Steel Screw Pile Design

How it calculates

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) ≤ 1.0

Long-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) ≤ 1.0