Impact of Construction Eccentricity on Direct Pier-to-Pile Connections for Permanently Cased Shaft (CFST) Piles

Project Information
Link to the Latest: December 2021 Progress Report

For seismic design of transportation structures, there are competing demands including: economy, strength, stiffness, inelastic deformation capacity, and seismic resilience. Prior research at the University of Washington (UW) demonstrates that concrete-filled steel tubes (CFSTs) can meet these competing demands. This proposed research builds on the prior CFST research to develop direct pier-to-pile connections specific for use in wide range of transportation systems including bridges, high speed rail (HSR), and port structures. Initially finite element analyses (FEA) were conducted to develop the connection and experimental test matrix. Specific study parameters includeembedment depth and the addition of a ring to enhance mechanical bond. This initial study resulted in an initial test matrix that is currently being conducted, to study these aspects of the connections. However, there is an important, yet unstudied parameter, which is the placement of the reinforcing steel cage. In construction, it is likely that the pier will not be placed at the exact center of the pile, but instead will be placed with some eccentricity relative to the center of the pile. This eccentricity is likely to be very important but is not possible to study analytically. Here, it is proposed to investigate the impact of eccentricity on the transfer mechanism and damage using large-scale experimental specimens. It is expected that two tests will be conducted which will be complementary with and extend current research that is being sponsored by Pacific Earthquake Engineering Research (PEER) center (referred to as PEER herein). The results will be used to determine design methods and nonlinear analytical models for these new connections.

The overall goals of the proposed research are to: Investigate impact of construction eccentricity on direct pier-to-CFST pile connections. Investigate the seismic response and resilience, including damage, of selected CFST connections using large-scale testing. Build on experimental study using validated FEA models to investigate unstudied parameters including pier-to-pile diameter ratio. • Develop, in collaboration with WSDOT and Caltrans as well as other interested transportation agencies, new design methods for these connections

Task 1 – Presentation to and Discussion with Transportation Agencies. The researchers will develop a one-hour course module summarizing the current research on the direct connection. This will include: (1) current state of practice for direct connections (this connection is used by WSDOT and likely other agencies), (2) analytical investigation of fundamental response parameters, (i.e., load-displacement response and damage) for salient design parameters, and (3) initial test results of concentric connections with different pile diameters with and without supplemental ribs. This will be followed by a discussion with these agencies, to provide feedback on the completed and proposed research. This will become another module in the CFST course. In addition, the research team will send out a survey to relevant DOTs to quantify the acceptable eccentricity for cage placement. The majority of Task 1 will be completed within the first two months, with additional time needed to determine the allowable eccentricity for each DOT (starting with WSDOT, ODOT and Caltrans).

Task 2 – Select and Design Test Matrix. The team, in collaboration with an oversight committee consisting of prominent engineers from WSDOT, ODOT and Caltrans, will select the test matrix. After two calls, a specimen test matrix will be designed and approved. It is expected that two different eccentricities will be tested.

Task 3 – Testing of Specimens. It is expected that two tests will be conducted, complementary to the testing being conducted as part of the PEER project and the prior FIU project. This project will focus on the eccentricity between the RC pier reinforcing cage and CFST pile, with two different eccentricities expected to be studied. The specimen is designed to investigate the impact of eccentricity on the transfer mechanism between the pier and CFST shaft with the objective of full hinging of the pier without extensive connection damage.

Task 4 – Development of Design and Analytical Tools. Using the FEA and experimental results, the team will develop both design methods and rotational springs that can be implemented in CSI Bridge. Both the design methods and nonlinear analytical model will account for bar size, material strengths, eccentricity as well as pier and pile geometries.

Task 5 – Interim and Final Reporting. The team will submit timely quarterly reports and present annually at the Research Days meeting. A final paper will be written that summarizes the methods used and the findings reached during the project. In addition, the results will be incorporated into the CFST course module, as indicated in Task 1.

Research Team:
Principal Investigator: Prof. Dawn Lehman
Co-Principal Investigator: Prof. Charles Roeder