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Building Code Reference

Got a CBC 1613 correction?

Seismic design requirements

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CBC 1613 is the California Building Code section that requires every structure to be designed for earthquake forces based on four site-specific parameters: mapped spectral accelerations (Ss and S1), site soil class, seismic risk category, and the resulting seismic design category (SDC). The seismic design category drives which lateral force-resisting system you're required to use, how it must be detailed, and what analysis procedures ASCE 7 requires. Most commercial and residential projects in California trigger a 1613 correction when the structural sheets don't show all the required seismic data.

How to fix it

  1. Pull the USGS seismic hazard values for your site

    Go to the USGS Seismic Design Maps tool and enter the project's latitude and longitude. Select the code edition that matches the permit jurisdiction (2025 CBC references ASCE 7-22 for new projects). The tool returns Ss (0.2-second spectral acceleration) and S1 (1-second spectral acceleration) for your exact location. Print or screenshot the results page with the coordinates and code edition shown — the plan-checker will want to see this in the calculations package. Do not use generic county-level values; coordinates within a few miles can shift Ss significantly in Southern California.

  2. Confirm the site class from the soils report

    The geotechnical report must explicitly state the ASCE 7 site class. Classes range from A (hard rock) to F (liquefiable or highly sensitive soils), including the intermediate BC, CD and DE classes added in ASCE 7-22. If the geotech report doesn't state a site class directly, it's not safe to assume Site Class D as a default without confirming average shear wave velocity (Vs30) or blow count data from the report. If you don't have a soils report, the engineer of record needs to justify the assumed class in writing or require a soils investigation before proceeding. Mismatching the site class on the plans vs. the soils report is one of the most common 1613 corrections in Orange County.

  3. Calculate design spectral values SDS and SD1

    ASCE 7-22, which the 2025 CBC references, no longer uses site coefficients Fa and Fv: SMS and SM1 come directly from the USGS seismic design data for your site class. Then: SDS = (2/3) x SMS and SD1 = (2/3) x SM1. SS, S1, SDS and SD1 must all appear on the title sheet or the structural general notes (CBC 1603.1.5). The design spectral values are what the engineer uses to size the lateral system; the plan-checker checks them for internal consistency with the raw Ss/S1 values.

  4. Determine the risk category and seismic design category

    CBC Table 1604.5 assigns a risk category (I through IV) based on occupancy. Most single-family and standard commercial buildings are Risk Category II. Hospitals and essential facilities are Category III or IV. From the risk category and SDS/SD1 values, use ASCE 7 Tables 11.6-1 and 11.6-2 to find the seismic design category (SDC A through F). SDC determines how much detailing is required on shear walls, hold-downs, and connections. This determination must be explicitly shown on the plans — not left for the plan-checker to calculate.

  5. Add the complete seismic data block to the title sheet

    Place a clearly labeled block on the structural title sheet (or Sheet S0.1 / general notes) that includes every item required by Section 1603.1.5: risk category, seismic importance factor Ie, Ss, S1, site class, SDS, SD1, seismic design category, basic seismic force-resisting system (e.g., 'Special reinforced concrete shear walls'), design base shear V, seismic response coefficient Cs, response modification factor R, and the analysis procedure used. Label each row clearly. Many corrections come from disorganized notes that have some values but not all — put them in one table so the plan-checker doesn't have to hunt.

  6. Verify detailing requirements match the SDC

    Higher seismic design categories (D, E, F) require specific detailing that SDC A and B don't. If your project is SDC D, for example, shear wall hold-downs, anchor bolts, and collector elements all have prescriptive requirements under ASCE 7 Chapter 12 and CBC Chapter 16. Make sure the structural details on the plans reference and meet those requirements. A 1613 correction often pulls in related corrections on shear wall schedules, diaphragm connections, and foundation anchor bolt specs — fixing 1613 first helps you see which details to upgrade.

  7. Cross-check the soils report site class against the plans before resubmitting

    Before you send the package back, open the soils report and the structural title sheet side by side. Confirm the site class letter matches exactly. If the geotech used a different classification method or the report is old enough that it predates ASCE 7-16/22 site class definitions, the structural engineer should note the reconciliation. Plan-checkers in most California jurisdictions will reject a resubmittal if this mismatch is still present — they see it as an unresolved structural assumption.

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Why you got flagged

Plan-checkers flag CBC 1613 most often because the required seismic design data block is missing from the structural title sheet or the general notes. Section 1603.1.5 lists exactly what must appear on construction documents: the mapped spectral response accelerations Ss and S1, the design spectral values SDS and SD1, the site class, the seismic design category, the basic seismic force-resisting system, the design base shear, the seismic response coefficient Cs, and the response modification factor R. If any of those are absent, the plan-checker stamps it as a 1613 correction. Contractors often get the response modification factor R added by an engineer but forget to carry through to the design base shear calculation on the title sheet.

