Wood diaphragm

A wood structural panel diaphragm to AWC SDPWS 2021 Section 4.2, under a uniform wind or seismic load, with the ASCE 7-22 load factors: one span between two shear wall lines, several spans, or a cantilever with its open-front limits. Unit shear, chord force, collector (drag) force, openings, aspect ratio, deflection and flexibility.

Source note. SDPWS 2021 (Chapter 4) was read against this module on 2026-11-08: every Table 4.2A / 4.2C vn it holds, the design factors (4.1.4), the aspect ratios (Table 4.2.2), the deflection equations (Table 4.2.3), the open-front rules (4.2.6) and the construction rules (4.2.7, 4.2.8). The apparent shear stiffness Ga is an input (transcribe it from Table 4.2A, 4.2B or 4.2C).

Inputs of the wood diaphragm #

• Sheathing and nailing: panel grade, thickness and nail; blocked or unblocked; for blocked diaphragms the nailing column of Table 4.2A (boundary 6 / 4 / 2-1/2 / 2 in with the other panel edges at 6 / 6 / 4 / 3 in; the boundary spacing also applies along continuous panel edges in cases 3 and 4 and along all panel edges in cases 5 and 6); the layout case 1 to 6 (unblocked: case 1 or cases 2 to 6 of Table 4.2C); the framing width at panel edges (2 or 3 in nominal); the framing species (species factor 1 − (0.5 − G) ≤ 1.0, table footnote 2). Entered replaces the table: type the one vn SDPWS 2021 prints; the second box takes a seismic vs from an SDPWS 2015 table or report only, and seismic uses the smaller of vn and 1.4 vs. Ga (kips/in) is entered from the table's OSB or plywood column; 0 skips the deflection.
• Diaphragm: the span L between the shear wall lines and the depth b between the chords; the length of the shear wall on each support line and its offset from the diaphragm edge (shorter than b: a collector drags the shear into the wall).
• Support: one span (the default; a calculation saved before this choice existed reads the same). Several spans: up to 12, with your judgment on the model: each span simple and tributary to the wall lines (a flexible diaphragm, the usual), or continuous on rigid wall lines (HUD Example 6.4's model; the middle line takes more, and the example itself warns how much this depends on that assumption). Cantilever: one wall line and the far edge free, with a load at the free edge as well as the uniform load.
• Open front (cantilever): on by default for seismic load: the L/W limit and the 35 ft limit of SDPWS 2021 4.2.6.1. Torsionally irregular is on by default, the stricter limit (1:1 one story, 0.67:1 above); turn it off, 1.5:1, when your analysis shows no Type 1 irregularity. Enter the elastic drift at the free edge from the walls, with torsion, from your rigid diaphragm analysis to check the drift of 4.2.6.1; the added unit shear from torsion also comes from there. The amplification of the diaphragm's own deflection is an input, 3 by SDPWS Commentary's example (WoodWorks calls Cd or the overstrength factor both acceptable).
• Openings: up to 6, each by its position and size and the transfer length lt on each side, where the ties run past the opening. Strips of at least 1 ft.
• High-load diaphragms: choose High-load and enter the vn of Table 4.2B (used for wind and seismic); the table is not reproduced here, its construction rules (4.2.8.1.2) are in a note.
• Roof slope: nothing to enter. Enter the span and depth of the diaphragm and the load along it; the table capacities and the aspect limits do not depend on the slope (Table 4.2.2 covers flat and sloped diaphragms).
• Loads: the uniform wind and seismic loads along the span (strength level, or ASD level 0.6W / 0.7E); the seismic load comes from Fpx (ASCE 7-22 12.10.1.1) and ρ = 1.0 for diaphragms (12.3.4.1). Ω0 for collectors is an input (12.10.2.1; 1.0 where the light-frame exception applies).
• Chords and deflection: chord E and area, the chord splice positions (each splice taken in both chords) and the splice slip Δc, optional chord and collector capacities (in the method's terms), the serviceability wind factor and an optional deflection limit (SDPWS 4.2.1 leaves the permissible deflection to the engineer). ADVE, the average drift of the adjoining walls, gives the flexibility classification.

Auto-design (editor toolbar) varies the sheathing and nail (table row), boundary nailing and blocking. Objectives: lowest cost index (sheathing + nails + blocking). Every other input is kept; see Auto-design.

Checks performed #

Drawings #

The drawing sheet (Drawing sheet / DXF buttons, and the report appendix) shows the diaphragm plan with the span running down the sheet: the shear walls along the two support lines (their length and offset as entered), the chords along the other two edges with ticks at the entered splices, the wind and seismic loads on the loaded edge, the sheathing and nailing callout, the unit shear at the supports, the largest chord force and, when the wall is shorter than the depth, the collector force; the span, splice positions, wall and depth are dimensioned. For the governing case it adds the unit shear and chord force diagrams along the span and the collector force along the support line. The chord and collector members, splice details, panel layout, framing and the shear walls themselves are not part of the calculation; the sheet notes say so.

