How to determine your project's Seismic Design Category

Helonic is an AI construction drawing analysis platform for people determine seismic design category during plan review.

Risk Category, Site Class, mapped spectral acceleration, and the two-table cross-reference in ASCE 7

Seismic Design Category (SDC) is a letter classification, A through F, that determines which seismic design and detailing requirements apply to a structure. It's calculated from two inputs: the building's Risk Category and the seismic hazard at its site. Getting it wrong early in design cascades into every structural system decision downstream.

What are the five steps to determine seismic design category?

Seismic design category is determined by risk category, site class, mapped accelerations, design spectral response, and the more severe of the two ASCE 7 tables.

Determine Risk Category

IBC Table 1604.5 assigns Risk Category I through IV based on occupancy and importance. A standard office is typically Risk Category II; a hospital or emergency response facility is Risk Category IV.

Determine Site Class

ASCE 7 Chapter 20 classifies the site A through F based on soil properties from a geotechnical report. If no geotechnical report exists yet, Section 11.4.3 sets Site Class D as the default.

Look up mapped Ss and S1

Enter the site's latitude and longitude into the USGS Seismic Design Maps tool to get the mapped short-period (Ss) and 1-second (S1) spectral acceleration values for the applicable ASCE 7 edition.

Apply site coefficients

Apply site coefficients Fa and Fv (from ASCE 7 Tables 11.4-1 and 11.4-2, based on Site Class) to get SMS = Fa x Ss and SM1 = Fv x S1, then SDS = 2/3 x SMS and SD1 = 2/3 x SM1.

Cross-reference both SDC tables

Check SDS against Table 11.6-1 and SD1 against Table 11.6-2, each cross-referenced against Risk Category. The final Seismic Design Category is whichever of the two results is more severe.

Why does the two-table SDC cross-reference matter?

A common shortcut is checking only Table 11.6-1 (based on SDS) and stopping there. ASCE 7 requires checking both tables, 11.6-1 for SDS and 11.6-2 for SD1, and using whichever result is more severe as the final SDC. A site can produce a Category C result from the short-period table and a Category D result from the 1-second table; the project is Category D.

Drawing Review Checkpoint

The design criteria block on the structural general notes sheet should show the SDC alongside the Risk Category, Site Class, and mapped Ss/S1 values it was derived from. If any of those upstream values changed during design, the SDC on the notes sheet needs to be reverified, not carried forward from an earlier design pass.

What does seismic design category control downstream?

SDC drives which structural systems are permitted, height and irregularity limits, and how much seismic-specific detailing (special moment frames, ordinary vs. special reinforced concrete shear walls, and similar requirements) applies to the project. It also determines seismic bracing requirements for nonstructural components, which is where structural and MEP coordination tends to break down; see our guide on seismic bracing coordination for how that plays out on a drawing set.

Practitioner insight

“The error that shows up most is a Site Class D assumed early because there was no geotech report yet, and then the geotech report comes back Site Class E and nobody reruns the SDS/SD1 tables. The notes sheet still says the original SDC, and everything downstream, bracing, detailing, was designed to the wrong category.”

Common finding in structural drawing coordination review.

Seismic Design Category FAQs

What is Seismic Design Category (SDC)?

Seismic Design Category is a classification (A through F) assigned to a building based on its Risk Category and the seismic hazard at its site, defined in ASCE 7 Chapter 11. SDC determines which structural design and detailing requirements apply, including permitted structural systems, height limits, and how much of the code's seismic-specific detailing a project has to follow.

How do I find Ss and S1 for a project site?

Ss (mapped short-period spectral acceleration) and S1 (mapped 1-second spectral acceleration) come from the site's latitude and longitude, looked up through the USGS Seismic Design Maps tool referenced by ASCE 7. Enter the site coordinates and reference document (ASCE 7-22 or whichever edition the jurisdiction has adopted) to get the mapped values used in the SDC calculation.

What Site Class should I use if there's no geotechnical report yet?

ASCE 7 Section 11.4.3 specifies Site Class D as the default when site-specific soil properties are not known, unless the jurisdiction's building official determines Site Class E or F soils are likely present. Site Class D is a conservative default, not a substitute for a geotechnical report once one becomes available, since a different Site Class changes the site coefficients and can shift the final SDC.

Does Risk Category affect Seismic Design Category?

Yes. Risk Category (I through IV, from IBC Table 1604.5, based on the building's occupancy and importance) is one of the two inputs to SDC, alongside the site's SDS and SD1 values. The same site with the same seismic hazard can land in a different SDC depending on whether the building is a standard office (Risk Category II) or a hospital (Risk Category IV).

Why does the general notes sheet show a different SDC than my calculation?

The most common causes are an outdated code edition reference (SDC values change slightly between ASCE 7 editions as the maps get updated) or a Site Class that was assumed early in design and never revised after geotechnical data came back. Both tables, 11.6-1 for SDS and 11.6-2 for SD1, need to be checked against the current Risk Category and Site Class, using whichever result is more severe.

Manas Gandhi

Co-founder & CTO, Helonic

Manas is the co-founder and CTO of Helonic, where he leads engineering and AI research for construction drawing analysis. He works directly with structural, MEP, civil, and fire protection engineers to translate the way they review drawings into AI systems that flag the issues that actually matter in the field. Before Helonic, he built machine learning pipelines for technical document understanding and has spent the last several years interviewing licensed design engineers and discipline leads to ground product decisions in real practice rather than industry assumptions.

Areas of focus

AI for technical document understanding
Cross-discipline coordination workflows
Code compliance automation (IBC, NEC, NFPA, IPC, IMC, ASCE)
Structural and MEP drawing review systems

How this page was researched: Based on the ASCE 7 Chapter 11 SDC determination procedure and structural general notes review patterns across drawing sets.

Last reviewed by Manas Gandhi · August 25, 2026