Bridges In California Research Articles

Effects of Pounding and Skewness on Seismic Responses of Typical Multispan Highway Bridges Using the Fragility Function Method

This paper uses the fragility function method to study the effects of pounding and skewness on the seismic behaviors of typical multispan RC highway bridges. Three-dimensional numerical models are built for typical three-span bridges in California that include nonlinear pier columns, distributed gap elements simulating pounding at both expansion joints and end abutments, and soil-structure interaction effects. The spatial differences of seismic excitations at various supports caused by varied soil profile properties and wave propagation effects, etc., are also included. As indicated by the nonlinear time history analyses using a large set of three-component earthquake ground motion records, fragility functions are derived and compared for bridges with various structural details, including the location of gaps, deck skew angle, and gap size. Although pounding causes significant local damage, the fragility functions show relatively moderate change in damage probability of the entire bridge. When pounding does not happen, skewed bridges outperform straight ones because of their coupled responses. For straight bridges, the pounding has a limited effect on bridge level damage and can be negligible. However, pounding results in increased damages of skewed bridges which aggravate with large skew angles. Using fragility functions, this paper clarifies the interactive roles of skewness and pounding for seismic damage probability of multispan highway bridges. These findings can provide valuable guidance for future bridge design.

Acoustic Emission Method for Real-Time Detection of Steel Fatigue Crack in Eyebar

After the discovery of a significant crack in an eyebar (fracture-critical element) on the San Francisco–Oakland Bay Bridge in California, the California Department of Transportation (Caltrans) explored possible remote monitor solutions to provide the greatest possible safeguards for the some 200,000 vehicles that used the bridge daily. Caltrans selected an acoustic emission (AE) monitoring system that allowed the detection and localization of crack initiation and growth in real time. The AE method relies on the propagation of elastic waves released by a sudden stress–strain change at the crack tip. The challenge of the AE method in a field application is the disturbance of the data by extraneous noise sources. A robust damage detection algorithm is required to differentiate relevant damage data from secondary noise sources, such as friction. Caltrans required that the proposed monitoring solution be validated through laboratory testing. This paper presents the full-scale laboratory testing of two eyebars loaded under constant amplitude fatigue to detect and locate crack growth under intense friction-type noise sources with use of the AE method. Verification is presented of AE results with ultrasonics testing, dye penetrant, and visual methods to detect damage. Linear location accuracy with simulated signal sources is discussed. The pattern recognition algorithm to differentiate noise signals and crack growth signals is presented. On the basis of this study, the AE method will be used to continuously monitor 384 eyebars with 640 AE sensors on the San Francisco–Oakland Bay Bridge.

Traffic Impacts of Variable Pricing on the San Francisco–Oakland Bay Bridge, California

The raising of bridge tolls in the peak period on the San Francisco-Oakland Bay Bridge in California in mid-2010 provides a rare opportunity to assess traffic impacts. Carpoolers who previously traveled for free during peak hours were charged an electronic toll under this variable pricing scheme. On the basis of 29 months of time series data, the introduction of carpool charges had a stronger impact on traffic volumes than did peak period pricing of regular traffic. The estimated short-term elasticity of about −0.30 suggests significant numbers of peak hour carpoolers did not travel, switched routes, shifted to public transit, or opted to drive alone. More than half the loss in carpool traffic was estimated to be attributable to the toll increase, a far stronger influence than factors such as rising gasoline prices and unemployment. The estimated elasticity of regular traffic in response to variable pricing was −0.23, an indication that peak period motorists were fairly insensitive to pricing and a reflection of the nondiscretionary nature of many peak hour journeys. It will be important to track trends over time to gain insights into the longer term sensitivity of motorists to variable pricing on natural corridors such as the San Francisco–Oakland Bay Bridge.

Vector-Valued Probabilistic Seismic Hazard Assessment for the Effects of Vertical Ground Motions on the Seismic Response of Highway Bridges

The vertical ground motion component is disregarded in the design of ordinary highway bridges in California, except for the bridges located in high seismic zones (sites with design horizontal peak ground acceleration greater than 0.6 g). The influence of vertical ground motion on the seismic response of single-bent, two-span highway bridges designed according to Caltrans Seismic Design Code (SDC-2006) is evaluated. A probabilistic seismic hazard framework is used to address the probability of exceeding the elastic capacity for various structural parameters when the vertical component is included. Negative mid-span moment demand is found to be the structural parameter that is most sensitive to vertical accelerations.A series of hazard curves for negative mid-span moment are developed for a suite of sites in Northern California. The annual probability of exceeding the elastic capacity of the negative mid-span moment is as large as 0.01 for the sites close to active faults when the vertical component is included. Simplified approaches based on the distance to major faults or the median design peak acceleration show that there is a large chance (0.4 to 0.65) of exceeding the elastic limit if the current 0.6 g threshold is used for the consideration of vertical ground motions for ordinary highway bridges. The results of this study provide the technical basis for consideration of a revision of the 0.6 g threshold.

