Dynamic Response of Bridges to Near-Fault Forward Directivity Ground Motions
Author: Rodriguez-Marek, Adrian | Size: 2.82 MB | Format:PDF | Quality:Original preprint | Publisher: Washington State Transportation Center | Year: 2008 | pages: 84
Research over the last decade has shown that pulse-type earthquake ground motions that result from forward-directivity (FD) effects can result in significant damage to structures. Three typical post-1990 Washington State Department of Transportation (WSDOT) monolithic concrete bridges were chosen to investigate their nonlinear response to FD ground motions (FDGMs) and non-FDGMs. Results showed that significant seismic damage may occur if the structural response is in tune with the period of the velocity pulse of the FDGM. This velocity pulse is a result of fault propagation effects in the near-fault, and occurs when the direction of slip and rupture propagation coincide. The period of the velocity pulse is proportional to the magnitude of the earthquake. The severity of the demand is controlled by the ratio of the pulse period to bridge fundamental periods. As a consequence of this, damage in a bridge with moderate periods (T=0.1s to 1.0s) may be more significant in smaller magnitude earthquakes where the pulse period is closer to the fundamental period of the structure. This was the case for both the MDOF and SDOF analyses of all three bridges in this research. The results showed also that the occurrence of high PGA and/or PGV is only one of several conditions that can cause high demand on the bridges. Of the three bridges considered, all typical concrete overpasses ranging from 50 m to 91 m in length, all generally survived the earthquake motions with only minor damage to their columns. However, column flexural failure was predicted for one model when subjected to two of the forward directivity ground motions. SDOF bridge models for preliminary analyses were found to yield slightly unconservative base shears and displacements compared to that of the full bridge models under non-FDGM. For FDGM, the results of a simple SDOF bridge model ranged from very conservative to slightly unconservative. Therefore, nonlinear SDOF analyses are specifically not recommended in the case of FDGM since the results were not consistent. A more detailed MDOF model should be used to assess bridge seismic performance so that SSI and the interaction of the longitudinal and transverse responses of the bridges can be included, particularly if a performance based design or assessment of the bridge is required.
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Development of Guidelines for Incorporation of Vertical Ground Motion Effects in Seismic Design of Highway Bridges
Author: Kunnath, Sashi K | Size: 2.72 MB | Format:PDF | Quality:Original preprint | Publisher: California Department of Transportation | Year: 2008 | pages: 108
This report describes a study which was conducted in order to assess the current provisions in the California Department of Transportation's Standard Design Criteria 2006 (SDC-2006) for incorporating vertical effects of ground motions in seismic evaluation and design of ordinary highway bridges. A series of simulations was carried out on a range of typical bridge configurations for the purpose of isolating the effects of vertical motions. Results from the simulations reveal that vertical ground motions can have a significant effect on the axial force demand in columns, moment demands at the face of the bent cap, and moment demands at the middle of the span.
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Bridge abutments are designed to provide resistance to deformation and earthquake-induced inertial forces from the bridge superstructure. The passive earth pressure of the abutments' structure backfill is an integral part of the force-resistance mechanism of bridge abutments in the longitudinal direction. Current design practices by the California Department of Transportation (Caltrans) do not take into account the structure backfill properties of bridge abutments. This report describes an experimental and analytical research program that investigated the role that soil properties, abutment geometry, and structure backfill have on the ultimate capacity and stiffness of bridge abutments. Specifically, it examined the effects of structure backfill properties, area of structure backfill, backfill height, and vertical wall movement. In addition, the report evaluates the current design procedures by Caltrans, and also proposes an improved soil spring model for predicting the stiffness and capacity of bridge abutments in longitudinal direction for cases where post-peak softening behavior is important in system modeling efforts.
