MCEER HIGHWAY PROJECT
Task C2-2: Pile Group Effects
Subject Area: Special
Bridges - Ground Motion & Geotechnical Studies
|Principal Investigator(s) and Institution(s)
Po Lam, Earth Mechanics, Inc.
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 possible 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.
In Research Year 1, the following subtasks were accomplished:
The scope for Research Year 2 is as follows:
The extension of the above research program to address dynamic response issues by applying equivalent linear soil properties in FLAC to evaluate the importance of dynamic group effects such as radiation damping and kinematic interaction, will be deferred until Year 3. The concept of using hybrid models to study these effects where non-linear interface models are combined with elastic for field models would also be part of a Year 3 scope. The consideration of design guidelines for caissons will also be deferred to Year 3.
08/01 Report describing analysis results and design guidelines for determining the stiffness characteristics of large pile groups.
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