A Discussion of Back-Calculated Rigidity Methods for Deep Foundation Load Testing-PPT Presentation

PPT Presentation

Pdf Summary

The document discusses methods for interpreting strain-gage data from deep-foundation static load tests to estimate a pile’s axial rigidity, \(AE\).

It distinguishes <strong>stiffness</strong>, a spring property measured as force per unit length, from <strong>rigidity</strong>, the product of cross-sectional area \(A\) and elastic modulus \(E\), measured in units of force. The author adopts “rigidity” to describe \(AE\).

Because the load–strain response may be nonlinear, the inferred rigidity can vary with strain and with the calculation method. The document presents three approaches:

- <strong>Secant rigidity:</strong> load divided by strain, \(F/\varepsilon\).
- <strong>Tangent rigidity:</strong> the slope of the load–strain curve at a given strain.
- <strong>Incremental rigidity:</strong> the change in load divided by the change in strain between successive readings.

For a quadratic load–strain relationship, the methods yield different expressions and therefore different rigidity estimates. The accompanying graph plots rigidity against microstrain for two strain gauges, showing measured estimates, linear fits, and an ACI reference value.

Overall, the discussion emphasizes that calculated \(AE\) depends on both the assumed load–strain relationship and the method used to interpret the measurements.

Keywords

strain-gage data

deep-foundation

static load tests

axial rigidity

pile analysis

load-strain relationship

secant rigidity

tangent rigidity

incremental rigidity

ACI reference value