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A Damage Identification Procedure for Bridge Structures with Energy Dissipation Devices
Author: Bozorgzadeh, Azadeh | Size: 10.15 MB | Format:PDF | Quality:Original preprint | Publisher: University of California, San Diego | Year: 2008 | pages: 211
Bridge abutments are designed to provide resistance to deformation and earthquake-induced inertial forces from the bridge superstructure. The passive earth pressure of the abutments' structure backfill is an integral part of the force-resistance mechanism of bridge abutments in the longitudinal direction. Current design practices by the California Department of Transportation (Caltrans) do not take into account the structure backfill properties of bridge abutments. This report describes an experimental and analytical research program that investigated the role that soil properties, abutment geometry, and structure backfill have on the ultimate capacity and stiffness of bridge abutments. Specifically, it examined the effects of structure backfill properties, area of structure backfill, backfill height, and vertical wall movement. In addition, the report evaluates the current design procedures by Caltrans, and also proposes an improved soil spring model for predicting the stiffness and capacity of bridge abutments in longitudinal direction for cases where post-peak softening behavior is important in system modeling efforts.
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A Damage Identification Procedure for Bridge Structures with Energy Dissipation Devices
Author: Benzoni, Gianmario University of California, San Diego Amaddeo, Carmen DiCesare, Antonio Palermo, Gene | Size: 6.38 MB | Format:PDF | Quality:Original preprint | Publisher: University of California, San Diego | Year: 2008 | pages: 143
This report describes research which focused on developing an effective health monitoring approach for applications to bridges that are protected with the most common seismic response modification devices (SRMD). Emphasis is on those devices tested extensively in the full-scale range of dimension, displacement, velocity and applied load. Devices considered in the project included viscous dampers or energy dissipators. The research included the following tasks: 1) parametric analysis of the effects of damper characteristics; 2) laboratory test on a full scale viscous damper to artificially introduce increasing levels of response degradation; 3) selection of an existing methodology for the assessing existing bridge conditions; 4) validation of the modified methodology using finite element models of an existing isolated bridge; and, 5) validation of the overall procedure with records from a bridge with energy dissipators in new and damaged conditions.
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In this study the post-earthquake residual displacements of reinforced concrete bridge bents were investigated. The system had mild steel that was intended to dissipate energy and an unbonded, post-tensioned tendon that was supposed to remain elastic and re-center the column. The columns tested had different mild steel to prestress ratios, which affected their re-centering ability. A re-centering ratio developed by Hieber (2005), which took into account the external axial load, initial prestress force, and the mild steel ratio, was used to predict these re-centering capabilities. Two 40 percent scale specimens with large-bar connection details and a central unbonded, post-tensioned tendon were tested by using pseudo-static loading. The large-bar system is a rapidly constructible precast system for use in seismic regions. The test columns had re-centering ratios of 1.6 and 1.2. A column with the same connection details but no prestress and a re-centering ratio of 0.9 was used as reference. The displacement at zero force in the test was used as a proxy for the residual displacement after an earthquake. The tests showed that columns with a larger re-centering ratio did experience lower residual drifts, although this distinction only became clear for drift ratios that exceeded 2 percent. The tests also showed that increases in post-tensioning force led to slight increases in damage at high drift ratios.
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Author: Pang, Jason B K | Size: 7.77 MB | Format:PDF | Quality:Original preprint | Publisher: Washington State Transportation Center | Year: 2008 | pages: 255
The use of precast components in bridge bents can accelerate bridge construction, but their use in seismic systems is challenging. Such systems must have connections that are both easy to assemble on site, and have sufficient strength and ductility during earthquakes. A precast bridge bent beam-column connection that is suitable for rapid construction in seismic regions has been developed and tested. The connection features a small number of large (#18) vertical column bars grouted into large corrugated ducts embedded in the cap-beam. This combination provides speed and simplicity of erection, as well as generous construction tolerances. Lateral-load tests on the system showed that it has strength and ductility similar to those of a comparable cast-in-place connection, and that deliberate debonding of a short length of the bars has little effect on its seismic performance.
