AWC NDS Wood Column Design Calculator
Check wood columns in axial compression and bending using NDS-style equations. Quickly evaluate slenderness, adjusted capacities, and combined loading unity checks.
For educational and preliminary design only. Always verify results against the latest AWC NDS and applicable building codes.
1. Geometry & Material
2. Design Loads
Set to 1.0 to auto-estimate from slenderness; override if you have a code-based value.
Optional lateral stability factor for bending (simplified).
3. Results (Summary)
Show detailed section properties & stresses
How this wood column design calculator works
This tool follows the design philosophy of the AWC National Design Specification (NDS) for wood construction. It is intentionally simplified so you can quickly explore column behavior, check slenderness, and estimate axial and bending capacities with combined loading.
You can use it for solid-sawn, glulam, or built-up wood columns by entering appropriate reference design values \(F_b\), \(F_c\), and \(E\). For final design, always refer to the current NDS and project-specific requirements.
1. Section properties
For a rectangular column with width \(b\) and depth \(d\) (major axis about y-y):
\( A = b \cdot d \)
\( I_y = \dfrac{b d^3}{12} \)
\( I_x = \dfrac{d b^3}{12} \)
\( r_y = \sqrt{\dfrac{I_y}{A}}, \quad r_x = \sqrt{\dfrac{I_x}{A}} \)
2. Slenderness ratios
Effective length is taken as \( kL \), where \(k\) depends on end conditions (pinned-pinned, fixed-free, etc.). Slenderness ratios are:
\( \left(\dfrac{kL}{r}\right)_y = \dfrac{k_y L}{r_y} \)
\( \left(\dfrac{kL}{r}\right)_x = \dfrac{k_x L}{r_x} \)
The NDS limits column slenderness (for example, \(kL/r \le 50\) for many cases). This calculator reports both major and minor-axis slenderness so you can compare against code limits.
3. Axial compression capacity (simplified)
The calculator uses your input compressive design value \(F_c\) and a column stability factor \(C_P\) to get an adjusted compressive stress:
\( F'_c = F_c \cdot C_P \)
\( P_{allow} = F'_c \cdot A \)
If you leave \(C_P = 1.0\), the tool will estimate a stability factor based on slenderness using a simple Euler-style approximation. For precise design, you should compute \(C_P\) directly from NDS tables or equations for your specific member.
4. Bending capacity
For each axis, the reference bending stress \(F_b\) is adjusted by a lateral stability factor (here denoted \(C_L\) for simplicity):
\( F'_{b,y} = F_b \cdot C_L \)
\( F'_{b,x} = F_b \cdot C_L \)
\( M_{allow,y} = F'_{b,y} \cdot S_y = F'_{b,y} \cdot \dfrac{I_y}{d/2} \)
\( M_{allow,x} = F'_{b,x} \cdot S_x = F'_{b,x} \cdot \dfrac{I_x}{b/2} \)
The calculator compares your applied moments \(M_y\) and \(M_x\) to these capacities and reports a unity ratio for the governing axis.
5. Combined axial and bending interaction
For members with both axial compression and bending, NDS uses interaction equations. A common simplified form for ASD is:
\( \dfrac{\sigma_c}{F'_c} + \dfrac{\sigma_{b,y}}{F'_{b,y}} + \dfrac{\sigma_{b,x}}{F'_{b,x}} \le 1.0 \)
where
\( \sigma_c = \dfrac{P}{A} \)
\( \sigma_{b,y} = \dfrac{M_y \cdot c_y}{I_y} \), \( c_y = d/2 \)
\( \sigma_{b,x} = \dfrac{M_x \cdot c_x}{I_x} \), \( c_x = b/2 \)
The calculator evaluates this interaction and reports a combined ratio. Values below 1.0 indicate that, under the assumptions made, the member meets the simplified criterion.
6. Practical tips for wood column design
- Check both axes. Wood columns are often much weaker about the minor axis; ensure bracing and loads are realistic.
- Account for unbraced length. Use the actual distance between lateral supports when choosing \(L\) and \(k\).
- Use correct design values. Reference design values depend on species, grade, moisture, duration of load, temperature, and size factors.
- Built-up columns. For multi-ply members, also check fasteners, load sharing, and stability per NDS and manufacturer literature.
- Serviceability. This tool focuses on strength; for tall or heavily loaded columns, also check deflection and vibration where relevant.
Disclaimer
This calculator is provided for educational and preliminary design purposes only. It does not implement the full AWC NDS and does not account for all adjustment factors, load combinations, or detailing requirements. Use of this tool does not replace the need for a licensed professional engineer and the official design standard.