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

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The presentation examined the incremental rigidity (IR) method for interpreting unusual strain-gauge readings from a bidirectional static load test on a 30-inch-diameter, 60-foot drilled shaft. IR uses changes in applied load and measured strain after shaft resistance above a gauge is mobilized to calculate internal force directly, without requiring the pile’s cross-sectional area or elastic modulus. Three gauge levels showed atypical, often bilinear responses, indicating changing rigidity and strain-dependent modulus. Despite differing data patterns, IR interpretations consistently indicated that nearly the full jack load reached all three levels, with little apparent load transfer. The presenter argued that IR can model changing rigidity more effectively than a constant-modulus estimate and can produce useful interpretations even with noisy data. The cause of the apparent lack of load transfer remained uncertain; the unusually rigid core beam used to mount the gauges may have affected load transmission. The team recommended against using this mounting arrangement.

Keywords

incremental rigidity

strain gauge readings

bidirectional static load test

drilled shaft

load transfer

shaft resistance