Experiments, Analysis, Displacement-based Assessment, and Database of Rocking Foundations PDF Download

Author: Andreas Gerasimos Gavras
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ISBN: 9781658413060
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Languages : en
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Book Description
The concept of intentional mobilization of controlled soil inelasticity and foundation uplift, as a rational and economical seismic protection strategy, has much matured as a result of extensive research over the last two decades. This dissertation provides further evidence and tools that contribute toward the implementation of rocking foundations in practice with emphasis on bridges applications. In response to the lack of high-quality experimental data on the seismic response of full-scale rocking foundations, a series of large-scale tests, involving reinforced concrete bridge columns with footings embedded into dense sand, was completed. The experimental program investigated the alignment of the footings to the shaking direction and included varying groundwater table elevations and footing backfill conditions. This study yielded results that corroborate the cumulative centrifuge experiments-based understanding of the dynamic behavior of rocking foundations and confirmed that loose and dry cohesionless backfill soil can ravel under the rocking footing at large rotations, resulting in reduced settlement, reduced stiffness degradation, enhanced energy dissipation, but potentially also causing permanent rotations. A modified beam-on-nonlinear-Winkler-foundation (BNWF) modeling scheme, representing a departure from previous attempts to capture both the vertical and rotational nonlinear responses of a shallow footing, was proposed. The considered scheme is cast around the foundation critical contact area ratio and the trilinear moment-rotation backbone of Deng et al. (2014), and is calibrated to four physical model tests. The calibrated scheme captured successfully the experimental response in terms of moment-rotation envelope, hysteretic energy dissipation and recentering across a range of rotation amplitudes and rocking-induced soil inelasticity. Settlement accumulation was captured for rotation amplitudes less than 0.025 rad but was overestimated at larger rotations. Displacement-based assessment (DBA) guidelines for bridges with rocking foundations were developed. Nonlinear response history analyses of idealized footing-column-mass models, using the calibrated BNWF model, were conducted to determine the fraction of the area-based hysteretic damping that is effective in reducing the seismic displacement demand and evaluate the importance of P-[delta] effects, in refinement of the Deng at al. (2014) study for elastic cantilever bridge columns on rocking foundations. The DBA guidelines were subsequently extended to bridge bents that also include plastic hinging at the top of the column. The bent-level equivalent-linear properties are derived, considering independent foundation-rocking and column-hinging subsystems and constant location for the contraflexure point based on their inelastic capacities. Comparison with a detailed nonlinear pushover analysis-based procedure for representative cases showed that the simplified procedure is practical and sufficient for estimating the total displacement demand, but less accurate at the component level. Lastly, provisions for multi-span bridges that follow a system-level approach for the longitudinal direction and an isolated-bent approach with modified mass and stiffness characteristics for the transverse direction were presented. The guidelines were applied to hypothetically redesign two existing bridges with rocking foundations. Numerical analyses of the full bridges demonstrated the accuracy of the procedure, except for the transverse response of bridges with high strength and energy dissipation at the abutments. Finally, a new rocking shallow foundation performance database, containing dynamic experimental results from five centrifuge and three 1g shaking table test series, was created. Its usefulness was illustrated through example correlations between the peak drift ratio demand of the structures and selected ground motion intensity measures. The fundamental value of the database is that it allows easy access to summary information and derived time series data that can be used to obtain empirical correlations, validate numerical models and design guidelines, and identify future experimental priorities.