MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

Task E1-3: Moment Ductility of Prestressed Concrete Piles

Subject Area: Geotechnical and Foundation Engineering
Research Year 3

Yan Xiao and Geoffrey R. Martin, University of Southern California

Objective

The efforts of geotechnical and structural engineers will be combined in this task in order to investigate the behavior of bridge piles subjected to differential lateral spread displacements of liquefied soils. During the 1995 Kobe earthquake it was found that a considerable number of piles suffered failure due to the differential lateral spread of liquefied soils, resulting in significant damage to a number of bridges. Insight into this type of failure is needed and protective measures must be developed. The results of this effort will be the development of a database of performance data of prestressed piles and their connections, seismic evaluation guidelines for assessing the seismic demand on piles subjected to differential lateral spread displacement of liquefied soils, and guidelines for preventing failure of pile foundations.

Approach

A significant amount of research has been conducted in order to provide a better understanding of seismic behavior and ductility of bridge columns. However, only limited a number of studies related to the behavior and ductility of pile foundations have been done. In the past two years, with support from the FHWA and MCEER, experimental studies have been conducted to assess the seismic behavior of pile to pile-cap connections. This research has resulted in several important findings related to the behavior of pile foundations, particularly concerning the effects of axial loading on the behavior of these connections.

The research to be conducted under this task will result in the development of a database of performance data of prestressed concrete piles and their connections, seismic evaluation guidelines for assessing the seismic demand on piles subjected to differential lateral spread displacement of liquefied soils, and guidelines for preventing failure of pile foundations. This will be accomplished via the following subtasks:

Subtask 1 - Test Program   Various pile types will be tested up to failure to establish moment-curvature response characteristics. The focus of the study on prestressed piles, which are more widely used in bridge construction, but for which sufficient performance data is still considered lacking. Test parameters will include pile section shape (e.g., circular, square, or octagonal) and size (14" to 24", or slightly smaller). Between six and ten specimens will be tested. 

As shown in the figure, piles will be tested as simply supported beams with applied axial load. The axial load is provided by using a hydraulic jack, two end loading stubs, and two to four high-strength rods. It is possible to vary the axial load during the testing. An actuator is employed to apply transverse force to generate pure bending and, thus, the moment can be calculated as:

M = ½ L_sF

where F is the applied transverse force and L_s is the length of the shear span. All the pile specimens used in this task will be donated by the manufacturers.

Subtask 2 - Analytical Modeling  An existing moment-curvature program for analyzing reinforced concrete columns will be modified to handle the prestressed concrete piles used in this program, and the results of the program will be compared with the test results. The prestressed concrete piles will be modeled using discretized beam elements. Sensitivity studies will be then carried out to evaluate the length of the beam elements as well as various types of interface spring models for piles and surrounding soils.

Subtask 3 - Analytical Studies of Lateral Spread of Liquefied Soils  This task will continue the previous efforts, with a focus on assessing the lateral demand on piles due to the lateral spread movement of liquefied soils. The analysis results of this subtask will be implemented and combined with the pile models and soil foundation.

Products

• Performance data of prestressed piles and their connections.
• Seismic evaluation guidelines for assessing the seismic demand on piles subjected to differential lateral spread displacement of liquefied soils.
• Guidelines for preventing failure of pile foundations.

Technical Challenges

The primary technical challenge is associated with ensuring adequate consideration of both structural and geotechnical concerns and contributions to this task. The testing program must be carefully planned in order to realistically reflect the seismic demand calculated from the geotechnical analyses. Another challenge in ensuring that the structural test results have been properly modeled and implemented in the soil liquefaction analysis.