Seismic base shear is one early number in a much larger structural design workflow. A source-aware screen can help organize ASCE 7 ELF-style inputs such as Ss, S1, site class, risk category, structural-system factors, period, Cs, and V = Cs × W, but it cannot replace current ASCE 7 text, project hazard-tool output, geotechnical data, adopted code, AHJ amendments, or licensed structural engineering review.
This guide explains the local prompts used by the ToolGrit seismic app and the source gaps that must stay visible before any number is carried into design software, permit calculations, construction documents, or sealed output.
Spectral Accelerations and Site Coefficients
The starting hazard prompts are mapped spectral accelerations Ss and S1, plus TL, risk category, selected standard edition, and site class. Use current ASCE Hazard Tool or USGS Seismic Design Web Services output for the project location and confirm the result with the geotechnical report, adopted code, AHJ, and engineer of record.
The app exposes local Fa and Fv rows so users can see how site class changes SDS and SD1:
SDS = (2/3) × Fa × Ss SD1 = (2/3) × Fv × S1
Those rows are source prompts only. They do not prove that Site Class D or E exceptions, site-response analysis, liquefaction, fault rupture, slope, soil-structure interaction, or local amendments have been addressed.
Seismic Base Shear Calculator
ASCE 7-22 equivalent lateral force procedure. Design spectral accelerations, seismic design category, response coefficient Cs, and base shear V from building weight and structural system.
The Seismic Response Coefficient Cs
The local Cs prompt starts with:
Cs = SDS / (R / Ie)
and shows upper and lower prompt rows:
- Upper prompt: Cs ≤ SD1 / [T × (R/Ie)] for T ≤ TL, or Cs ≤ SD1 × TL / [T^2 × (R/Ie)] for T > TL
- Lower prompt: Cs ≥ max(0.044 × SDS × Ie, 0.01)
- Additional prompt: where S1 ≥ 0.6g, Cs ≥ 0.5 × S1 / (R/Ie)
R, Ω0, Cd, and Ie are not approval switches. They depend on actual structural system, detailing, height limits, irregularities, load path, diaphragm behavior, connection design, and current ASCE 7 requirements.
Building Period and Its Effect on Base Shear
The app can use a local approximate period prompt:
Ta = Ct × hn^x
or a user-entered manual period. Manual periods need a clear source, model assumptions, and the applicable upper-period limit before they are used to reduce base shear. The app does not apply every Cu cap, modeling limitation, torsional requirement, modal-response trigger, or response-history trigger.
Use period output as a review item. Drift, P-delta, nonstructural components, diaphragms, collectors, foundations, and vertical force distribution still need full structural analysis.
Seismic Design Category: What It Controls
The app shows a local SDC prompt from SDS, SD1, S1, and risk category. That prompt can flag when a project may fall into stricter review territory, but it is not a final code determination.
SDC can affect permitted systems, height limits, irregularities, analysis method, redundancy, detailing, special inspection, and nonstructural-component requirements. The adopted building code, amendments, owner/project criteria, and engineer of record control how those requirements apply.