Wood diaphragm

A wood structural panel diaphragm to AWC SDPWS 2021 Section 4.2, spanning simply between two shear wall lines under a uniform wind or seismic load, with the ASCE 7-22 load factors: unit shear, chord force, collector (drag) force, aspect ratio and deflection.

Source note. SDPWS 2021 itself is not yet on file at ILX Studio (AWC offers it view-only). The SDPWS 2021 design factors (4.1.4), Eq. 4.2-1 and part of Tables 4.2A / 4.2C are confirmed by the free published examples listed under Verification examples; the other values and the provisions marked "[verify]" in the steps will be re-checked against the document. The apparent shear stiffness Ga is an input (transcribe it from Table 4.2A 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). Entered replaces the table values (vs and vw as in SDPWS 2015; the seismic check uses vn = 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).
• 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 4.2.2 leaves the permissible deflection to the engineer).

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 #

• One simple span between two shear wall lines, uniform load, symmetric reactions. Cantilever and open-front diaphragms (4.2.5.2), openings in the diaphragm, continuous or multi-span diaphragms and concentrated loads are not covered.
• High-load blocked diaphragms (Table 4.2B) and lumber sheathing (4.2.8) are 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 flexibility classification of the diaphragm (ASCE 7-22 12.3.1) and the distribution to the walls are not done here.
• Confirmed from published examples (2026-11-01): the SDPWS 2021 factors 2.0 / 2.8 and 0.80 / 0.50 (4.1.4); vn = 1.4 vs rounded to 5 plf; the blocked rows of 7/16 in (3 in framing), 15/32 in (8d and 10d) and 19/32 in (10d) rated sheathing (SDPWS 2015 Table 4.2A as reproduced by WoodWorks) and vn = 895 plf (19/32 in, 10d at 6 in); vn = 505 plf of 3/8 in Structural I unblocked, 8d, 2 in framing, cases 2 to 6 (Table 4.2C); Eq. 4.2-1 including the chord splice term. Still [verify] until SDPWS 2021 is on file: the other rows, the footnotes, Table 4.2.4.
• 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 #

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 examples themselves are outside the module's scope (simple spans only); only their capacities are compared.

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 2015 vs = 580 plf).