Code Reference
StructuralASCE 7-22

Section 11.4.1/Mapped Acceleration Parameters

ASCE 7-22 Section 11.4.1 establishes how the mapped spectral acceleration parameters SS and S1 are determined, the starting point for all seismic design.

What this section requires

The mapped spectral acceleration parameters SS (short period) and S1 (1-second period) must be determined from Figures 22-1 through 22-8 or from the ASCE 7 Hazard Tool for the project site location. These parameters represent the maximum considered earthquake (MCER) ground motion and are used to determine the seismic design category, base shear, and all other seismic design parameters.

Why this section exists

Seismic ground motion varies dramatically by location. SS and S1 are the foundation of the entire seismic design process in ASCE 7. Every subsequent seismic calculation (site coefficients, design spectral accelerations, seismic design category, base shear) derives from these two parameters. An error in SS or S1 propagates through every structural member and connection on the project.

What plan reviewers look for

Plan reviewers check that SS and S1 are stated on the structural design criteria sheet and verify the values against the ASCE 7 Hazard Tool for the project latitude and longitude. They also check that the values are used consistently throughout the calculations and that the correct ASCE 7 edition maps were used (the maps changed between 7-16 and 7-22).

Common violations

SS and S1 not shown on design criteria
The structural drawings state the seismic design category but do not show the underlying SS and S1 values used to determine it. The reviewer cannot verify the SDC without the source parameters.
Values from wrong ASCE 7 edition
SS and S1 values from ASCE 7-10 or 7-16 maps are used when the jurisdiction requires ASCE 7-22. The seismic hazard maps were updated between editions and the values can differ significantly.
Wrong site coordinates
The SS and S1 values were looked up using coordinates that do not match the actual project site. Even a few miles of error can produce different values in high-seismicity regions.
Compliance tip
State SS and S1 on the design criteria sheet with the site coordinates used for the lookup. Reference the ASCE 7 edition and note whether values were obtained from the maps or the ASCE 7 Hazard Tool. Show the full chain from SS/S1 through site coefficients to SDS/SD1 to SDC.

Related ASCE 7 requirements

Section 11.4.3 covers site coefficients (Fa and Fv) that modify SS and S1 for local soil conditions. Section 11.6 determines the seismic design category from SDS and SD1. Section 12.8.1 uses SDS to calculate the seismic base shear.

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Related sections

12.8.1Seismic Base ShearASCE 7-2212.12.1Story Drift LimitsASCE 7-22