Step through the ASCE 7-22 Equivalent Lateral Force (ELF) procedure: enter site spectral values, site class, and building system to compute SDS, SD1, Cs, design base shear V, and story force distribution.
Step through the ASCE 7-22 Equivalent Lateral Force (ELF) procedure for multi-story buildings. Enter site spectral accelerations Ss and S1, site class, risk category, and lateral force-resisting system to compute design spectral values SDS and SD1, seismic response coefficient Cs, design base shear V, and the vertical story force distribution Fx.
The ELF procedure (ASCE 7 §12.8) follows seven steps: (1) Look up site coefficients Fa and Fv from site class and spectral values. (2) Compute MCE spectral accelerations SMS = Fa·Ss and SM1 = Fv·S1. (3) Scale to design level: SDS = 2/3·SMS and SD1 = 2/3·SM1. (4) Determine Seismic Design Category (SDC). (5) Compute approximate period Ta = Ct·h^x. (6) Find Cs = SDS/(R/Ie), bounded by minimum values. (7) Compute base shear V = Cs·W and distribute to stories via Cvx = wi·hi^k / Σ(wi·hi^k).
Base shear: V = Cs·W. Seismic coefficient: Cs = SDS/(R/Ie) ≤ SD1/(Ta·R/Ie). Minimum: Cs ≥ max(0.044·SDS·Ie, 0.01). Approximate period: Ta = Ct·hn^x (Ct = 0.028, x = 0.8 for steel moment frames). Story force: Fx = Cvx·V where Cvx = wx·hx^k / Σ(wi·hi^k), k = 1 for Ta ≤ 0.5 s, k = 2 for Ta ≥ 2.5 s.
The ELF procedure applies to most regular buildings in SDC B through D when the fundamental period is within specified limits (Ta ≤ 3.5·Ts for SDC D-F). For irregular structures, long-period buildings, or SDC E/F, a response spectrum analysis (RSA) or response history analysis is required. The tool is ideal for preliminary design and education; final design always requires reference to the published ASCE 7-22 tables and maps.
Cs = SDS/(R/Ie) is the fraction of the building weight W that becomes the design base shear. It is bounded by minimum values to prevent under-design of buildings in low-seismicity regions. A lower R (less ductile system) gives a higher Cs and therefore larger forces.
R is the response modification factor that accounts for ductility and overstrength of the lateral force-resisting system. Higher R values (e.g., R = 8 for special steel moment frames) allow the building to be designed for lower forces by relying on ductile behavior during the design-level earthquake. Lower R values (e.g., R = 3 for ordinary moment frames) require design for larger forces.
SDC (A through F) is assigned based on the design spectral accelerations SDS and SD1 combined with the Risk Category. Higher SDC means stricter requirements for system selection, detailing, height limits, and analysis procedure. SDC A has minimal requirements; SDC D, E, and F require special seismic detailing and may restrict system selection.
The k exponent in Cvx = wi·hi^k / Σ(wi·hi^k) accounts for higher-mode effects in taller buildings. For periods ≤ 0.5 s, k = 1 (linear triangle distribution). For periods ≥ 2.5 s, k = 2 (parabolic distribution with more force at the top). Between 0.5 and 2.5 s, k interpolates linearly between 1 and 2.