Limitations #

• Uniform load on the same depth over all spans; concentrated loads only at a cantilever's free edge. Offsets and re-entrant corners are not covered.
• The torsion of a cantilever and its distribution to the walls come from your rigid diaphragm analysis (the torsion calculation); the rules for a torsionally irregular structure that is not open front (4.2.5.1: L/W at most 1.5:1 with wood structural panels, drift at each edge) are stated in a note, not checked. The open-front limits are applied also to a cantilever of 6 ft or less, which 4.2.6.1 exempts. Chord forces from walls along the chord lines are not included (the WoodWorks example shows about 365 lb in its case).
• Openings: no published free numbers; the method is hand-checked statics and the strips and zones follow Diekmann and Malone's reasoning, and two openings at one section are refused. Malone's transfer diaphragm is a judgment: ties and zone shears depend on lt.
• Table 4.2B values are entered, not tabulated. Lumber sheathing (4.2.8.2 to 4.2.8.4, Table 4.2D) is not offered.
• The chords are the diaphragm edges (b between them); chord and collector members and their splices are checked only against an entered capacity.
• The distribution to the walls is not done here.
• Read against SDPWS 2021 on 2026-11-08: every vn of Tables 4.2A and 4.2C held here, their footnotes, 4.1.4, Tables 4.2.2 and 4.2.3, 4.2.5, 4.2.6, 4.2.7 and 4.2.8.1. That reading corrected rated sheathing 5/16 in 6d (blocked vn 475 / 630 / 940 / 1,065 plf with 2 in framing and 530 / 700 / 1,065 / 1,205 with 3 in, were 460 / 600 / 910 / 1,035 and 520 / 700 / 1,050 / 1,175; unblocked case 1 with 3 in framing 475, was 460) and rated sheathing 15/32 in 10d unblocked case 1 with 3 in framing (810, was 800): all were low, on the safe side. The cantilever splice term now counts each listed splice in both chords, as the simple span does (it was half). Entered values: seismic takes the smaller of vn and 1.4 vs; High-load takes its one vn for both.
• From NDS 2024 (on disk) the module takes only the framing specific gravity G (Table 12.3.3A; Redwood corrected to G = 0.37 at verification, which lowers its species factor from 0.94 to 0.87) and the common nail dimensions (Table L4); both are confirmed. NDS 2024 does not contain the SDPWS provisions.

Verification examples #

Verification status: 71 values compared in 19 examples (13 published, 6 hand calculations from the cited equations): 71 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.

Published worked examples, all free from WoodWorks (research/sources/design_examples): Introduction to Wood: Structural Lateral Framing Design (January 2025 slides, SDPWS 2021), Wood Diaphragm Deflections and Flexibility (March 2024 slides, SDPWS 2021) and Cantilever Wood Diaphragm Webinar Series Part 3 (slide 16 reproduces SDPWS 2015 Table 4.2A). The Cantilever Webinar slides' cantilever is compared on capacities only. The full cantilever is compared with Design Example of a Cantilever Wood Diaphragm (WoodWorks, May 2026), and two spans with HUD's Residential Structural Design Guide, Example 6.4.

| WWDE-ASD | Right cantilever 35 × 40 ft, ASD, Method 2B forces: 185.63 plf and 553.2 lb at the free edge | 176.3 plf, 200.5 plf with 24.3 of torsion, 289.3 plf, chord 3,327 lb, L/W 0.875 | 176.3, 200.6, 289.3, 3,327, 0.875 | | WWDE-STR | Same, strength level, v = 236 plf: Eq. 4.2-2, flexibility, drift | 0.027 + 0.165 + 0.075 = 0.267 in; 2 Δ = 0.428 in; Δ = 2.26 in, limit 2.4 in | same | | HUD-6.4 | Two 24 ft spans continuous, 24 ft deep, 200 plf | V 3,000 lb, v 125 plf, middle line 6,000 lb, M 14,400 ft-lb, chord 600 lb | same |

The example's forces come from its rigid diaphragm analysis, entered here as loads. Its v = 236 plf is applied as printed (w = 236 × 40 / 35), and its 0.075 in of slip, from its spreadsheet, is entered as one splice at the support, with half the slip because each listed splice counts in both chords.

The seismic capacity follows SDPWS 2021 4.1.4 since 2026-11-01 (φD = 0.50 on vn, was 0.80 on vs: the LRFD seismic capacity drops by about 12 %; ASD vn / 2.8 is within 1 plf of the former vs / 2.0).

Hand calculations derived step by step in backend/tests/test_verify_wood_diaphragm.py (15/32 in sheathing, 10d, blocked 6 / 6 in, 2 in framing: vn = 810 plf, SDPWS 2021 Table 4.2A).

No free published example of a diaphragm opening with numbers was found, nor of Table 4.2B; the WoodWorks Malone papers describe the method only.