Calculated Load-Transfer Responses from Atypical Strain Readings Using the Incremental Rigidity Method-Paper

Paper

Pdf Summary

The paper examines how to calculate internal forces in deep foundations when strain-gauge readings are atypical or a pile’s effective rigidity changes during a static load test. Strain gauges measure strain—not force—so converting readings to internal forces requires assumptions about the pile’s stiffness and cross-section. The conventional approach uses the product of elastic modulus and area (EA), often estimating concrete modulus with an ACI formula.<br /><br />The authors present the Incremental Rigidity (I.R.) method, which uses changes in test load relative to changes in strain (ΔQ/Δε) to derive an effective, potentially strain-dependent rigidity and calculate internal forces without requiring a predetermined EA.<br /><br />A bi-directional load test on a 60-foot drilled shaft provided the case study. Strainmeters were mounted on a steel core beam welded to the jack assembly. At three evaluated gauge levels, the ΔQ/Δε plots showed bilinear responses, indicating changes in effective rigidity; two were also noisy. Applying the I.R. method produced plausible internal forces that closely matched the relevant upward or downward test loads. In contrast, the ACI-based calculation yielded physically impossible forces at one level, including forces exceeding the applied load and those calculated at a location closer to the load source.<br /><br />The authors conclude that the I.R. method can interpret atypical readings and account for rigidity changes, potentially preserving gauge levels that might otherwise be discarded. They caution that interpretation remains uncertain and depends on load-transfer conditions, including whether shaft resistance is fully mobilized. They also recommend avoiding core-beam-mounted sister-bar strainmeters in bi-directional tests because this arrangement can produce atypical readings. Overall, the I.R. method is presented as more suitable than a constant-modulus ACI calculation for these atypical responses.

Keywords

deep foundations

internal forces

strain-gauge readings

incremental rigidity method

ΔQ/Δε

effective rigidity

bi-directional load test

drilled shaft

ACI modulus

pile stiffness