Seismic Performance of Benchmark Highway Bridge with Variable Friction Pendulum System

Dynamic response of a benchmark highway bridge isolated with variable friction pendulum system (VFPS) is investigated under six earthquake motions. The study is based on the finite-element model of the benchmark highway bridge in California. The seismic response of bridge isolated with VFPS is compared with the conventional friction pendulum system (FPS). A parametric study is carried out to study the effects of the maximum coefficient of friction, initial time period and isolation period of VFPS. The response of bridge is also compared with the corresponding uncontrolled case, with FPS, and that controlled by alternate sample control strategies. It is concluded that with the installation of VFPS, the seismic response of the bridge under near-fault motions can be controlled significantly. The VFPS is quite effective in reducing the peak response quantities of the bridge to a level comparable to / better than that of the sample controllers.

Shear wave velocity as function of standard penetration test resistance and vertical effective stress at California bridge sites

Shear wave velocity, V s , is defined as a statistical function of SPT blow count, N 60, and vertical effective stress, σ ′ v , using a data set collected at various California bridge sites. At each site, V s measurements were recorded by suspension logging in the same borehole in which N 60 was measured. Regression analysis was used to derive statistical relations for sand, silt, and clay soil types. The relation between V s and N 60 is shown to depend strongly on σ ′ v since V s and N 60 normalize differently with overburden, which has been mostly omitted in previously published correlations. A random effects regression model is used to separate the error into intra- and inter-boring terms. Inter-boring errors are shown to depend weakly on geologic age. The average shear wave velocity in the upper 30 m, V s30 , is computed directly from the suspension logs and compared with V s 30 ¯ computed from the statistical relations. The relations are shown to provide unbiased estimates of V s30 , with standard deviation of the error equal to the standard deviation of the inter-boring error term. Ground motion prediction equations require V s30 as an input parameter, and the statistical relations may be useful for estimating V s30 at sites where only penetration resistance data are available. The proposed relations should not substitute for more accurate geophysical measurements when predicted ground motions are sensitive to the uncertainty in V s30 , but may be useful for identifying whether geophysical measurements should be performed to better refine the V s30 estimate.

Seismic Retrofit of California's Auburn–Foresthill Bridge

The Auburn–Foresthill Bridge in California was built by the Bureau of Reclamation in 1973. The 2,428-ft-long bridge is made up of three spans—639 ft, 862 ft, 639 ft—and sits 730 ft above the north fork of the American River; it would have spanned the reservoir created by the proposed, but never constructed, Auburn Dam. The superstructure is a parabolic haunched deck truss bridge that has fracture-critical, high-strength steel (100 ksi) main members. As part of the Local Agency Seismic Retrofit Program, Placer County, California, embarked on the seismic retrofit design of this bridge. This project expanded on the concepts and design methodologies that were developed during the retrofits of the California Toll Bridge Program. Strain-based design criteria were developed for all concrete and steel members of the bridge. The project-specific design criteria may serve as the “next generation” for complex steel seismic retrofit projects. The time history model incorporated material and geometric nonlinearity components and was capable of capturing the global buckling of steel members and reporting member strains. The gusset plates were evaluated to ensure that any ductile behavior was limited to the members themselves and not the connections. The project incorporated the use of large buckling restrained braces (BRBs) to improve the performance of the bridge during large seismic events. BRBs are simple, low-maintenance devices that dissipate energy and improve the seismic behavior of steel structures. This paper summarizes the innovative design concepts and their application throughout the project.

Comparison of seismic performance of three restrainers for multiple-span bridges using fragility analysis

Steel restrainer cables for multiple-frame bridges in California in the United States showed the effectiveness to prevent the unseating at the internal hinges during the past several earthquakes. After that, the steel restrainer cables are being tried to apply for multiple-span-simply-supported (MSSS) bridges in the central and the southeastern regions in the United States. In addition, shape memory alloy (SMA) bars in tension are being studied for the same application. In multiple-frame bridges, the developed seismic forces are transferred to piers through the restrainers; however, in MSSS bridges, the seismic forces are transferred to abutments by the restrainers. Therefore, the abutment’s behavior should be investigated as well. This study assesses the seismic performance of the three types of restrainers—steel restrainer cables, SMA bars in tension, and SMA bars in bending for MSSS bridges—due to moderate to strong ground motions using a deterministic seismic analysis. Also, a fragility analysis is conduced to assess the seismic resistance for the overall bridge system. For these analyses, the bending test of an SMA bar is conducted and its analytical model is determined. Then, nonlinear time history analyses are conducted to assess the seismic responses of the as-built and the retrofitted bridges. The deterministic analysis illustrates how the restrainers influence the components of the bridge. However, it could not explain the effects on the whole bridge system. The fragility analysis shows the effects of the three restrainers on the overall bridge system. The fragility analysis indicates that the SMA bars in bending are the most effective ones.

