AWC NDS Wood Bolt Design Calculator (Bolted Connections)
Preliminary design of wood-to-wood and wood-to-steel bolted connections per AWC NDS yield equations. Compute lateral design value per bolt and total connection capacity with basic spacing checks.
Wood Bolt Design Calculator
Connection Type
Single-shear wood-to-wood or wood-to-steel, lateral load only.
Design Basis
Simplified NDS yield equations (Zmode) with user-defined adjustment factor Ctotal.
Disclaimer
For preliminary design and education only. Always verify with the current NDS and a licensed engineer.
How the wood bolt design calculator works
This calculator follows the philosophy of the AWC National Design Specification (NDS) for Wood Construction for dowel-type fasteners in lateral shear. It estimates the lateral design value per bolt by evaluating simplified yield equations for several potential failure modes and selecting the minimum value.
1. Yield modes considered
For a single-shear bolted connection, the NDS considers several possible yield modes. This tool approximates:
- Mode Im – bearing (crushing) in the main member only.
- Mode Is – bearing in the side member only.
- Mode II – single plastic hinge in the bolt plus bearing.
- Mode III – double plastic hinge in the bolt plus bearing.
The design lateral value per bolt is taken as the minimum of these modes, multiplied by the overall adjustment factor Ctotal and the resistance factor ϕ (or 1.0 for ASD).
Simplified per-bolt design value
Let:
- d = bolt diameter (in)
- ts, tm = side and main member thickness (in)
- Fes, Fem = dowel bearing strengths (psi)
- Ctotal = product of NDS adjustment factors
- ϕ = resistance factor (LRFD) or 1.0 (ASD)
The calculator uses the following simplified expressions:
Mode Im: \( Z_{Im} = F_{em} \, d \, t_m \)
Mode Is: \( Z_{Is} = F_{es} \, d \, t_s \)
Mode II: \( Z_{II} = 1.15 \, \sqrt{F_{es} F_{em}} \, d \,
\sqrt{t_s t_m} \)
Mode III: \( Z_{III} = 2.3 \, F_{es} \, d \, t_s \)
Then: \( Z_{design} = \phi \, C_{total} \, \min(Z_{Im}, Z_{Is}, Z_{II}, Z_{III}) \)
These equations are intentionally conservative and simplified for quick checks. For final design, always use the exact NDS equations and tabulated values for your specific species, grade, and loading direction.
2. Bolt group capacity
Once the per-bolt design value is known, the total connection capacity is:
Number of bolts: \( n = n_{rows} \times n_{cols} \)
Group design capacity: \( R_n = n \times Z_{design} \)
Utilization: \( U = \dfrac{V}{R_n} \)
If the utilization U ≤ 1.0, the connection passes the shear capacity check for the given load V. If U > 1.0, you need more bolts, larger bolts, higher-strength material, or a different configuration.
3. Spacing, end distance, and edge distance checks
The NDS specifies minimum spacing and edge/end distances to control splitting and bearing failures. This calculator performs simple checks using typical minimums for bolts loaded parallel to grain:
- End distance e ≥ 7d (loaded end)
- Edge distance e′ ≥ 1.5d
- Spacing parallel to grain s ≥ 4d
- Spacing perpendicular to grain s′ ≥ 3d
If any of these are not satisfied, the calculator flags a warning. You should adjust the layout or consult the NDS for the exact requirements for your connection type and loading direction.
4. Wood-to-wood vs. wood-to-steel
For wood-to-wood connections, both Fes and Fem are dowel bearing strengths for wood members (often similar values).
For wood-to-steel connections, the steel side plate is usually much stiffer, so the controlling bearing is in the wood member. In this simplified tool, you still enter Fes and Fem as wood dowel bearing strengths; the steel plate is assumed not to govern bearing.
Design tips for bolted wood connections
- Use multiple smaller bolts rather than a few very large bolts to reduce splitting risk.
- Keep bolts away from loaded ends and edges; increase end distance where possible.
- Stagger bolts in adjacent rows to improve performance and reduce splitting.
- Consider load duration (CD) and moisture (CM) factors carefully.
- Check both shear and withdrawal (if applicable), as well as member net section tension.
Limitations
- Single-shear connections only; multiple shear planes are not modeled.
- No checks for net section tension, block shear, or group tear-out.
- Assumes uniform load distribution among bolts.
- Does not compute individual NDS adjustment factors; you must supply Ctotal.
For detailed design examples and background, refer to the AWC NDS, AWC design examples, and technical publications on bolted wood connections.
Frequently asked questions
Can I use this calculator for mass timber (CLT, glulam) connections?
You can use it for rough preliminary checks, but mass timber products often have specific dowel bearing strengths, layups, and detailing requirements. Always use manufacturer data and the latest NDS provisions for engineered wood products.
How do I choose Fes and Fem?
Dowel bearing strengths depend on species, grade, specific gravity, and loading direction (parallel or perpendicular to grain). Use NDS tables (e.g., Table 11.3.1) or manufacturer data to obtain appropriate values for your members.
Should I design in ASD or LRFD?
The NDS supports both ASD and LRFD. This calculator can be used with either approach by choosing ϕ = 1.0 for ASD (and using allowable loads) or a code-appropriate ϕ for LRFD (and using factored loads). Be consistent with the rest of your structural design.
Does the calculator handle combined shear and tension?
No. It assumes pure lateral shear in the plane of the connection. For combined shear and tension or out-of-plane loading, more advanced analysis and code checks are required.
Is this calculator suitable for final design?
No. It is intended for conceptual design, quick checks, and education. Final designs must be checked against the current NDS, project specifications, and reviewed by a qualified structural engineer.