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

Principal Investigator(s) and Institution(s)
John B. Mander, University at Buffalo
Yan Xiao and Geoffrey Martin, University of Southern California

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 to date indicate 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.) It is the intent of this task to extend this work to include other pile types such as prestressed/reinforced concrete and timber. Improved methods of detailing the pile-to-cap connection for new construction will be proposed, along with economical methods of retrofitting existing seismically deficient construction. The approach used here will be essentially the same as in the previous investigation. Several timber pile tests will be conducted at the University at Buffalo while the University of Southern California will conduct test on reinforced/prestressed concrete piles.

A second aspect of this task is to investigate the rotational capacity of plastic hinges in piles where the hinges are located at some depth below the ground surface. The behavior of such hinge zones is quite different from free-standing 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.

Anticipated Start Date and Duration
January 1, 1999 - 24 months