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

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The presentation describes the Incremental Rigidity method for converting measured pile strains into calculated internal forces. Rather than requiring separate values for elastic modulus and cross-sectional area, the method estimates their product, axial rigidity (EA), from changes in applied load and measured strain. It uses data collected after shaft resistance above a strain-gauge level has been mobilized, when changes in applied load correspond to changes in internal force.<br /><br />The case history concerns a 30-inch-diameter, 60-foot-long drilled shaft tested using a bi-directional loading system. Six levels of vibrating-wire sister-bar strainmeters were installed, with sensors mounted between the flanges of a W16x31 core beam around the jack assembly. The method was applied to strain readings—including noisy and atypical responses—to interpret internal forces at selected gauge levels.<br /><br />The authors conclude that Incremental Rigidity can account for changes in pile rigidity, including bilinear load–strain behavior and strain-dependent cementitious elastic modulus, and can produce reasonable internal-force estimates despite uncertainty in loads, strains, resistance mobilization, and stress history. They note that conventional modulus estimates using the ACI formula cannot capture these effects, and that the method may remain useful when other approaches fail. Finally, mounting sister-bar strainmeters on a core beam during bi-directional testing may produce atypical readings and should be avoided.

Keywords

Incremental Rigidity

pile strains

internal forces

axial rigidity

drilled shaft

bi-directional loading

vibrating-wire strainmeters

sister-bar strainmeters

bilinear load-strain behavior

ACI modulus formula