Seismic Vulnerability of the Highway System

Task E1-1: Behavior and Displacement Capacity of Pile and Pile-to-Cap Connections

Subject Area: Geotechnical and Foundation Engineering 
Research Year 2

Principal Investigator(s) and Institution(s)

Yan Xiao and Geoffrey Martin, University of Southern California
John B. Mander, University at Buffalo


The strength and deformability limit states of existing pile foundations, particularly due to soil-foundation interaction in the inelastic range, are presently not well understood. The objective of this task is to investigate the soil-pile interaction effects on the ductility capability of common pile types and the strength and ductility of pile-to-cap connections for short-to-medium span bridges as well as long span bridges.


Inertia loads in bridges caused by earthquake ground shaking must be transferred to the ground. This is normally achieved through pile foundations. In cases where the pier consists of a series of strong columns or is a wall-type structure, the piled foundation is often the weakest link in the chain of lateral load resistance. This implies that under large lateral loads, in the limit, a plastic mechanism must form within the piled foundation. Two sets of plastic hinges must form: an upper set at the pile-to-cap connection, and a lower set some four to eight pile diameters below the bottom of the cap in the soil. This type of inelastic behavior is not well understood in terms of either seismic demands or capacities. This task will assess the inelastic capacity (particularly the rotational limit states) of plastic hinges in piled bridge foundations. 

Some work was initiated on this topic as part of the prior FHWA/MCEER Contract DTFH61-92-C-00106 where the rotational capacity of steel pile-to-cap connections was experimentally investigated. Results show that considerable inelastic rotation can be achieved, even with small pile embedments. For good performance, however, it is desirable to have at least 2.5 pile diameter embedment into a reinforced concrete cap. (Note - if thinner caps are desired or required, special anchorage and confinement detailing should be provided.) 

Year 1 of this research task extended the above-mentioned earlier work by investigating the seismic resistance of two other types of pile caps as follows:

  1. The University at Buffalo investigated seismic resistance of timber pile foundations with a particular emphasis on the failure mechanisms and detailing requirements for timber pile-to-concrete cap connections. Results of this study demonstrated that existing practice, although not ideal, provides reasonably ductile connections with failure occurring in the connection itself as a result of crushing of the timber perpendicular to the grain. Additional experiments were conducted with enhanced reinforcement detailing where some additional U-bars were used in the foundation to help anchor the pile within the cap. Results of these experiments demonstrated that it is possible for timber piles to sustain drift angles of 6% without undue loss of strength from cyclic loading. 

  2. At the University of Southern California, experiments have been conducted on steel and concrete piles. Deficiencies due to inadequate anchorage have been identified and improved detailing strategies proposed.

During Year 2 of this task the following research activities are planned:

  1. At the University at Buffalo it is proposed to investigate the rotational capacity of the plastic hinge zones in piled foundations where the hinges are located at some depth below the ground surface. The behavior of such hinge zones is quite different from freestanding concrete columns as the curvature distribution, and hence pile performance, is modified by the surrounding soil. Plastic hinge zones tend to be longer (2 to 3 pile diameters, instead of diameter for columns) due to the surrounding soil pressures that modify the shear force, maximum bending moment, and inelastic curvature distributions in each pile of a group. It is difficult (although not impossible) to investigate such behavior experimentally. Therefore, an analytical approach will be adopted where inelastic curvatures are integrated over a length of pile to assess the total plastic rotation. Then, if feasible, some limited experimentation will be undertaken in the University at Buffalo soil testing pit to verify theoretical predictions. Recommendations will be derived from these studies to provide a method of assessing the rotational capacities of piles in soil as a function of pile type (ductile detailing), soil type, and depth to the hinge within the soil. 

  2. At the University of Southern California continuation of the experimental research will be undertaken-a unique emphasis of this work is on the axial pull-out capacity of steel and concrete piles. Detailing improvements for new structures and retrofit strategies for existing structures will also be proposed. 


Seismic evaluation guidelines for assessing the rotational/ductility capacity of plastic hinges for piles within soil and for assessing the strength and inelastic rotational capacity of existing pile-to-cap connections; detailing recommendations for improving the ductility capability of new pile designs; and retrofit guidelines for improving the inelastic rotational capacity of existing pile-to-cap connections.

Technical Challenges

A large variety of piled foundation types exist. This makes generalization into design and retrofit guidelines difficult. The main challenge is to keep the recommendations fundamental, elegantly simple, and yet easy to implement for the widest variety of design and retrofit situations.


return to Task Statement List