Economic Pier-to-Pile Connections for Permanently Cased Shaft (CFST) Piles

Background:
Transportation systems including elevated bridges for vehicles, high-speed rail and ports are moving towards modulated systems that promote and facilitate accelerated construction. Accelerated construction (AC) of transportation systems, more commonly referred to as accelerated bridge construction, is important and advantageous because it (1) reduces traffic interruption and downtime of the system, (2) reduces labor and (3) reduces on-site construction time, which in turn, reduces cost. However, most AC techniques use precast components as the piers, which is advantageous from the perspective of schedule but requires that heavy equipment which can increase the cost and thereby reduce the cost-effectiveness of AC. In addition, most AC studies ignore the foundation construction cost and schedule which misses a critical point because foundations typically make up more than 50% of the cost of the structural system.

A solution to reduce equipment cost and promote AC of transportation systems is to use concrete-filled steel tubes (CFSTs) An alternative system has been investigated for AC. This system uses CFSTs as piles and/or piers. These components meet new seismic and construction performance requirements, as follows:

  1. Reduced downtime and post-earthquake repair. The seismic performance is excellent. Prior work shows that CFST connections can sustain connection rotation demands in excess of 8% without requiring repair. Because of their superior shear strength (relative to RC components), CFST piles resist high-demands resulting from lateral spreading or liquefiable soils without damage.
  2. Improved constructability and reduced cost. A priority of states and transportation authorities (including the CA HSRA, ports and DOTs) is reduction in cost and traffic interruption for new systems. CFTs meet these needs in three important ways. First, replacing RC piers with CFST pier components and their connections results in between 25 and 60% cost savings. Second, CFSTs reduce the need for internal reinforcement and formwork (for the pier) which greatly reduces schedule, cost and improves worker safety since long (sometimes over 100’) cages do not have to be placed. Finally, CFSTs do not require heavy equipment and thereby reduces the equipment costs.
Figure 2. (a) Embedded Ring and (b) Headed, Debonded Dowel
CFST Pier-to-Superstructure Connections

With all of these advantages, it would seem that CFST systems are a natural structural solution for HSR, ports and other transportation infrastructure. However, their use is curtailed by the lack of standard, tested connections with proven seismic resistance. The PIs have developed two new connections for CFST component to pile cap or cap beams, as shown in Figure 1. The first connection uses a welded ring to transfer the shear, axial and flexural demands; this ring acts similarly to a stud. The second connects the CFST using welded high-strength (80-ksi reinforcement). Both of these connections dissipate energy through elongation and shortening of the steel tube and/or reinforcement. By design, these connections promote AC and reduce damage through elongation of the steel without damage to the concrete, promoting ductile response without permanent damage, thereby meeting higher performance objectives.

Objective:
A missing piece of this new structural system is the foundation system including the direct pier-to-pile connection and the contribution of soil-structure interaction on the this is an economical solution that reduces the cost of the structural foundation system. The overall goals of the proposed research are to:

  • Investigate CFST connections and other column-to-pile connections through a literature review.
  • Select column-to-pile connections for study based on the FEA results.
  • Investigate the seismic response and resilience, including damage, of selected CFST connections using large-scale testing.
  • Develop, in collaboration with WSDOT and Caltrans as well as other interested transportation agencies, new design methods for these connections.
  • Develop a simplified nonlinear spring element to simulate the connection and pair the engineering demand parameters from the model to the damage states to provide tools for PBEE.

Scope:
The following research tasks are proposed to achieve these objectives.

  • Task 1 –  Literature Review and Agency Discussions

    • A comprehensive review of past experimental research will be completed.  Experimental results evaluating resistance, stiffness, and force-deflection of direct column-to-pile connections will be studied.  This task is expected to be completed quickly, because the researchers are familiar with most of the past work through research performance on other CFST research projects funded by Caltrans and WSDOT. It is anticipated that Task 1 will be completed within the first two months of the research study.
  • Task 2 – Select and Design Test Matrix
Figure 2 Pier-to-Pile ConnectionTest
    • The team, in collaboration with an oversight committee consisting of prominent engineers from WSDOT, ODOT and Caltrans, will select the test matrix using the results of the FEA. The UW team will arrange a call or calls to present the research results from the prior study funded by the FIU ABC center, including the impact of cyclic loading, bar size, relative pier to pile geometry and the connector size and layout. After two calls, a specimen test matrix will be designed and approved.
  • Task 3 – Testing of Specimens

    • PBEE of CFST piles foundation systems require validated design and connection models. This task will provide the fundamental tests data for both. It is expected that two tests will be conducted, complementary to the testing being conducted as part of the PEER project. That project is focusing on one CFST pier-to-CFST pile and one RC pier-to-CFST pile connection. This project will focus on the connection between the RC pier and CFST pile specifically investigating the impact of connector layout and relative pier/pile diameter. The specimen is designed to test the transfer mechanism between the pier and CFST shaft with the objective of full hinging of the pier without connection damage. Figure 2 shows the test  setup for this task.
  • 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, 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.

Research Team:
Principal Investigator:  Dr. Dawn Lehman
Co-Principal Investigator: Dr. Charles Roeder
Research Assistant:  Spencer Lindsey