Screw Anchor Sizing: The Complete Guide

Screw anchors perform optimally in cohesive soils like clay and silty sands where they can achieve high holding power. They are also effective in loose sands if proper torque verification is applied. However, rocky or boulder-filled soils may require specialized drilling or pre-augering techniques.

Load capacity is determined by soil shear strength, anchor geometry, and helix diameter. Engineers typically use empirical formulas or geotechnical analysis software to estimate ultimate bearing capacity. It is essential to account for both axial tension and compression loads when designing the foundation.

Depth is dictated by the required load capacity and the stability of the underlying soil strata. Typically, anchors are installed until they reach competent soil layers below the frost line or active zone. The number of helices and their spacing are adjusted to achieve the necessary depth and performance.

Installation torque serves as a primary indicator of anchor capacity and soil density during placement. Higher torque values generally correlate with greater load-bearing potential in granular soils. Engineers often use torque-capacity correlations to verify that the installed anchor meets the design specifications without excessive testing.

Corrosion protection, such as hot-dip galvanizing or epoxy coatings, adds thickness to the anchor shaft. This extra material must be considered when calculating the effective cross-sectional area for load-bearing purposes. In aggressive environments, thicker coatings or stainless steel components may be specified to ensure long-term integrity.