**Principal Investigator(s) and
Institution(s)**
Po Lam, Earth Mechanics, Inc.
Geoffrey R. Martin, University of Southern California
**Objective**
Evaluations of the earthquake response of bridges normally divide the bridge system into
two equivalent substructure types, a bridge substructure and a foundation substructure,
usually separated at a pile cap or at mudline pile nodes. The foundation substructure
requires the development of an appropriate foundation stiffness matrix and foundation
forces (reflecting excitation due to ground motion) for use in the bridge substructure
dynamic analysis. As described in a prior FHWA/MCEER task (Task 106-E-4.10 of Project
DTFH61-92-C-00106), substructuring methods may be deployed in the time domain or the
frequency domain, or possibly via a hybrid frequency-time domain. Each of these methods
have unique merits and degrees of difficulty.
Whereas engineers have a good understanding of the pros and cons of these methods and the
associated questions of the relative importance of inertial and kinematic interaction in
the case of smaller (say 3x3) pile groups, the appropriate application of these methods in
the case of very large groups involving hundreds of piles normally associated with long
span bridges, remains to be resolved. For this case, ignoring group effects can lead to
serious errors, and the use of an elasticity approach is known to be deficient.
By incorporating advances in constitutive modeling and the application of 3-D finite
element analysis, this task will establish a practical approach to determine stiffness
matrices appropriate for very large pile groups, and examples where the use of the
developed approach will be illustrated by application to pile groups representative of
long span bridges in both the Western and the Eastern U. S.
**Approach**
The following subtasks will be undertaken during the first year of this two-year study:
Subtask 1 - Large Pile Group Data Base. To provide a baseline for analytical studies, a
data base will be compiled describing pile configurations, pile types and related site
soil conditions for long span bridges representative of various regions of the country.
Subtask 2 - Development of Analysis Methodology. Initially, nonlinear 3-D finite element
solutions for smaller pile groups will be back fitted to smaller (say 3x3) pile group
data. The associated calibration of the constitutive model will then be used to predict
group effects of much larger pile groups by using periodic boundary conditions. These
solutions will then be used to evaluate equivalent linear soil properties to match static
group effects, and the results will be compared with the more conventional elasticity
approach to establish group effects. The results of this analysis will then be used to
establish design guidelines for the determination of a static stiffness matrix for large
pile groups as a function of pile configuration and soil type.
In Year 2, the research will be extended by applying the equivalent linear soil properties
available in elasto-dynamic programs (such as SASSI) to extend static group effects to the
evaluation of the importance of dynamic group effects, including radiation damping and
kinematic interaction
In addition, a preliminary evaluation of the research framework described above will be
extended during Year 1 to bridge caisson systems, as an initial first step in refining the
scope of work for future Task C2-5 on "Soil-Structure Interaction for Caissons."
Included in the evaluation will be the establishment of a
bridge caisson data base.
**Anticipated Start Date and Duration**
January 1, 1999 - 24 months
9/23/99 |