Dissertation/Thesis Abstract

Centrifuge Modeling, Numerical Analyses, and Displacement-Based Design of Rocking Foundations
by Deng, Lijun, Ph.D., University of California, Davis, 2012, 232; 3511714
Abstract (Summary)

This dissertation investigated the use of rocking shallow foundations in improving the seismic performance of ordinary bridges. Centrifuge model tests and numerical analyses were conducted and displacement-based design guidelines for rocking foundations were proposed.

Three series of centrifuge model tests of lumped-mass models and bridge system models were completed. Slow cyclic loading tests and dynamic shaking tests were performed. It was observed that rocking bridges were more stable than conventional hinging-column bridges. Rocking foundations had the re-centering ability that resulted in less residual rotations and showed non-degrading moment capacity. Rocking foundations were a good energy dissipater. The rocking-induced settlement increased with the cumulative footing rotation and decreased with the factor of safety for vertical bearing capacity. Concrete pads beneath rocking foundations were effective in reducing the rocking-induced settlement.

A beam-on-nonlinear-Winkler-foundation finite element model was developed in OpenSees to study the collapse potential of bridges with rocking foundations or hinging columns. Parametric studies including large deformation effects compared the performance and stability of stiff, flexible, tall and short rocking-foundation and hinging-column systems. The rocking and hinging systems had similar pushover curves for fair comparison. A suite of pulse-like and broadband motions were used as the input motions and incremental dynamic analysis was performed. The results showed that, in a probabilistic sense, bridges with rocking foundations were more stable than bridges with hinging columns if their fundamental periods were the same and if base shear coefficients to initiate hinging or rocking mechanisms were the same. Maximum drifts were not significantly affected by changing between rocking and hinging mechanisms except near collapse, but residual drifts are smaller for rocking systems. The results also challenged the notion that rocking systems require a different design approach than hinging column systems.

Direct displacement-based design (DDBD) guidelines were proposed for the design of rocking shallow foundations. A multi-linear hysteretic model for rocking foundations was proposed based on prior experimental studies and was decomposed into an elastic element and a plastic element to account for the radiation and soil hysteretic energy dissipation during foundation rocking. An ordinary bridge composed of a column and a nonlinear rocking foundation was integrated into a single visco-elastic element and then the displacement-based design was applied. Step-by-step design procedure was developed and elaborated with a design example in detail. The design example showed the feasibility of the proposed guidelines.

Indexing (document details)
Advisor: Kutter, Bruce L.
Commitee: DeJong, Jason T., Kunnath, Sashi K.
School: University of California, Davis
Department: Civil and Environmental Engineering
School Location: United States -- California
Source: DAI-B 73/10(E), Dissertation Abstracts International
Subjects: Geotechnology, Engineering, Civil engineering
Keywords: Bridge, Centrifuge modeling, Displacement-based design, Earthquake, Numerical analysis, Rocking foundation
Publication Number: 3511714
ISBN: 978-1-267-39834-5
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