| Case | Units | Shape | Section | L | K | Fc | E | Factors (CD·CM·Ct·Cf·Ci) | Design format | Capacity (approx.) |
|---|---|---|---|---|---|---|---|---|---|---|
| Example A | Metric | Rectangular | b=150 mm, h=150 mm | 3.0 m | 1.0 | 18 MPa | 10,000 MPa | 1.00 | ASD (Ω=1.0) | Calculated after submit |
| Example B | Imperial | Round | d=6 in | 10 ft | 1.0 | 2,200 psi | 1,600,000 psi | 1.00 | LRFD (φ=0.90) | Calculated after submit |
Formula used
This calculator combines a material crushing limit with elastic buckling about the weak axis. It applies your adjustment factors to the base material properties.
Imin = weak-axis second moment of area
rmin = √(Imin/A)
Le = K·L
Fc′ = Fc · (CD·CM·Ct·Cf·Ci)
E′ = E · (E adjustment factor)
Pc = Fc′ · A
Pe = π² · E′ · Imin / Le²
Rankine interaction: Pr = 1 / (1/Pc + 1/Pe)
ASD: Pd = Pr / Ω
LRFD: Pd = φ · Pr
Notes: This is a practical estimation approach for engineering comparisons. Always confirm with your governing code requirements, connection details, load combinations, and bracing conditions.
How to use this calculator
- Pick a unit system and select the section shape.
- Enter dimensions and the unbraced length between restraints.
- Set K based on end restraint and bracing assumptions.
- Provide Fc and E for the timber grade being checked.
- Adjust factors (leave as 1.0 if not applicable).
- Select ASD or LRFD, then press Submit.
- Review the capacity, slenderness, and utilization above the form.
- Use CSV/PDF buttons to save calculation records.
Timber column capacity checks for axial compression
1) What this calculator evaluates
This tool estimates axial capacity for a timber column by combining crushing strength and elastic buckling. It reports section properties (area, weak-axis inertia, radius of gyration), slenderness (KL/r), and three capacities: Pc (material limit), Pe (Euler buckling), and Pr (Rankine interaction).
2) Input data you should prepare
Gather dimensions, unbraced length, end restraint assumptions (K), and material properties for the selected grade. Use design values for Fc (compression parallel to grain) and E (modulus of elasticity). If your specification applies modifiers, enter CD, CM, Ct, Cf, and Ci, plus an E adjustment factor.
3) How buckling influences capacity
As unbraced length increases, the Euler term Pe = π²E′I/Le² decreases rapidly. For a rectangular member, weak-axis buckling may govern if one side is smaller or unbraced. The calculator uses Imin automatically, so the reported capacity reflects the most critical buckling direction.
4) Reading the results for design decisions
If Pe is much smaller than Pc, the column is buckling-controlled and improving bracing or reducing K can be more effective than increasing grade. If Pc is smaller, strength governs and increasing cross-section or material grade is typically the fastest improvement. Add an applied axial load to see utilization and a clear pass/fail status.
5) Documenting calculations with exports
Construction reviews often require transparent calculation records. Use the CSV export for spreadsheets and peer checks, and the PDF export for submittals. Both include the exact inputs and the reported capacities so teams can reproduce results. For final design, confirm code-specific stability provisions, load combinations, eccentricity, and connection restraint.
FAQs
1) Which axis does the calculator use for buckling?
It computes both principal inertias for rectangles and uses the smaller value (weak axis). For round sections, the inertia is the same in all directions.
2) What does K represent?
K is the effective length factor reflecting end restraint and sway conditions. Lower K means more restraint and higher buckling capacity, while higher K indicates less restraint.
3) Why are Pc, Pe, and Pr all shown?
Pc highlights the material crushing limit, Pe shows elastic buckling, and Pr provides a combined estimate between them. Seeing all three helps you understand what governs and how to improve capacity.
4) Do adjustment factors always increase capacity?
No. Some factors may reduce design properties depending on service conditions. Enter factors that match your specification; if a factor is not applicable, keep it at 1.0.
5) Can I use this for short columns?
Yes. When the column is short, Pc typically governs and the Rankine value approaches Pc. The tool still reports slenderness and confirms that buckling is not controlling.
6) How should I choose Fc and E values?
Use design values from your timber grade/specification or code tables, consistent with the chosen unit system. Ensure Fc and E correspond to the same reference conditions and adjustment framework.
7) Does the calculator include eccentricity or bending?
No. It is an axial-only estimate. If the column carries bending, eccentric load, notches, or connection offsets, perform a combined stress check and apply the governing code interaction equations.