Health monitoring of a bridge system using strong motion data

In this paper, the acceptability of system identification results for health monitoring of instrumented bridges is addressed. This is conducted by comparing the confidence intervals of identified modal parameters for a bridge in California, namely Truckee I80/Truckee river bridge, with the change of these parameters caused by several damage scenarios. A challenge to the accuracy of the identified modal parameters involves consequences regarding the damage detection and health monitoring, as some of the identified modal information is essentially not useable for acquiring a reliable damage diagnosis of the bridge system. Use of strong motion data has limitations that should not be ignored. The results and conclusions underline these limitations while presenting the opportunities offered by system identification using strong motion data for better understanding and monitoring the health of bridge systems.

Influence of vertical earthquake motion and pre-earthquake stress on joint response of precast concrete segmental bridges

This paper investigates the seismic response of precast concrete segmental bridges using detailed, 2-D nonlinear time-history analyses and focuses on the behavior of segment-to-segment joints constructed using the balanced-cantilever method. Analytical models of full-scale precast concrete segmental bridges with geometries and characteristics similar to the Otay River Bridge in California were used in this study. 20 near-field earthquake records were used to determine the median joint response as well as to quantify the effect of vertical motion on the joint response. Four different pre-earthquake stress conditions were studied to determine whether the effects of creep, shrinkage, and temperature affect the seismic response of segment joints. Results indicated that vertical earthquake motions and the pre-earthquake stress state can significantly alter the response of segment joints.

Lateral Performance of Full-Scale Bridge Abutment Wall with Granular Backfill

Bridge abutments typically contain a backwall element that is designed to break free of its base support when struck by a bridge deck during an earthquake event and push into the abutment backfill soils. Results are presented for a full-scale cyclic lateral load test of an abutment backwall configured to represent the dimensions ( 1.7 m height), boundary conditions, and backfill materials (compacted silty sand) that are typical of California bridge design practice. An innovative loading system was utilized that operates under displacement control and that assures horizontal wall displacement with minimal vertical displacement. The applied horizontal displacement ranged from null to approximately 11% of the wall height (0.11H) . The maximum earth pressure occurred at a wall displacement of 0.03H and corresponded to a passive earth pressure coefficient of Kp =16.3 . The measured force distribution applied to the wall from hydraulic actuators allowed the soil pressure distribution to be inferred as triangula...

Effect of abutment modeling on the seismic response of bridge structures

Abutment behavior significantly influences the seismic response of certain bridge structures. Specifically in the case of short bridges with relatively stiff superstructures typical of highway overpasses, embankment mobilization and inelastic behavior of the soil material under high shear deformation levels dominate the response of the bridge and its column bents. This paper investigates the sensitivity of bridge seismic response with respect to three different abutment modeling approaches. The abutment modeling approaches are based on three increasing levels of complexity that attempt to capture the critical components and modes of abutment response without the need to generate continuum models of the embankment, approach, and abutment foundations. Six existing reinforced concrete bridge structures, typical of Ordinary Bridges in California, are selected for the analysis. Nonlinear models of the bridges are developed in OpenSees. Three abutment model types of increasing complexity are developed for each bridge, denoted as roller, simplified, and spring abutments. The roller model contains only single-point constraints. The spring model contains discrete representations of backfill, bearing pad, shear key, and back wall behavior. The simplified model is a compromise between the efficient roller model and the comprehensive spring model. Modal, pushover, and nonlinear dynamic time history analyses are conducted for the six bridges using the three abutment models for each bridge. Comparisons of the analysis results show major differences in mode shapes and periods, ultimate base shear strength, as well as peak displacements of the column top obtained due to dynamic excitation. The adequacy of the three abutment models used in the study to realistically represent all major resistance mechanisms and components of the abutments, including an accurate estimation of their mass, stiffness, and nonlinear hysteretic behavior, is evaluated. Recommendations for abutment modeling are made.

