Large-Scale Test Results and Evaluation of the In-ground Hinge Development of Steel Pipe Piles

Abstract: The University of California, San Diego has nearly completed a series of large-scale tests conducted to evaluate the performance of the in-ground hinge for steel pipe piles with diameter-to-thickness (D/t) ratios ranging from 22 to 43. The testing consists of reversed-cyclic lateral loading with constant applied axial loading on four piles, each of which were individually tested after being driven and instrumented into an outdoor soil pit. The reversed-cyclic lateral loading was displacement-controlled and remained consistent amongst each test. The constant axial load varied amongst the four tests.
The pile performance and post-buckling behavior were evaluated through post-processing of the recorded instrumentation data installed along the pile. An extensive array of large-strain electrical foil strain gauges and less dense array of accelerometers were installed in the anticipated plastic hinge region to better evaluate the area of interest. In-ground hinge locations were identified allowing for the determination of plastic rotations at two critical points, (1) when the pile exceeds the estimated theoretical yield and (2) prior to the post-peak moment dropping below 80% of maximum moment.
All four piles were fabricated in accordance with API 5L standards, which is commonly accepted in the industry in place of ASTM A252. Two piles are considered per AISC 360 as non-compact sections with D/t equal to 43 and two are compact sections with D/t equal to 34 and 22. As marine structures designed to resist gravity and seismic demands utilize steel pipe piles with D/t ratios ranging from 20 to 60, the outcome of this research will provide useful information for industry practitioners to determine whether the use of non-compact sections could be more widely implemented.
Results from a blind prediction contest will also be summarized in the paper. The blind prediction encourages academics and industry personal to compete by self-evaluate the pile test performance through the performance of decision engineering and analytical computational methods.
This paper will outline the testing performed by focusing on the individual experimental results as well as comparative results of the pile performance. Testing limitations and assumptions will be clearly described for the reviewers understanding. Finite element analysis (FEA) was performed concurrently to validate the initial two large-scale tests consisting of non-compact piles with varying axial loading. The FEA results are compared against an FEA parametric study for piles with D/t ranging from 20 to 60. These collective results are used to determine critical strains; which are compared with critical strains found in relatable but not directly comparable literature.
The outcomes of this research will directly benefit the upcoming updates, as applicable, to the ASCE 61 standards and UFC.

How does this project / topic contribute to the advancement of the industry and profession?: This topic has been a well debated topic amongst industry practitioners, specifically those who hold positions on code writing committees. The results presented provide an update and progress summary of the well desired research which has been anticipated since the 2019 and 2022 Port Conferences. Results of this research will be used in future updates, as applicable, to the ASCE 61 standards and UFC.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The summary of the paper could potentially result in committee members evaluating and modify the current ASCE/COPRI and UFC recommendations regarding local buckling for compact and non-compact sections.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: Scheduling and working with academic labs and competing university projects has caused significant delays to the overall testing and post-processing of the results. This has been over-come by continuing to involve the industry-advisors in periodic meetings to determine solutions and remain in-line with what is the desired project outcome.

Who is the target audience for this paper?: Designers/Structural Engineers, Port authorities, Codes and Guidelines Committee members