
|---|---|
| Ordinary Moment Frame (OMF) | A, B (height limit applies in C) |
| Intermediate Moment Frame (IMF) | A through C (with height limits) |
| Special Moment Frame (SMF) | A through F (no height limit unless noted) |
Shear Walls (Concrete and CLT)
| System | Permitted SDC |
|---|---|
| Ordinary Reinforced Concrete Shear Wall | A, B |
| Intermediate Precast Shear Wall | A through C |
| Special Reinforced Concrete Shear Wall | A through F |
Braced Frames (Steel)
| System | Permitted SDC |
|---|---|
| Ordinary Concentrically Braced Frame (OCBF) | A through C; limited height in D |
| Special Concentrically Braced Frame (SCBF) | A through F |
| Eccentrically Braced Frame (EBF) | A through F |
| Buckling-Restrained Braced Frame (BRBF) | A through F |
| Special Plate Shear Wall (SPSW) | A through F |
Approximate Height Limits (ASCE 7-22 Table 12.2-1)
| System | SDC B/C | SDC D | SDC E | SDC F |
|---|---|---|---|---|
| OMF (steel) | NL | 35 ft | NP | NP |
| IMF (steel) | NL | NL | NP | NP |
| SMF (steel) | NL | NL | NL | NL |
| OCBF (steel) | NL | 35 ft | NP | NP |
| SCBF (steel) | NL | NL | NL | NL |
| BRBF (steel) | NL | NL | NL | NL |
NL = No limit. NP = Not permitted.
Seismic Design Process Overview
The following flowchart summarizes the SDC determination process per ASCE 7-22:
1. Determine Risk Category (IBC Table 1604.5)
|
2. Obtain S_S and S_1 from USGS hazard maps
|
3. Determine Site Class (ASCE 7-22 Ch. 20)
|
4. Determine Fa and Fv (Tables 11.4-1 and 11.4-2)
|
5. Compute site-modified values:
SMS = Fa × SS
SM1 = Fv × S1
|
6. Compute design values:
SDS = (2/3) × SMS
SD1 = (2/3) × SM1
|
7. Check S1 >= 0.75?
YES → SDC E (RC I/II) or SDC F (RC III/IV) → STOP
NO → Continue
|
8. Determine SDC from SDS (Table 11.6-1)
Determine SDC from SD1 (Table 11.6-2)
|
9. Assign the MORE SEVERE of the two SDCs
|
10. Select permitted lateral system per
ASCE 7-22 Table 12.2-1 and AISC 341-22
Frequently Asked Questions
How do I determine the SDC for my project?
Start by looking up S_S and S_1 for the project address using the USGS Seismic Design Values web tool (https://earthquake.usgs.gov/hazards/designmaps). Then classify the site per ASCE 7-22 Chapter 20 using available geotechnical data or defaulting to Site Class D. Apply F_a and F_v to obtain S_MS and S_M1, then compute S_DS = (2/3)S_MS and S_D1 = (2/3)S_M1. Run both Tables 11.6-1 and 11.6-2 against the project's Risk Category and take the more severe result. Check separately whether S_1 ≥ 0.75 to see if SDC E or F is triggered.
What is the difference between SDC D and SDC E?
SDC D is driven by high design spectral accelerations (S_DS ≥ 0.50 or S_D1 ≥ 0.20) and applies to Risk Categories I through IV. SDC E is reserved for Risk Category I and II buildings at sites where the mapped 1-second acceleration S_1 ≥ 0.75, regardless of S_DS and S_D1. SDC E represents near-fault or very high-hazard sites where the long-period content of ground shaking is particularly severe. Practically, SDC E prohibits several systems allowed in SDC D — including OCBF and OMF in steel — and generally requires Special systems with no height restrictions removed.
Can I use an Ordinary Moment Frame in SDC C?
It depends on the building height and the specific system. ASCE 7-22 Table 12.2-1 permits steel OMF in SDC C only up to 35 ft in height for Risk Category I/II structures. Above that height limit or for Risk Category III/IV occupancies, OMF is not permitted in SDC C and an Intermediate or Special Moment Frame must be used. In reinforced concrete, Ordinary moment frames are not permitted in SDC C at all per ACI 318.
