Steel Pipe-Pile Experiments and Analyses: In-Ground Flexural Deformation Capacity in the Presence of Axial Load

The ASCE/COPRI 61-14 seismic design standard for pier and wharves prescribes an in-ground life safety (LS) strain limit for steel pipe-piles. The adequacy of this limit has been the topic of much discussion within the marine structural engineering community. It has been argued that steel pipes cannot achieve the LS strain limit before undergoing local buckling. In response, researchers have proposed critical strain ( ε c r ) equations for replacement of the standing LS strain limit. The critical strain has been obtained by curve fitting data from numerous pipe flexure tests. Although these studies have provided valuable insights into pipe flexural behavior, most were conducted using three- or four- point in-air bending tests and in the absence of axial load; conditions that differ from those encountered in the field. This paper presents two 1:3-scale experiments on axially loaded soil-embedded steel pipe-piles: one subject to quasi-static monotonic and another to reversed cyclic lateral loading. The test data were used to calibrate a detailed nonlinear finite-element model (FEM) for soil-embedded steel pipe-piles. The model was employed to perform a numerical study on pile deformation capacity within the context of critical strain ε c r . Practical ranges for select variables that steer pier-wharf design were considered in the numerical study. Results show that ε c r is overly conservative, inconsistent, and fails to capture the influence of axial load; an undesirable attribute for capacity-driving metrics. Furthermore, in practice, the use of strain as a damage-indicating engineering parameter requires the use of an equivalent plastic hinge length, a parameter which adds uncertainty. The paper proposes the use of a strength-targeted plastic rotation capacity ( θ p c ) which is shown to be an intuitive capacity indicator and makes use of a regression analysis on a subset of numerical results to postulate analytical θ p c expressions for displacement-based design of steel pipe-piles.