Abstract:
Deep foundation elements subject to load testing are often instrumented with strain gages, to determine the force distribution within the element. Strain itself is not a measure of force, but must be converted to force via a function, sometimes mis-labeled as the ‘stiffness’ but more accurately described as the ‘rigidity’ of the foundation element.
The rigidity function is often assumed to be a constant multiplier of the element’s cross?sectional area and Young’s modulus. However, cementitious materials have a nonlinear stress-strain relationship at strains typically encountered during load testing, and this must be accounted for in order to analyze strain gage data properly. Several back-calculation methods exist which estimate the rigidity as a function of strain from a linear regression to the strain gage data itself. These methods all derive from the same basic assumption about the nonlinear stress-strain curve, and can be shown to be interrelated. A proper understanding of their derivation and underlying assumptions is necessary in order to implement and use these techniques. This paper presents a derivation and discussion of these methods, including their advantages and limitations.