Enter post details
Formula used
- Area: A from the selected cross‑section geometry.
- Weak‑axis inertia: Imin = min(Ix, Iy) for rectangular sections.
- Effective length: Le = K × L (K from end condition).
- Euler buckling: Pcr = π² × E × Imin ÷ Le².
- Crushing limit: Pm = (fc,allow × condition) × A.
- Allowable design load: min(Pcr/SFbuck, Pm/SFmat).
- Utilization: applied ÷ allowable (≤ 1.0 passes).
This calculator provides screening-level checks for garden structures. For critical builds, consult local codes and verified engineering tables.
How to use this calculator
- Select your unit system and post shape.
- Enter dimensions and the unsupported length between restraints.
- Choose a material preset, or enter custom E and allowable stress.
- Pick an end condition (K factor) that matches your supports.
- Enter applied compressive load and safety factors.
- Press Calculate to view results above the form.
- Download CSV or PDF for records or sharing.
Tip: If the result fails, try reducing the unsupported length, increasing section size, adding bracing, or selecting a stiffer material.
Example data table
| Scenario | Shape | Dimensions | Length | Material | Applied | Allowable | Status |
|---|---|---|---|---|---|---|---|
| Pergola corner post | Solid square | 100 × 100 mm | 2400 mm | Structural timber | 8.0 kN | ≈ 10–14 kN* | Often PASS |
| Planter support | Round pipe | D 89 mm, t 3 mm | 2000 mm | Mild steel | 12.0 kN | ≈ 30–60 kN* | PASS |
| Light trellis | Solid round | d 75 mm | 2100 mm | Pressure-treated wood | 3.0 kN | ≈ 4–7 kN* | Check |
*Approximate ranges depend on end conditions, safety factors, and actual material properties.
What the allowable load represents
Compressive capacity is limited by two checks: material stress and column stability. The calculator reports an allowable design load as the smaller of the stress-based capacity (A × f_c) and the Euler buckling capacity reduced by an effective-length factor. This mirrors how a garden post can crush locally or suddenly bow when it is slender. Include self‑weight, beam reactions, and planter loads.
Material inputs that matter outdoors
Use an allowable compressive stress that already includes your chosen safety margin or code method. For treated timber posts, values often reduce when moisture, knots, and long‑term creep are considered. For steel, corrosion protection and connection detailing can govern. Modulus of elasticity (E) affects buckling only, so a stiffer material can carry more load at the same size and height. Typical E: timber 9000–14000 MPa, steel about 200000 MPa.
Why length and bracing change everything
Slenderness is driven by the effective length (K × L) divided by the radius of gyration (r). Doubling the unbraced height can reduce buckling strength by about four times. Adding a mid‑height rail, diagonal brace, or rigid panel shortens the unbraced length and greatly increases capacity without changing the post size. For round posts, small diameter increases r noticeably; avoid notches that reduce section stiffness.
Footings and load path in garden builds
A strong post still needs a stable base. Ensure the footing, soil bearing, and post‑to‑base connector can transfer the compressive force without rotation or settlement. Loads that are off‑center—such as a pergola beam connection or a trellis with wind pressure—introduce bending and reduce true capacity. Keep brackets centered and provide lateral restraint where possible. On concrete piers, keep anchors centered and tight.
Using the result for quick decisions
Treat the output as a screening value for layout choices: spacing, post size, and bracing strategy. If your expected load is close to the limit, increase section size, reduce height, add bracing, or select a stiffer material. For quick checks, compare multiple scenarios and pick the worst‑case. Record the input set and downloads with your project notes for consistent repeat checks after site changes.
FAQs
1) What loads should I enter for a garden post?
Enter the total vertical force carried by the post, including beam reactions, roof or shade elements, and any hanging items. Use a conservative estimate if loads vary during use.
2) Does the calculator include wind or side loading?
No. It evaluates axial compression only. Wind on screens, trellises, or pergolas creates bending, which can reduce capacity. Add bracing and check combined loading if lateral forces are significant.
3) How do I choose the effective length factor K?
K reflects end restraint. Pinned–pinned is near 1.0, fixed–fixed can be near 0.5, and cantilevered cases are higher. If you are unsure, use 1.0 for a cautious estimate.
4) Why does bracing increase the allowable load so much?
Bracing reduces the unbraced length L. Buckling capacity scales with 1/L², so even a modest reduction in unbraced height can produce a large increase in capacity.
5) Can I use this for hollow steel posts?
Yes. Select “Hollow square” or “Hollow round” and enter outer size plus wall thickness. Make sure the wall thickness is realistic, and confirm connector details so the load is delivered concentrically.
6) My stress capacity is high, but the result is low. Why?
Your post is likely slender, so buckling governs. Increase the section size, shorten the unbraced height, improve end fixity, or choose a stiffer material to raise the buckling capacity.