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ASCE 7-22 Diaphragm Seismic Design Forces

Three methods side by side: the traditional F_px of §12.10.1–12.10.2, the alternative §12.10.3 procedure with the design acceleration coefficient C_px and the diaphragm reduction factor R_s, and the §12.10.4 procedure for one-story rigid-wall, flexible-diaphragm (RWFD) buildings. Defaults reproduce the FEMA P-2192 examples.

Multi-story building (§12.10.1–2 and §12.10.3)

S_DS

S_D1

I_e

R (vertical SFRS)

Ω₀

Period T (s)

Stories N

Story height (ft)

Roof weight (kips)

Typical floor weight (kips)

z_s (Table 12.10-1 note)

Roof diaphragm R_s

Floor diaphragms R_s

Level	h (ft)	w (kips)	F_x	Traditional F_px	C_px	Alternative F_px	Alt. collector ×1.5

w_px is taken equal to w_i (the FEMA example does the same "for simplicity"; walls parallel to the force may be excluded). Collectors with the traditional method use Ω₀ (§12.10.2.1); with the alternative method ×1.5 (§12.10.3.4); neither needs to exceed E_ci.

One-story RWFD building (§12.10.4)

Diaphragm

S_DS

S_D1

I_e

Span L (ft)

Depth B (ft)

w_px (kips)

Collector length (ft)

Scope limits of §12.10.4.1 apply: one story; wood structural panel or bare (untopped) steel deck; rigid vertical elements (concrete, precast, masonry walls; steel or composite braced frames or walls); rectangular segments; no horizontal irregularities except Type 2. Defaults: FEMA P-2192 §7.6.2 warehouse (600 × 360 ft, S_DS 1.0, S_D1 0.5).

Sources and verification

MethodThe step-by-step procedures, the R_s table (ASCE 7-22 Table 12.10-1) and the z_s table are taken from FEMA P-2192-V1, 2020 NEHRP Recommended Seismic Provisions: Design Examples (2021, public domain), §7.3. ASCE 7-22 is cited by clause number only.

AnchorsAll three FEMA P-2192 examples are reproduced (tests/asce7-diaphragm.test.js): one-story wood assembly hall (both methods govern at the lower bound, 19,072 / 17,072 lb); six-story steel building (traditional 302 / 510 / 460 / 411 / 377 / 377 kips; alternative roof 778 kips and floors 4–2 at 392 / 385 / 385 kips); one-story RWFD warehouse (traditional 1,712 kips; §12.10.4 with S400 detailing 1,266 kips; without it 3,800 kips — the detailing decision changes the force threefold).

Errata foundChecking the six-story example revealed two slips in FEMA P-2192 §7.5.2. (1) "C_pi = C_p0 = 0.48 (Eq. 12.10-8)" — Eq. 12.10-8 is 0.8·C_p0 = 0.384; the result is unaffected because 0.9·Γ_m1·Ω₀·C_s = 0.51 governs. (2) At the 6th floor the book interpolates C_p6 over 12 ft (the story height) instead of the 0.2·h_n = 14.4 ft between 0.8·h_n and h_n: 0.60 instead of 0.582, i.e. F_p6 = 471 instead of about 457 kips (conservative by 3 %). This tool uses the corrected interpolation. Also, the traditional F_px is not floored at F_x: in a one-story building F_x = V includes the walls parallel to the force, which w_px excludes (FEMA §7.6.1 uses 1,712 kips, not V = 2,090).