Round timber poles and piles
A round timber pole (ASTM D3200) or pile (ASTM D25) standing as a cantilever from the ground, as in a pole building or a sign. It is designed by NDS 2024 Chapter 6 with the tapered and round column rules of 3.7.2 and 3.7.3. Its embedment is by IBC 2024 1807.3. LRFD or ASD.
Inputs of the pole #
- Member and species:
- Construction pole: Supplement Table 6B (p. 91), eight species.
- Pile: Table 6A, Pacific Coast Douglas Fir, Red Pine and Southern Pine.
- The values are for normal load duration and wet service, so CM = 1.0 (6.3.3).
- Conditioning before treatment: Cct of Table 6.3.5 (air dried 1.0, kiln dried 0.90, Boulton 0.95, steaming 0.80 or 0.74 marine).
- Diameters at groundline and at the tip: from the pole class (ASTM D3200 / ANSI O5.1 circumferences, not built in) or measured.
- Height above grade and Ke: Appendix G gives 2.1 for a free-standing cantilever and 1.0 when the roof braces the top.
- Critical section factor Ccs: permitted by 6.3.9, so it is an option, off by default. Fc is multiplied by 1 + 0.004 Lc, at most 1.10, with Lc the pole's height above grade.
- Piles in a capped group: Cls of Table 6.3.11.
- Loads: axial dead, live and snow at the top; lateral wind and live loads at a height above grade. They are combined by ASCE 7-22: the strength combinations for the pole, the ASD combinations for the embedment.
- Embedment:
- Restraint at grade: nonconstrained (Eq. 18-1) or constrained by a slab or pavement (Eq. 18-2).
- Backfill: concrete, where b is the footing diameter; or compacted soil or sand, where b is the pole diameter.
- Soil: an IBC Table 1806.2 class or your geotechnical values.
- Isolated pole: the doubling of 1806.3.4 for isolated poles that tolerate 1/2 in. of motion at grade (your judgment, off by default).
- Embedment depth: provided, checked against the required depth.
Auto-design (editor toolbar) varies the diameter at groundline (the tip keeps the entered taper) and the embedment depth. Objective: the smallest pole, then the shallowest embedment.
Checks performed #
| Check | Basis |
|---|---|
| Column stability | Representative diameter by Eq. 3.7-2 with a = 0.70 (large end fixed, small end free); the equal-area square for le/d (3.7.3); CP with c = 0.85; le/d ≤ 50 |
| Compression at the tip | fc at the tip ≤ Fc* (3.7.2) |
| Bending at groundline | fb = M / (π d³/32) against Fb' with CF above 13.5 in. diameter; no CL (6.3.6) |
| Shear at groundline | Vr' = (3/4) F'v A for a circular section (3.4.2.1) |
| Combined | Eq. 3.9-3 with the column's FcE |
| Embedment | IBC Eq. 18-1 (S1 at d/3) or Eq. 18-2 (S3 at d), solved for the depth that returns itself; the depth for the pressure at most 12 ft; the pressure at most 15 times the tabular value |
| Vertical bearing | P over the footing or pole bottom against Table 1806.2 (the footing's weight not included) |
The drawings are the pole elevation (taper, grade, footing, loads, height and embedment) and the section at groundline.
Limitations #
- Factors not in NDS 2024 Chapter 6: an untreated factor and a form factor for round sections are not in the 2024 edition (Table 6.3.1), so none is applied.
- Ct: taken for wet service, our reading for poles and piles.
- Combined stress: Eq. 3.9-3 uses the stress on the representative section and the moment at groundline. This is our reading for a tapered cantilever.
- Not covered:
- pole classes by circumference;
- the concrete footing's design and weight;
- soil-structure analysis beyond IBC 1807.3;
- the pole's connections and bracing;
- decay and marine borers;
- CD above 1.6 for water-borne treated poles (6.1.4.1).
Verification examples #
Verification status: 15 values compared in 9 examples (6 published, 3 hand calculations from the cited equations): 15 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
Published values:
- the Supplement's Table 6B values and NDS Tables 6.3.5 and 6.3.11;
- two embedment examples on disk: PDHonline S235 (nonconstrained, Eq. 18-1, 7.31 ft) and CalcBook's IBC 2021 example (constrained, 6.60 ft).
Hand calculations: the column, bending and Ccs values. No worked NDS example of a round pole is on disk.