Seismic Analysis (ASCE 7)

How it calculates

The Seismic Analysis calculator determines seismic design forces for US buildings using the Equivalent Lateral Force (ELF) procedure per ASCE 7-16 Chapter 12. It converts spectral acceleration parameters from the USGS hazard maps into a seismic base shear and distributes that force to each building story.

Site-adjusted spectral accelerations (ASCE 7-16, Cl. 11.4)

Starting from the mapped short-period acceleration (S_s) and long-period acceleration (S_1), the calculator applies site coefficients F_a and F_v to obtain the maximum considered earthquake (MCE) spectral accelerations:

S_MS = F_a × S_s (site-adjusted short period) S_M1 = 1.5 × F_v × S_1 (site-adjusted long period, with Supplement 3 50% increase for Site Class D when S_1 > 0.2)

Design spectral accelerations are two-thirds of the MCE values:

S_DS = 2/3 × S_MS S_D1 = 2/3 × S_M1

Note: Supplement 3 to ASCE 7-16 is applied, which may significantly increase seismic loads for Site Class D (when S_1 > 0.2) and Site Class E (when S_s > 1.0 or S_1 > 0.2).

Seismic design category (SDC)

SDC is determined from Tables 11.6-1 and 11.6-2 as the more severe of the category based on S_DS and S_D1, considering the building's Risk Category. Both tables are always evaluated. Site class F and SDC A special design requirements are not supported.

Seismic base shear (ASCE 7-16, Cl. 12.8)

The seismic response coefficient C_s is calculated from S_DS, the response modification coefficient R, and the importance factor I_e. It is also checked against the S_D1-based upper bound for long-period buildings and a minimum floor. Base shear:

V = C_s × W_total

Where W_total is the effective seismic weight. The fundamental period T is either computed as the approximate period T_a = C_t × h_n^x (using "all other structural systems" parameters per Table 12.8-2) or entered as a custom value, subject to the C_u upper bound.

Vertical distribution of seismic forces

The base shear is distributed to each story level using the vertical distribution exponent k (1.0 for T ≤ 0.5 s, 2.0 for T ≥ 2.5 s, interpolated between):

F_x = C_vx × V, where C_vx = w_x × h_x^k / sum(w_i × h_i^k)

The lateral load at each story and the cumulative shear are tabulated and can be exported for diaphragm and lateral system design.

Assumptions

The building is assumed to have no torsional irregularities. No height limit increases or structural limit modifications are considered for any seismic force-resisting system. The simplified design procedure per Cl. 12.14 is not used.