A second common trigger is a mismatch between the site class on the structural plans and what's in the soils report. The geotechnical engineer assigns a site class based on soil testing — Site Class A through F. When the structural engineer uses a different class (usually defaulting to D without testing confirmation), the plan-checker catches the conflict. This matters because the site class directly sets the design spectral values: under ASCE 7-22, SMS and SM1 come from the USGS data for that class, and softer soils generally raise them.

Projects that change scope mid-design also get flagged. An addition or tenant improvement that increases the footprint can shift the structure's risk category — which may change the seismic design category and trigger more stringent detailing. If the engineer didn't update the seismic parameters on the plans when the scope changed, the plan-checker will cite 1613 on resubmittal.

The official code

California Building Code 2025 — Chapter 16 Structural Design (Section 1613 Earthquake Loads)

Read the source →

Common questions

Does CBC 1613 apply to residential additions and ADUs?
Yes. Any new structure or addition that requires a building permit in California is subject to CBC 1613, including accessory dwelling units and residential additions. For standalone ADUs, the engineer of record must run the seismic parameters and include the full data block on the structural plans. If you're adding square footage to an existing house, the project triggers a re-evaluation of the entire structure's risk category and may require upgrading the lateral system if the addition materially changes the building's mass or geometry. Smaller additions under certain thresholds may qualify for prescriptive seismic requirements under the CRC instead of a full CBC 1613 analysis, but the structural engineer makes that call.
What site class should I use if there's no soils report?
ASCE 7 allows using Site Class D as a default when no site-specific data is available, with one major exception: you cannot use Site Class D as the default if Site Class E or F conditions are known or suspected. In practice, many California plan-checkers in cities with expansive clay soils, liquefiable fill, or known soft ground conditions will flag a default Site Class D and ask for justification or a soils investigation. For projects in Orange County near coastal areas, alluvial flood plains, or areas with mapped liquefiable soils, assume the plan-checker will push back. Budget for a geotechnical investigation on anything other than solid bedrock sites if you want to avoid a 1613 correction on site class.
How do I find Ss and S1 values for my project address?
Use the USGS Seismic Design Maps web tool at the U.S. Geological Survey's website. Enter the project latitude and longitude (not just a street address — use coordinates for accuracy) and select the applicable ASCE edition. The tool returns Ss, S1, TL, and other parameters directly. Some structural engineers also use the ASCE 7 Hazard Tool, which is the official tool tied to the current standard. Either way, print the output with the coordinates displayed and include it in your calculations package. Plan-checkers frequently ask to see the source for Ss and S1 values, and a printout from the official USGS or ASCE tool is the cleanest answer.
What's the difference between Ss and SDS?
Ss is the mapped short-period spectral acceleration — a raw value based on the seismic hazard at your location from the USGS hazard maps. SDS is the design spectral acceleration after adjusting Ss for site amplification (Fa) and applying a two-thirds factor. The formula is SDS = (2/3) x Fa x Ss. SDS is what you actually use to size the lateral force-resisting system and calculate design base shear. The plan-checker wants to see both Ss (the input) and SDS (the design value) on the title sheet, along with the Fa factor and the intermediate calculation SMS = Fa x Ss, so they can verify the math. If Ss and SDS are the only two values on the plans, the calculations are incomplete.
What triggers a change in seismic design category?
The seismic design category (SDC) is determined by two inputs: the risk category of the building (from CBC Table 1604.5) and the design spectral values SDS and SD1. A change in either input can shift the SDC. Common triggers: upgrading a storage building to an occupied use (raises the risk category), discovering the site is on softer soil than assumed (a softer site class can raise SDS and SD1), or a code edition change that updates the hazard maps. On commercial tenant improvements or change-of-use permits, the plan-checker will verify that the SDC was re-evaluated if the occupancy changed. Getting bumped from SDC C to SDC D is a big deal — it triggers prescriptive requirements on hold-downs, collectors, and connections that SDC C doesn't require.
Can the structural engineer of record choose the seismic force-resisting system?
Yes, but the system must be permitted for the building's seismic design category. ASCE 7 Table 12.2-1 lists allowed systems by SDC. For example, ordinary moment frames are not permitted in SDC D. The engineer selects from the allowed systems, which sets the response modification factor R and the deflection amplification factor Cd used in the design. These values must appear on the structural title sheet. If the engineer lists a system that ASCE 7 restricts for that SDC, the plan-checker will flag it under 1613 even if the seismic data block is otherwise complete.

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