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Bridge Foundations: Modeling Large Pile Groups and Caissons for Seismic Design

I. Po Lam, H. Law and G.R. Martin (Coordinating Author)

MCEER-07-0018 | December 1, 2007

The report synthesizes and summarizes current FHWA-sponsored bridge research and related technical reports on the seismic design of large pile groups and caissons into one design-oriented guideline report on seismic design, suitable for applications to the retrofit of existing long span bridges or for new design. The characterization of ground motions for seismic soil-foundation-structure interaction analyses is an important component of the foundation modeling process, and is discussed in detail. Discussion includes methods for generating spectrum compatible time histories, spatial variations of ground motion and effects of local soil conditions on site response. Modeling approaches for large pile groups focus on the use of the substructuring approach, the use of p-y curves and associated pile group effects to develop linearized Winkler springs and the assembly of pile group stiffness matrices. The analysis approach is illustrated with several case history examples, including the San Francisco-Oakland Bay Bridge East Span project. The complexity of kinematic soil-pile interaction as related to modification of input ground motions is also described. Modeling approaches for large caisson foundations often employed as piers for large over-water bridges are discussed in detail, with an emphasis on the use of linear stiffness matrices, and nonlinear lumped spring and nonlinear distributed spring approaches. The influence of kinematic interaction on input ground motions is also described. Two case history examples are used to illustrate the modeling approaches, including one used for the new Tacoma Narrows Bridge.

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