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Visual Inspection & Capacity Assessment of Earthquake Damaged Reinforced Concrete Bridge Elements
Author: Veletzos, Marc J | Size: 34.19 MB | Format:PDF | Quality:Original preprint | Publisher: University of California, San Diego | Year: 2008 | pages: 350
Caltrans geotechnical engineers initiated a research project aimed at broadening their perspective from simple geotechnical site response analyses to a more comprehensive seismological approach. The project was centered on a series of seminars on seismological theory and analyses using a pair of stochastic numerical ground motion models that allowed uniform treatment of uncertainties in recognized earthquake source, path, and site effects. The project was not intended to produce a “report,” but seminar notes and a portion of the analyses have been scanned and compiled for archival and educational value. Seven sets of seminar notes and two application examples are presented. Two seminars provide an overview of site specific specification of ground motion from a seismological perspective. Another seminar provides background on seismological instrumentation and processing of strong-motion recordings. A pair of seminars addresses empirical attenuation models and outlines the variety of numerical ground motion modeling approaches. The final pair of seminars systematically explore source, path and site effects on ground motion and various strategies employed to capture these effects for purposes of prediction. The two application examples use the stochastic model to explore the impacts and uncertainties of geotechnical site effects within the context of the broader seismological problem.
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Evaluation and Implementation of an Improved Methodology for Earthquake Ground Response Analysis: Uniform Treatment of Uncertainties in Source, Path and Site Effects
Author: California Department of Transportation | Size: 18.53 MB | Format:PDF | Quality:Original preprint | Publisher: California Department of Transportation | Year: 2008 | pages: 552
Caltrans geotechnical engineers initiated a research project aimed at broadening their perspective from simple geotechnical site response analyses to a more comprehensive seismological approach. The project was centered on a series of seminars on seismological theory and analyses using a pair of stochastic numerical ground motion models that allowed uniform treatment of uncertainties in recognized earthquake source, path, and site effects. The project was not intended to produce a “report,” but seminar notes and a portion of the analyses have been scanned and compiled for archival and educational value. Seven sets of seminar notes and two application examples are presented. Two seminars provide an overview of site specific specification of ground motion from a seismological perspective. Another seminar provides background on seismological instrumentation and processing of strong-motion recordings. A pair of seminars addresses empirical attenuation models and outlines the variety of numerical ground motion modeling approaches. The final pair of seminars systematically explore source, path and site effects on ground motion and various strategies employed to capture these effects for purposes of prediction. The two application examples use the stochastic model to explore the impacts and uncertainties of geotechnical site effects within the context of the broader seismological problem.
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A pre-cast concrete bridge bent designed to re-center after an earthquake
Author: Cohagen, L Pang, J B Eberhard, M O Stanton, J F | Size: 2.97 MB | Format:PDF | Quality:Original preprint | Publisher: Washington State Department of Transportation | Year: 2008 | pages: 105
In this study the post-earthquake residual displacements of reinforced concrete bridge bents were investigated. The system had mild steel that was intended to dissipate energy and an unbonded, post-tensioned tendon that was supposed to remain elastic and re-center the column. The columns tested had different mild steel to prestress ratios, which affected their re-centering ability. Two 40 percent scale specimens with large-bar connection details and a central unbonded, post-tensioned tendon were tested by using pseudo-static loading. The large-bar system is a rapidly constructible precast system for use in seismic regions. The test columns had re-centering ratios of 1.6 and 1.2. A column with the same connection details but no prestress and a re-centering ratio of 0.9 was used as a reference. The displacement at zero force in the test was used as a proxy for the residual displacement after an earthquake. The tests showed that columns with a larger re-centering ratio did experience lower residual drifts, although this distinction only became clear for drift ratios that exceeded 2 percent. The tests also showed that increases in post-tensioning force led to slight increases in damage at high drift ratios.
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