Wood Diaphragm Calculator, SDPWS 2021

What it checks

The module designs wood diaphragms for lateral loads, evaluating shear capacity, chord forces, and aspect-ratio limits. It applies the Allowable Stress Design (ASD) method, converting nominal capacities using a specified ASD conversion factor and load-duration factors. The sequence of checks begins with determining the nominal unit shear capacity based on sheathing type, nail size, and spacing. It then calculates the apparent shear stiffness for deflection analysis and evaluates the diaphragm's aspect ratio. Finally, the module assesses out-of-plane gravity deflection using a Mindlin Q4 plate finite element model if gravity loads are provided.

Inputs

Geometry - Diaphragm dimension parallel to lateral load (ft), dimension perpendicular to load (ft), and total applied shear force (kips).
Sheathing - Panel sheathing type key (e.g., "3/8_str1", "7/16_osb"), where Structural I panels have higher values than OSB.
Nailing - Nail size ("6d", "8d", or "10d"), boundary nail spacing (in), and interior field nail spacing (in).
Configuration - Boolean flag indicating if the diaphragm is blocked or unblocked.
Factors - ASD conversion factor (default 2.0) and load-duration factor CD (1.0 for seismic, 1.6 for wind).
Chord Properties - Chord cross-sectional area (in²), modulus of elasticity (psi), nail slip per nail (in), and chord splice slip (in).
Gravity FEM - Optional gravity load (psf), panel elastic modulus (ksi), equivalent plate thickness (in), Poisson's ratio, and mesh dimensions (nx, ny).

What it reports

Nominal Shear - The nominal unit shear capacity (plf) looked up from SDPWS-21 Table 4.2A, falling back to the next-larger boundary spacing if an exact match is not found.
Shear Stiffness - The apparent shear stiffness Ga (lb/in) derived from SDPWS-2021 Table 4.2A footnotes and Commentary Table C4.2.2A.
Gravity Deflection - The FEM out-of-plane midpoint deflection (in) under uniform gravity load, reported only if the FEM check is executed.
Gravity Moment - The FEM maximum bending moment Mx at the panel center (k·ft/ft), reported only if the FEM check is executed.
Deflection Verdict - A boolean indication of whether the FEM deflection is less than or equal to span/360, reported only if the FEM check is executed.

Code references

The engine cites SDPWS 2021 §4.2 and Table 4.2A. It also references SDPWS-2021 Table 4.2A footnotes and Commentary Table C4.2.2A for apparent shear stiffness, and SDPWS Table C4.2.2D for the nail slip equation.

Limits

The module assumes a simply-supported condition for the plate FEM, with w=0 at boundary nodes and free rotations. The FEM gravity deflection and moment results are None if the FEM is skipped. The nail slip input is an estimated value, typically ranging from 0.005 to 0.040 in. Results are engineering aids; the engineer of record is responsible for the design.

This module ships in the desktop app

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