AustraliaAS 2870:2011AS 2159:2009
Steel Screw Pile Design
Run the calcPile loads link from connected column and beam calculations so load changes propagate automatically. Checks ultimate bearing capacity and long-term serviceability to AS 2870:2011 and AS 2159:2009, with geotechnical risk rating (AS 2159 Table 4.3.2) and 50-year corrosion allowances.
The Calcs.com Steel Screw Pile Design Calculator enables the fast design and analysis of screw piles to AS2870 and AS2159. The calculator privides fast results for ultimate load, bearing capacity, and serviceability load based on provided pile spacing, embedment depth, and screw diameter.
Method & scope
Calculation method
Load demand
The calculator derives design actions from AS/NZS 1170.0:2002+A5 load combination factors. Total permanent loads G and imposed loads Q are summed from distributed loads, point loads, and footing self-weight across the tributary area defined by pile spacing s. The ultimate limit state demand uses the governing combination: ULS = max(1.35 × G, 1.2 × G + 1.5 × Q) The long-term serviceability demand combines permanent load with a sustained fraction of imposed load: SLS = G + 0.6 × Q The 0.6 imposed action factor provides a conservative estimate of sustained residential loads.Geotechnical strength reduction factor
The basic geotechnical strength reduction factor φ_gb is taken from AS 2159:2009 Table 4.3.2(A) based on the geotechnical risk rating. Nine individual risk indicators (IRR₁-IRR₉) covering site investigation thoroughness, design method, and construction control are combined into a weighted average risk rating: ARR = (2·IRR₁ + 2·IRR₂ + 2·IRR₃ + 1·IRR₄ + 2·IRR₅ + 1·IRR₆ + 2·IRR₇ + 2·IRR₈ + 0.5·IRR₉) / 14.5 Lower ARR values indicate higher certainty and produce higher φ_gb. Engineers can enter φ_gb directly if the risk category is known from a geotechnical report.Corrosion allowances
Both shaft diameter and screw diameter are reduced for a 50-year design life per AS 2159:2009 Table B2, based on the selected exposure classification. These reduced long-term diameters are used in all serviceability capacity calculations.Ultimate bearing capacity , alpha method
Short-term bearing capacity R_u is calculated using the alpha method, appropriate for undrained conditions in cohesive soils. End bearing uses Skempton’s (1959) bearing capacity coefficient N_c,α , which decreases with increasing shaft diameter , and skin friction uses the NAVFAC DM 7.2 adhesion factor α: R_u = q_b × π × d_l² / 4 + q_s × π × d_shaft,l × D For cohesive soils: q_b = c_u × N_c,α and q_s = α × c_u. For granular soils: q_b = γ × D × N_q (Janbu, 1976) and q_s = 0 (conservative short-term assumption). The ultimate bearing utilization check must satisfy: ULS / (φ_gb × R_u) ≤ 1.0Long-term serviceability capacity , beta method
Long-term capacity R_l uses the beta method to account for drained soil conditions. Skin friction is based on the soil-pile interface friction coefficient μ = tan(δ) and horizontal effective stress at the pile tip, where K₀ is the at-rest earth pressure coefficient (Terzaghi formula for cohesive soils, Jaky expression for granular soils): R_l = q_b,l × π × d_l² / 4 + q_s,l × π × d_shaft,l × D For granular soils, long-term skin friction uses effective horizontal stress. For cohesive soils, the beta method includes both drained bearing and cohesion terms. The interface friction angle δ = 20° is recommended for steel piles per NAVFAC DM 7.2. The long-term serviceability utilization check must satisfy: SLS / (φ_gb × R_l) ≤ 1.0How to use it
1
Open the calculator
Open it from Run calc in the About this calculator panel above.
2
Enter your inputs
Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See Checks, References, Conditions and Assumptions.
3
Review the results and export
Check the utilization of each governing check in the summary, then export a PDF report. See Views and Export.
Common questions
What design standards does this calculator use?
What design standards does this calculator use?
The calculator applies AS 2870:2011 for residential footing design and AS 2159:2009 for piled foundation geotechnical design. Load combinations follow AS/NZS 1170.0:2002+A5. Geotechnical strength reduction factors are from AS 2159:2009 Table 4.3.2 and corrosion allowances from AS 2159:2009 Table B2.
What are the key inputs?
What are the key inputs?
Pile geometry: maximum pile spacing (for tributary area), embedment depth, screw diameter, shaft diameter, and exposure classification for corrosion. Soil properties: soil type (cohesive or granular), cohesion c’, and soil-pile interface friction angle. Loads: permanent and imposed as distributed, point, or footing loads. The built-in geotechnical risk worksheet covers nine AS 2159 criteria, or you can enter a risk rating directly.
What checks does the calculator perform?
What checks does the calculator perform?
Two bearing capacity checks: (1) ultimate bearing capacity, ULS demand versus design ultimate capacity using the alpha method (undrained, short-term); (2) long-term serviceability, SLS demand versus design long-term capacity using the beta method (drained conditions). Both utilization ratios must be 1.0 or less for the design to pass.
Can it handle both cohesive and granular soils?
Can it handle both cohesive and granular soils?
Yes. For cohesive soils, end bearing uses Skempton’s bearing capacity coefficient and skin friction uses the NAVFAC adhesion factor. For granular (cohesionless) soils, end bearing uses Janbu’s Nq factor with at-rest horizontal pressure, and short-term skin friction is taken as zero (conservative). Long-term skin friction for granular soils uses the beta method with effective horizontal stress and the soil-pile interface friction angle.
How do I determine the geotechnical risk rating?
How do I determine the geotechnical risk rating?
The calculator includes an optional Geotechnical Risk Worksheet based on AS 2159:2009 Table 4.3.2(A). You rate nine criteria, geological complexity, extent of ground investigation, quality of geotechnical data, design experience, design method, and construction control, on a scale of 1 to 5. The tool computes a weighted average risk rating (ARR) that drives the geotechnical strength reduction factor. If your geotechnical report already specifies a risk category, you can select it directly without completing the worksheet.
Does this calculator support load linking with column or beam calculations?
Does this calculator support load linking with column or beam calculations?
Yes, permanent and imposed loads from connected column or beam calculations in the same Calcs.com project link directly into the screw pile’s load inputs. When the structure above changes, different column section, revised spans, or updated live loads, the pile inputs update automatically. Ultimate bearing and serviceability checks update immediately with no manual transfer of reaction values.
Next steps
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