| Principal Investigator and Institution
I. Po Lam, Earth Mechanics, Inc.
Objective
Task C2-2 is completing the development of numerical approaches to
investigate soil-pile interaction effects of large closely spaced pile
groups subjected to lateral pseudo-static loading, expressed in terms of
p-y multipliers. Based on the results of parametric studies conducted
under the task, linearization then allows a determination of a static
stiffness matrix as a function of pile configuration and soil type for use
in structural analysis.
Under Task C2-3, an analytical framework for dynamic loading will be
advanced to model and evaluate wave propagation and wave scattering
effects for both large pile groups and large caissons, including
quantifying the effects of modifying the input motions due to wave
scattering and its influence on system damping from radiation of reflected
waves. In addition to developing design guidelines, this task will provide
an evaluation of the appropriate platform for implementation of
soil-structure interaction principles for bridge design and analysis.
Approach
At the onset of the pile group research being conducted under MCEER
Project 094, two fundamental objectives were identified to advance the
state-of-practice regarding analysis of the type of foundation systems
encountered for long span bridges, including large pile groups and
caissons. These were:
- Improved accounting of the interaction of the stress and strain
field in the soil due to superposition of loading from many adjacent
piles in close spacing, commonly referred as pile group effects.
Research under Task C2-2 has been focused on addressing pile group
effects for static loading conditions. Results from Task C2-2 will
directly apply to static pushover capacity analysis of large pile
groups, as well as the characterization of foundation stiffness and
material damping issues in a global model demand analysis. Results of
these studies can be assessed against data from available on-going
pile group experiments for typical 3x3 or 4x4 pile groups.
- Extending this effort to address pile group foundations under
dynamic loading conditions, particularly in relation to how earthquake
wave scattering of the foundation system will alter the near-field
ground motion transmitted to the superstructure. In addition to pile
groups, the wave scattering issue needs to be addressed for large
diameter drilled shafts and sunken caisson foundations (typical
foundations for long-span bridges) where the wave scattering effect
becomes increasingly important.
Item (2) above has traditionally been treated by frequency domain
solution computer codes such as SASSI or CLASSI. However, the overall
global bridge model analyses for major long-span bridges are commonly
analyzed by computer codes such as ADINA or ABAQUS using time history
analyses. Because of a lack of a common platform for analyses,
substructuring techniques are needed to implement results of wave
scattering and foundation impedances from frequency domain solutions into
the subsequent time domain global bridge analyses models. This two-step
approach, typically done by separate groups of engineers (i.e., structural
versus geotechnical analysts), conducted using different numerical
platforms, often leads to confusion and delays in projects, and is
generally not favored by practitioners.
There is a great interest in advancing the state-of-practice so that
the wave propagation and scattering analyses can both be conducted in time
domain, or in a numerical platform that is familiar to structural
engineers. Then, both groups (geotechnical professionals who is familiar
with wave propagation analyses, and structural engineers performing the
global structural model) can work simultaneously on the same computer
platform to contribute expertise to the total problem.
It is considered feasible to address many of the wave scattering or
soil-structure interaction issues using time domain general utility
computer programs that are commonly used by bridge designers working on
long-span bridge projects. The approach of using a common platform by both
structural engineers in structural response studies and by geotechnical
engineers in soil-structure interaction analyses is considered to be more
practical than the traditional substructuring method, and is likely to
help avoid mistakes when transferring large volumes of ground motion and
foundation submodel data to the subsequent structural model.
During Research Years 3 and Year 4, the knowledge obtained from Task
C2-2 for pile groups will be advanced to address dynamic loading issues,
particularly for wave scattering effects during the analysis of long-span
bridges. In Research Year 3, the work will require calibration solutions
for typical caissons and pile group configurations to ensure that the wave
scattering treatment in time domain can be compatible to proven approaches
(e.g., SASSI type analyses) within the context of linear elastic
half-space assumptions. After confirmation of the basic time domain
framework, an examination of the degree of change in the ground motion
input affected by the scattering issue will be examined. Results from the
Research Year 3 sensitivity studies will then be used to develop
guidelines to account for wave scattering and radiation damping issues.
Work during Research Year 3 will consist of the following:
- Select typical pile groups, large diameter drilled shafts, and large
caisson systems for soil-structure interaction analyses to be
conducted in both the frequency domain (using programs such as SASSI)
and in the time domain (using an analytical platform such as ADINA.
- Compare the results of these analyses to demonstrate that the time
domain computer codes can yield similar wave propagation and wave
scattering solutions as frequency domain solutions.
- Based on the results of sensitivity studies, develop guidelines for
modifying the free-field site response ground motion criteria to
account for wave scattering and radiation damping issues.
Products
- Assessment of whether frequency domain and time domain analysis
approaches provide similarly accurate results for long-span bridge
foundation systems.
- Guidelines for modifying the free-field site response ground motion
criteria to account for wave scattering and radiation damping.
Technical Challenges
Although the PI and agency staff are experienced in the use of the
computer program ADINA, they have no experience in using the SASSI
program. This will require careful modeling and results review
considerations to ensure that the analyses are conducted correctly.
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