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ASCE 7-22 — Equivalent Lateral Force

Seismic response coefficient Cs · Approximate period Ta · Base shear V = Cs·W · Vertical distribution of seismic forces Fx · Story shear Vx

Site data from USGS (optional)

Fetch SDS, SD1, S1, TL and the multi-period design spectrum for a site from the USGS Seismic Design Web Service (ASCE 7-22). With the spectrum loaded, Method 1 reads Sa at the building period automatically (§11.4.5.1).

Latitude

Longitude

Risk category

Site class (Table 20.2-1)

or Vs30 ft/s

Seismic Parameters

S_DS (design, g)

ASCE 7 Hazard Tool, for your site class

S_D1 (design, g)

S₁ (mapped MCE_R, g)

Used only for SDC E/F and the Eq. 12.8-7 floor

T_L (long-period transition) s

ASCE 7 Hazard Tool

S_a at T — optional, Method 1 (g)

From the multi-period design spectrum (§11.4.5.1) at the period T used below

ASCE 7-22 no longer uses the site coefficients F_a and F_v: site-adjusted S_MS, S_M1 and S_DS = ⅔S_MS, S_D1 = ⅔S_M1 come directly from the USGS Seismic Design Geodatabase (ASCE 7 Hazard Tool) for the site class. Entering ASCE 7-16 values (F_a·S_s) here would not be an ASCE 7-22 design.

R — Response modification factor

Ie — Importance factor

Structure type (for Ta)

Building Data

Total seismic weight W kip

Building height hn ft

Number of stories

T analytical (optional, from modal analysis) s

If given, T = min(T_analytical, Cu·Ta) per §12.8.2 / Table 12.8-1

Story weights are assumed equal (W/n per floor)

Seismic Design Results

SDS (input) §11.4.5—

SD1 (input)—

SDC Tables 11.6-1/2—

Ta (approx. period) Eq.12.8-8 s—

Cu · Ta (period cap) Table 12.8-1 s—

T used—

Governing Cs equation—

Cs (Eq.12.8-3)—

Cs,max (Eq.12.8-4/5)—

Cs,min (Eq.12.8-6/7)—

V = Cs·W Eq.12.8-1 kip—

k (distribution exponent)—

Code References

ASCE 7-22 updates (v547)Supplement 3 (ASCE) adds rules to the multi-period spectrum (§11.4.5.1) for periods longer than 10 s — they reach this tool through the Sa value you read from the ASCE 7 Hazard Tool. Short-period spike: in the central and eastern U.S. the multi-period spectrum peaks at 0.05–0.1 s; for ELF with T < 0.2 s the two-period spectrum (Method 2) is recommended by Soules and Guglielmo (STRUCTURE magazine, 2025) — Method 1 is conservative there. Soft-soil sites (central and eastern U.S., Site Classes D/E): the multi-period spectra can give markedly lower accelerations than ASCE 7-16 (e.g. Savannah, GA, Site Class E: SDC D → C) — the unconservative direction; a correction is being processed as a supplement to ASCE 7-22, and jurisdictions adopting the 2024 IBC are advised to adopt Code Change S102-25 (S. K. Ghosh Associates). Check the current ASCE 7-22 errata and supplements before final design.

Supplement 3ASCE 7-22 Supplement 3 (effective January 27, 2025) restates §11.4.5.1: S_a = 2/3 of the multi-period MCE_R spectrum from the USGS Seismic Design Geodatabase at the listed periods, linear interpolation between them, and beyond 10 s (T_L ≥ 10 s) S_a(10 s)·10/T up to T_L, then ·10T_L/T². Enter S_a at your period accordingly. Where the multi-period spectrum is available, ELF Method 1 or Method 2 may be used (S. K. Ghosh Associates, 2025).

§11.4.5SDS, SD1, TL from the USGS Seismic Design Geodatabase (ASCE 7 Hazard Tool) for the site class — ASCE 7-22 has no Fa/Fv site coefficients

Eq.12.8-8Ta = Ct·hn^x (approximate fundamental period)

Eq.12.8-2Method 1: Cs = Sa/(R/Ie), Sa from the multi-period spectrum at T (no upper cap)

Eq.12.8-3Method 2: Cs = SDS/(R/Ie)

Eq.12.8-4Cs ≤ SD1/(T·R/Ie) for T ≤ TL

Eq.12.8-6Cs ≥ max(0.044·SDS·Ie, 0.01) — both methods

Eq.12.8-1V = Cs·W (seismic base shear)

Eq.12.8-5Cs ≤ SD1·TL/(T²·R/Ie) for T > TL

Eq.12.8-7Cs ≥ 0.5·S1/(R/Ie) where S1 ≥ 0.6g — floor applied AFTER the cap (ordering verified by regression test)

Table 12.8-1Cu upper-limit coefficient on calculated period: T ≤ Cu·Ta (1.7…1.4, interpolated on SD1)

Tables 11.6-1/2Seismic Design Category from SDS and SD1 (higher governs); S1 ≥ 0.75g → E/F

Verificationengine/asce7-seismic.js — 50/50 tests: two external anchors (civilmat; SkyCiv doc example corrected via internal-consistency audit), all five Cs branches, Cu interpolation, 8 SDC cases. A third candidate source was REJECTED for an arithmetic error (78^0.8 misstated). One real ordering bug found in us-codes.js and fixed with a regression test.

§12.8.3Fx = Cvx·V ; Cvx = wx·hx^k / Σ(wi·hi^k)

Engine: us-codes.js + asce7-seismic.js · US-ASCE7-22.v2 · 50/50 external-anchor tests ·

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Vertical Distribution of Seismic Forces

Story	hx (ft)	wx (kip)	wx·hx^k	Cvx	Fx (kip)	Vx (kip)

Step-by-Step Solution

Preliminary calculation tool. Ss and S1 values must be obtained from USGS seismic design maps for the specific project site. Final design requires the full ASCE 7-22 text and review by a licensed PE.