Design Guidelines for ABC Column-to-Drilled-Shaft Foundation Connections in High Seismic Zones

Background:

Figure 1. Tran Tests of Connection Between Precast Column and Enlarged Drilled Shaft

Precast columns have the potential to be very cost- and time-efficient for Accelerated Bridge Construction (ABC), but they must be connected effectively to the foundation, particularly in regions of moderate or high seismicity. Most of the relevant research has been conducted using spread footings, but in many applications, drilled shafts are preferred, most commonly with a diameter larger than that of the column. Little research has been performed on connections between precast columns and enlarged drilled shafts.

The current AASHTO ABC design recommendations for shafts are based on the results of cast-in-place column behavior and a single cyclic test of a column-to-shaft subassembly. Some research has been conducted for cast-in-place column systems (e.g., Mclean and Smith 1997, Murcia-Delso et al. 2013), but cast-in-place column construction takes time and does not lend itself easily to accelerated bridge construction.

As part of his PhD thesis studies at the University of Washington, Hung Tran (2015) reported the results of three tests of connections between precast columns and drilled shafts. These tests (Fig. 1) provided valuable data, but the tests by themselves are insufficient to understand the complex force transfer mechanism in the real 3-D connection, and to develop specific design guidelines

As part of a study funded by the Pacific Earthquake Engineering Research center (PEER), additional tests of column-to-shaft connections will be performed. The tests already performed by Tran (2015) and those to be performed as part of the PEER project will provide much more data to calibrate analytical models and design procedures than has been available in the past.

A parametric study, with models calibrated by these tests, is required to investigate the wide range of conditions expected in practice.  The test results and parametric study will make it possible to develop design recommendations for precast-column-to-drilled-shaft connections.

Objective:
The main objective of the research is to develop guidelines for the design of ABC connections between precast columns and enlarged, cast-in-place drilled shafts in seismic regions.

Scope:
UW researchers will develop guidelines for the design of ABC connections between precast columns and enlarged, cast-in-place drilled shafts in seismic regions. These guidelines will be based on the results of three tests conducted previously by Tran, and on additional tests to be conducted with PEER funding. The team will perform a parametric study using strut-and-tie models and finite-element models, which will have been calibrated with the test results, to consider a wide range of column and shaft properties:

  • Task 1 – Analysis of Tran Test Data
    • All of the data collected by Tran has been preserved, but it is necessary to re-analyze the data, so that the data processing methodology for the new tests will be consistent with that used for the new, PEER-funded tests. This analysis will also make it possible to design the new test specimens to so that they will be most useful in developing new design recommendations..
  • Task 2 – Development/Calibration of Analytical Models
    • Strut-and-tie and finite-element models will be calibrated using the data available from Tran and the PEER-funded tests. Figure 3 shows the initial form of a possible strut-and-tie model that could be used in this application.
  • Task 3 – Parametric Study
    • The developed models will be used in a parametric study to evaluate the effects of key design variables, including the depth of embedment of the precast column, the relative diameters of the column and shaft, as well as the details of the longitudinal and transverse reinforcement.
  • Task 4 – Development of Design Recommendations
    • The results of the parametric study will be used to develop design recommendations that would be suitable for practice. These recommendations will be reviewed by bridge engineers from California and Washington states to verify that the form is appropriate.

Research Team:
Principal Investigator:  Professor Marc Eberhard
Co-Principal Investigator: Professor John Stanton
Research Assistant:  Michelle Chang