Does SDC apply to non-structural components?
Yes. ASCE 7-22 Chapter 13 governs nonstructural components and applies seismic design requirements that scale with SDC. In SDC A, essentially no nonstructural seismic requirements apply. Beginning in SDC B and escalating through SDC F, components such as mechanical equipment, cladding, suspended ceilings, piping, and electrical conduit must be designed and anchored for seismic forces. The seismic design force for a component is computed using the component amplification factor a_p and the component response modification factor R_p, both tabulated in ASCE 7-22 Table 13.5-1 and 13.6-1. Special inspections for nonstructural anchorage are triggered in SDC C through F.
What is the practical difference between SDC A and SDC D? SDC A represents very low seismic hazard — typically parts of the central and eastern United States far from active faults — and requires no seismic lateral force-resisting system at all; lateral loads from wind typically govern. SDC D represents high seismic hazard found in coastal California, the Pacific Northwest, and Alaska, where SDS ≥ 0.50g or SD1 ≥ 0.20g. In SDC D, Special structural systems are required for most occupancies, Ordinary Moment Frames are limited to 35 ft, Ordinary Concentrically Braced Frames are similarly restricted, special inspections are mandatory, and the design must explicitly consider stability under large inelastic deformations. The engineering design effort, material costs, and inspection requirements in SDC D are substantially greater than in SDC A.
How does site class affect SDC assignment? Site class amplifies the mapped spectral accelerations S_S and S_1 through the site coefficients F_a and F_v before computing the design values SDS and SD1. Soft soil conditions (Site Class E) produce the largest amplifications — F_v values up to 4.2 at low S_1 — which can push a building from SDC B to SDC D even on a moderate-hazard site. For example, a site with mapped S_1 = 0.15g could have SD1 = (2/3) × Fv × S_1 = (2/3) × 3.3 × 0.15 = 0.33g on Site Class E soil, yielding SDC D, whereas the same site on Site Class B rock would compute SD1 = (2/3) × 0.8 × 0.15 = 0.08g and yield SDC B. This is why a geotechnical investigation establishing the actual site class can significantly affect structural system selection and cost.
What detailing requirements change between SDC B and SDC C? In SDC B, ordinary structural systems with minimal special detailing are permitted, and special inspection of lateral systems is not generally required. Moving to SDC C introduces several key changes: Ordinary Reinforced Concrete Moment Frames and Ordinary Reinforced Concrete Shear Walls are no longer permitted per ACI 318; intermediate or special concrete systems must be used instead. Steel Ordinary Moment Frames are height-limited. Special inspection of welding, high-strength bolting, and concrete in the lateral system becomes required. Additionally, nonstructural components including mechanical and electrical equipment must be anchored for seismic forces, and certain types of irregularities trigger additional analysis requirements under ASCE 7-22 Section 12.3.
Run This Calculation
→ Seismic Load Calculator — compute ASCE 7 seismic base shear using your site's SDS, SD1, and Risk Category.
→ Load Combinations Calculator — apply ASCE 7 LRFD and ASD load combinations including seismic (E) load effects.
Related Reference Pages
- Load Combinations — ASCE 7-22 LRFD & ASD
- Wind Load Calculation — ASCE 7-22 MWFRS & C&C
- Snow Load Calculation — ASCE 7 Design Procedure
- Live Load Reference — ASCE 7 Floor Live Loads (psf)
- Steel Fy & Fu Reference — Yield and Tensile Strength by Grade
- Beam Capacity Calculator
- structural wind load calculator
- snow load calculator
The authoritative requirements for Seismic Design Category determination are found in ASCE 7-22 Sections 11.4 through 11.6. F_a and F_v tables are in Sections 11.4.3 and 11.4.4. Height limits and permitted systems are in Section 12.2 and Table 12.2-1. Always consult the current adopted edition of IBC and the applicable version of ASCE 7 for the jurisdiction of your project.
Disclaimer (educational use only)
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.
All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.
The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.