Wood Diaphragm Design Calculator (NDS / SDPWS)
Preliminary design tool for wood-sheathed diaphragms per NDS/SDPWS: unit shear demand, capacity, deflection, flexible/rigid classification, and chord forces.
Engineering disclaimer: this tool is for education and preliminary sizing only. Final designs must be checked and sealed by a licensed structural engineer using current codes (NDS, SDPWS, IBC, local amendments).
Wood Diaphragm Design
How the Wood Diaphragm Calculator Works
This tool is inspired by industry references from APA, WoodWorks, NEHRP and SDPWS provisions. It is meant to give a quick, transparent check of diaphragm unit shear, deflection and chord forces for wood-sheathed diaphragms in light-frame and engineered wood buildings.
1. Unit Shear Demand
For a diaphragm with story shear \(V\) and width \(W\), the average unit shear demand \(v\) is:
\[ v = \frac{V}{W} \]
where:
\(V\) = story shear (kips or kN)
\(W\) = diaphragm width (ft or m)
\(v\) = unit shear (plf or kN/m)
The calculator converts units automatically. For US units, \(V\) is in kips and \(W\) in feet, giving \(v\) in kips/ft, which is then converted to plf.
2. Diaphragm Shear Capacity
The design unit shear capacity \(v_n\) is taken from a small library of common nailing schedules, or entered manually if you have values from APA or manufacturer tables. The utilization ratio is:
\[ \text{Utilization} = \frac{v}{v_n} \]
Values above 1.0 indicate that the selected nailing schedule is not adequate for the applied shear and a stronger diaphragm (thicker sheathing, closer nailing, blocking, etc.) is required.
3. Diaphragm Deflection and Flexible vs Rigid Classification
SDPWS provides a deflection equation for wood diaphragms that includes panel shear deformation and fastener slip. A simplified form is:
\[ \Delta_{\text{dia}} \approx \frac{v L}{G t} + C_n \, v \, L \]
where:
\(\Delta_{\text{dia}}\) = diaphragm deflection at the chord
line
\(v\) = unit shear (plf or kN/m)
\(L\) = diaphragm span (ft or m)
\(G\) = panel shear modulus (psi or MPa)
\(t\) = panel thickness (in or mm)
\(C_n\) = nail slip coefficient (in/plf or mm/(kN/m))
The calculator uses this expression with user-specified \(G\), \(t\) and \(C_n\). It then compares \(\Delta_{\text{dia}}\) to the story drift \(\Delta_{\text{story}}\) you provide.
As a screening check, the diaphragm is flagged as:
- Flexible if \(\Delta_{\text{dia}} \le 0.5 \, \Delta_{\text{story}}\)
- Rigid otherwise
This is consistent with common interpretations used in practice but does not replace a full code-compliant analysis or the detailed guidance in SDPWS, IBC and WoodWorks/APA documents.
4. Chord Forces
For a simple-span diaphragm with span \(L\) and story shear \(V\), the maximum chord force \(T\) can be approximated as:
\[ M_{\text{max}} \approx \frac{V L}{2} \quad\Rightarrow\quad T \approx \frac{M_{\text{max}}}{L} = \frac{V}{2} \]
For a cantilever diaphragm, the chord force is conservatively taken as:
\[ T \approx V \]
The calculator then estimates the bending stress in the chord assuming a rectangular section of depth \(d\) and unit width:
\[ f_b \approx \frac{6 T}{d} \]
This is compared to the allowable bending stress \(F_{b,\text{allow}}\) you provide, giving a chord utilization ratio.
5. Typical Input Values
- G (panel shear modulus): 600,000–1,000,000 psi for common OSB/plywood panels.
- t (thickness): 7/16" (0.4375 in), 15/32" (0.46875 in), 19/32" (0.59375 in), etc.
- Nail slip coefficient Cn: 0.0002–0.0004 in/plf is typical for many systems.
- Chord Fb,allow: 1,000–1,600 psi for many dimension lumber grades (after NDS adjustments).
Limitations and Good Practice
- This tool assumes a single-span diaphragm with uniform shear; multi-span and irregular layouts require more advanced analysis.
- Collectors, drag struts, openings and re-entrant corners are not explicitly modeled.
- Overstrength, seismic load combinations and load duration factors must be handled per code.
- Always cross-check capacities with the latest APA and SDPWS tables and consider boundary nailing, blocking and chord detailing.
Wood Diaphragm Calculator FAQ
Can I use this for seismic diaphragm design?
You can use the calculator to get a quick feel for diaphragm unit shear, deflection and chord forces under seismic design shears. However, seismic design requires additional checks (overstrength, collector design, chord splices, detailing) that are not fully captured here. Use this as a starting point and complete the design per SDPWS, ASCE 7 and local amendments.
Where do the nailing schedule capacities come from?
The capacities in the dropdown are representative values based on common APA wood structural panel diaphragm tables. They are intentionally rounded and conservative. For final design, always pull exact values from the current APA or SDPWS tables for your specific panel grade, thickness, nailing pattern, and framing conditions.
How should I choose G and the nail slip coefficient?
Panel shear modulus G and nail slip coefficients depend on panel grade, thickness, fastener type and spacing. If you do not have test data, you can use typical values from APA or SDPWS commentary as a starting point and then perform sensitivity checks. For critical projects, rely on manufacturer data or project-specific testing where available.
Does the calculator consider blocked vs unblocked diaphragms?
Not explicitly. Blocked vs unblocked construction is implicitly reflected in the unit shear capacity you select or enter. If you are designing a blocked diaphragm, choose a capacity corresponding to a blocked system from APA/SDPWS tables. The deflection model is generic and does not distinguish between blocked and unblocked beyond the effect embedded in G and Cn.