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Fountain Valley, CA, United States

Taiebat M.,University of British Columbia | Jeremic B.,University of California at Davis | Dafalias Y.F.,University of California at Davis | Dafalias Y.F.,National Technical University of Athens | And 2 more authors.
Soil Dynamics and Earthquake Engineering

To predict the earthquake response of saturated porous media it is essential to correctly simulate the generation, redistribution, and dissipation of excess pore water pressure during and after earthquake shaking. To this end, a reliable numerical tool requires a dynamic, fully coupled formulation for solid-fluid interaction and a versatile constitutive model. Presented in this paper is a 3D finite element framework that has been developed and utilized for this purpose. The framework employs fully coupled dynamic field equations with a u-p-U formulation for simulation of pore fluid and solid skeleton interaction and a SANISAND constitutive model for response of solid skeleton. After a detailed verification and validation of the formulation and implementation of the developed numerical tool, it is employed in the seismic response of saturated porous media. The study includes examination of the mechanism of propagation of the earthquake-induced shear waves and liquefaction phenomenon in uniform and layered profiles of saturated sand deposits. © 2009 Elsevier Ltd. All rights reserved. Source

Shamsabadi A.,Office of Earthquake Engineering | Kapuskar M.,Earth Mechanics Inc.
Transportation Research Record

This paper investigates the behavior of skewed highway bridges subjected to earthquake loading with strong velocity pulses. The behavior of bridges with skewed abutments in the longitudinal direction is strongly coupled by transverse loading. The interaction between skewed bridge abutment foundations and backfill has a strong impact on dynamic bridge response. While bridge structures may remain in the linear range during seismic loading, local nonlinear behavior at the abutment-embankment interface can cause significant nonlinear bridge structure response. Skewed bridge models are presented incorporating soil-abutment-structure interaction using nonlinear abutment springs. As a case study, a global three-dimensional nonlinear finite element model of the skewed and seismically instrumented Painter Street overpass in Rio Dell, California, with monolithic abutments was developed. The model used nonlinear foundation-soil interaction based on approach soil properties from geotechnical tests. A soil continuum finite element analysis was performed using constitutive hardening soil material to evaluate abutment backfill passive resistance considering backwall skew. The bridge response resulting from seismic ground motion records was compared with structure response data. The computer models represented fairly well the overall seismic response of the skewed bridge. Bent pile foundations had much less impact on overall bridge response than the abutments. Near-fault ground motions with high-velocity pulses generated asymmetrical impulsive loading and large displacements in transverse directions leading to significant rotation and residual deck displacement. These permanent structure displacements can control the design of the abutment seat width and shear keys and could exceed column displacement capacity and impose additional moment at the column not considered in current design. Source

Sritharan S.,Iowa State University | Cox A.-M.,Raker Rhodes Engineering | Huang J.,Shanghai Xuhui Land Development Co. | Suleiman M.,Lehigh University | Arulmoli K.,Earth Mechanics Inc.
PCI Journal

The design of prestressed concrete piles in seismic regions is required to include confinement reinforcement in potential plastic hinge regions. However, the existing requirements for quantifying this reinforcement vary significantly, often resulting in unconstructible details. This paper presents a rational approach for designing minimum confinement reinforcement for prestressed concrete piles in seismic regions. By varying key variables, such as the concrete strength, prestressing force, and axial load, the spiral reinforcement quantified according to the proposed approach provides a minimum curvature ductility capacity of about 18, while the resulting ultimate curvature is 28% greater than an estimated target curvature for seismic design. This paper also presents a new axial load limit for prestressed piles, an integrated framework for seismic design of piles and superstructure, the dependency of pile displacement capacity on surrounding soils, and how further reduction to confinement reinforcement could be achieved, especially in medium to soft soils and in moderate to low seismic regions. Source

Law H.,Earth Mechanics Inc. | Wilson P.,Earth Mechanics Inc.
NCEE 2014 - 10th U.S. National Conference on Earthquake Engineering: Frontiers of Earthquake Engineering

Site-specific seismic coefficients are developed for use in pseudo-static analyses of tall walls and slopes for the Fourth Bore Caldecott Tunnel project in Oakland, CA. The site is located about 1.9 km from the Hayward fault, with expected peak ground acceleration for the 1500-year return period design earthquake in excess of 1 g. Due to the unusually tall walls and high ground acceleration, seismic coefficients are developed based on a rational approach incorporating effects of wavelength and wall height as well as coherency of spatially varying ground motions. Two-dimensional site response analyses are conducted for a suite of ground motion time series in order to develop average horizontal acceleration time series for the potential sliding mass over a range of wall/slope heights. The peak accelerations from those time series are found to decrease with increasing wall/slope heights. This is attributed mainly to reduction in the high frequency content of the overall sliding mass ground motions. Furthermore, the use of peak acceleration values is considered conservative in the context of a pseudo-static limit equilibrium approach, because the seismic coefficient is traditionally applied as a constant force on the potential sliding mass. In reality, the actual peak acceleration only occurs for a brief instant. In order to avoid this undue conservatism, a Newmark sliding block approach was implemented to determine yield acceleration values which would limit the predicted displacement levels to a certain threshold value (about 10 cm) which suggests overall wall/slope stability. After incorporating the above siteresponse effects as well as the Newmark deformations, seismic coefficients of 25% to 33% of the PGA were identified for use in pseudo-static analyses for design of wall/slope reinforcements. Source

Wilson P.,Earth Mechanics Inc. | Elgamal A.,University of California at San Diego
Journal of Geotechnical and Geoenvironmental Engineering

Passive earth pressure is recorded in two different tests, using a 6.7-m long, 2.9-m wide soil container. In these tests, sand with 7% silt content is densely compacted behind a moveable test wall to a supported height of 1.68 m (5.5 ft). Lateral load is applied to the vertical reinforced concrete wall section, which displaces freely along with the adjacent backfill in the horizontal and vertical directions. The recorded passive resistance is found to increase until a peak is reached at a horizontal displacement of 2.7-3% of the supported backfill height, decreasing thereafter to a residual level. In this test configuration, a triangular failure wedge shape is observed, due to the low mobilized wall-soil friction. Backfill strength parameters are estimated based on this observed failure mechanism. From these estimates, along with triaxial and direct shear test data, theoretical predictions are compared with the measured passive resistance. Using the test data, a calibrated finite-element model is employed to produce additional load-displacement curves for a wider range of practical applications (e.g., potential bridge deck displacement during a strong earthquake). Hyperbolic model approximations of the load-displacement curves are also provided. © 2010 ASCE. Source

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