Seismic site parameters and spectrum
Gives the seismic design values of a site: SDS, SD1, the design spectrum, the Seismic Design Category and, if you ask, the vertical spectrum and the site class from a velocity profile. ASCE 7-22 has no table of site coefficients any more: section 11.4.3 takes SS, S1, SMS, SM1 and the multi-period spectrum from the USGS Seismic Design Geodatabase. So those values are the input here. Copy the results into Seismic base shear or the other seismic types: nothing links them.
Inputs of the spectrum #
- Latitude, longitude, site class and risk category. The site class comes from your geotechnical report. If the soil is not known well enough, choose Not known: the most critical of Site Classes C, CD and D at each period (11.4.2.1).
- Get values from USGS: the button asks the USGS web service for that site and fills in everything below, with the date and model in Source of the values. The values are then part of your calculation and are saved with it. The calculation never asks again, so it works offline and gives the same answer later. If you change the location, risk category or site class afterward, an advisory tells you to get the values again. If the service does not answer, you get a message and can type the values.
- SS, S1, SMS, SM1, TL, PGAM: from USGS or typed. S1 sets SDC E or F at 0.75 or more. With SMS or SM1 at 0, the value comes from your table by the rule of 21.4, which is how USGS gets it too (FEMA P-2192). PGAM is carried for reference.
- Design spectrum: multi-period (11.4.5.1), or two-period (11.4.5.2), which the code permits only where the multi-period values are not available: that is your call.
- MCER spectrum table: period and SaM at up to 30 periods. The second and third values are other site classes, for 11.4.2.1 or 20.3: the largest governs at each period.
- Period of interest T: gives Sa at that period, with the rules past 10 s and past TL.
- Vertical spectrum: the option of 11.9 in place of 12.4.2.2, by region: west of 105° W, Alaska, Hawaii and the territories by 11.9.2; the rest of the conterminous US at 2/3 of the horizontal spectrum.
- Site class from a velocity profile: layers with thickness and shear wave velocity. It reports the average over the upper 100 ft and the class. With velocities estimated, it also finds the class at 1.3 times and 1/1.3 times the average (20.3). Extend the last layer is your judgment on the geology, for data past 50 ft and short of 100 ft. The profile does not change the values: they stay those of the site class you chose, and an advisory says if the two differ.
Checks performed #
| Result | Basis |
|---|---|
| SDS, SD1 | Eqs. 11.4-1, 11.4-2 |
| SMS, SM1 from the table | 21.4: 90 % of the largest SaM from 0.2 to 5 s; 90 % of the largest T SaM from 1 to 2 s above 1,450 ft/s or 1 to 5 s below, not less than SaM at 1 s |
| Multi-period spectrum | 11.4.5.1: 2/3 of the MCER values, straight lines between them, 10/T and 10 TL/T² past 10 s |
| Two-period spectrum | 11.4.5.2: Eqs. 11.4-3 to 11.4-5, T0 and TS |
| MCER spectrum | 11.4.6: 1.5 times the design spectrum |
| Seismic Design Category | 11.6, Tables 11.6-1 and 11.6-2, the more severe; E or F at S1 of 0.75 or more |
| Vertical MCER and design spectra | 11.9.2, 11.9.3, Table 11.9-1, Eqs. 11.9-1 to 11.9-8 |
| Average shear wave velocity, site class | Eq. 20.4-1, Table 20.2-1, 20.3 |
The type gives values and reads Calculated. Site Class F stops the calculation: it needs a site response analysis (21.1).
Limitations #
- USGS is the source of the values. The app checks only that they are numbers in range, and shows the 21.4 result from the table beside SMS and SM1 as a cross-check. USGS rounds what it returns.
- Site-specific procedures (11.4.7, Chapter 21) are not run. A site-specific spectrum can be typed into the table.
- The four conditions under which 11.6 permits Table 11.6-1 alone depend on the structure, so they are checked in Seismic base shear, not here.
- Site Class F soils (20.2.1) are not detected from velocities: that is your geotechnical engineer's call. Near-fault status (11.4.1) is not determined.
- For Site Class not known, Table 11.9-1 has no column of its own; the column of Site Class D is used, the largest of the three.
- Correlations from blow count or strength to velocity are not built in: enter the velocities your geotechnical engineer gives.
- Chart to 5 s; the calculation lists every period.
Verification examples #
Verification status: 64 values compared in 2 examples (2 published, 0 hand calculations from the cited equations): 61 within tolerance, 3 recorded deviations. Thin: only 2 published examples. Code basis: the provisions it uses were read in the target edition.
| Example | Source | Values |
|---|---|---|
| Vertical spectrum, Irvine, Site Class D | FEMA P-2192-V1 (2020), Table 3-2, p. 122 | SMS 1.63, Cv 1.426, SaMv at 23 periods, Fmd at 13 |
| Most critical of D, CD and DE, velocities estimated | FEMA P-2192-V1, Table 3-3, p. 124 | Final SaM at 23 periods; 850, 1,105 and 654 ft/s and their classes |
No published example covers one site from SS and S1 through SDS, the multi-period spectrum and the category with the numbers shown, so those steps are hand calculations in the tests. The button is tested against a stand-in for the service, and the shape of its answer was read from a live call on 2026-11-07 (latitude 33.66, longitude -117.82, Site Class D and the default class). One documented difference: Table 3-2 leaves out the lower limit of Eq. 11.9-4 (0.5 SaM / Fmd) at 1, 1.5 and 2 s, where it governs. This type applies it as the code says, so it gives 0.449, 0.313 and 0.232 g against the printed 0.425, 0.242 and 0.155 g. The other 20 periods agree. In the code copy on file, Table 11.9-1 prints "12.0" for the SMS row of 1.0 and Eq. 11.9-8 prints "1 < Tv ≤ 1.0" for 10 s. The example's Cv of 1.426 and its Fmd values confirm 1.0 and 10.