Effects of structural characterizations on fragility functions of bridges subject to seismic shaking and lateral spreading

This paper evaluates the seismic vulnerability of different classes of typical bridges in California when subjected to seismic shaking or liquefaction-induced lateral spreading. The detailed structural configurations in terms of superstructure type, connection, continuity at support and foundation type, etc. render different damage resistant capability. Six classes of bridges are established based on their anticipated failure mechanisms under earthquake shaking. The numerical models that are capable of simulating the complex soil-structure interaction effects, nonlinear behavior of columns and connections are developed for each bridge class. The dynamic responses are obtained using nonlinear time history analyses for a suite of 250 earthquake motions with increasing intensity. An equivalent static analysis procedure is also implemented to evaluate the vulnerability of the bridges when subjected to liquefaction-induced lateral spreading. Fragility functions for each bridge class are derived and compared for both seismic shaking (based on nonlinear dynamic analyses) and lateral spreading (based on equivalent static analyses) for different performance states. The study finds that the fragility functions due to either ground shaking or lateral spreading show significant correlation with the structural characterizations, but differences emerge for ground shaking and lateral spreading conditions. Structural properties that will mostly affect the bridges’ damage resistant capacity are also identified.

“Cooking the Books”—Behavior-Based Safety at the San Francisco Bay Bridge

Practitioners of Behavior-Based Safety (BBS) claim dramatic reductions in worker injuries and illnesses through modifying workers' "unsafe behaviors." This case study of a BBS program implemented by KFM, a giant construction consortium rebuilding the eastern span of the San Francisco Bay Bridge in California, documents how BBS was used to suppress reporting of worker injuries and illnesses on site. The key elements of KFM's BBS "injury prevention" strategy included: 1) cash incentives to workers and supervisors who do not report injuries; 2) reprisals and threats of reprisals against those employees who do report injuries; 3) selection and use of employer friendly occupational health clinics and workers compensation insurance administrators; 4) strict limits on the activities of contract industrial hygiene consultants; and 5) a secretive management committee that decides whether reported injuries and illnesses are legitimate and recordable. KFM reported injury and illness rates 55% to 72% lower than other bridge builders in the Bay Area, but the California Division of Occupational Safety and Health (Cal/OSHA) issued Willful citations to the consortium in June 2006 for failing to record 13 worker injuries on its "OSHA Log 300," as required by law.

R-Factor Parameterized Bridge Damage Fragility Curves

Damage fragilities describe the probability that a bridge will incur certain (discrete) damage states conditioned on the intensity of the earthquake it may experience. Reinforced concrete box girder highway overpass bridges are prevalent among the total inventory of bridges in California. For this class of bridges, a method for computing damage fragilities for three damage states (concrete cover spalling, longitudinal bar buckling, and column failure) based on the bridge force reduction factors ( R -factors) is derived in this paper. Bridge damage fragilities are described by equations relating the median intensity and uncertainty of ground motion to discrete damage states of column concrete cover spalling, column bar buckling, and column failure using the bridge R -factor as the principal parameter describing the bridge structure. Such damage fragility equations are furnished for earthquake intensities measured using pseudospectral acceleration (Sa) and cumulative absolute displacement (CAD) in both the ...

Performance‐based seismic bridge design for damage and loss limit states

AbstractRecent efforts in the earthquake engineering community have focused largely on developing a performance‐based earthquake engineering assessment methodology. However, equally important is applying such a methodology to the design of structures. Probabilistic performance‐based seismic design enables design and acceptance criteria to be specified by a continuum of limit states and incorporates uncertainty in all the sources of data. This paper introduces a non‐iterative method, based on the Pacific Earthquake Engineering Research Center framework, for design of bridge structures in terms of single or multiple physical design parameters, such as column height and diameter. Design parameter solutions are obtained from explicit consideration of uncertainty in the hazard, demand, damage, and loss to the structure by way of specifying a level of confidence in the resulting design under different performance objectives. Similar to other performance‐based methodologies, performance objectives consist of a performance level and a hazard level. Design equations are derived for performance levels defined in terms of bridge damage and loss. Examples are presented of both univariate and bivariate design parameter cases for typical reinforced concrete highway overpass bridges in California. Copyright © 2007 John Wiley & Sons, Ltd.

Nonlinear Static Procedure for Seismic Vulnerability Assessment of Bridges

Abstract: The impact of an earthquake event on the performance of a highway transportation network depends on the extent of damage sustained by its individual components, particularly bridges. Seismic damageability of bridges expressed in the form of fragility curves can easily be incorporated into the scheme of risk analysis of a highway network under the seismic hazard. In this context, this article focuses on a nonlinear static method of developing fragility curves for a typical type of concrete bridge in California. The method makes use of the capacity spectrum method (CSM) for identification of spectral displacement, which is converted to rotations at bridge column ends. To check the reliability of this current analytical procedure, developed fragility curves are compared with those obtained by nonlinear time history analysis. Results indicate that analytically developed fragility curves obtained from nonlinear static and time history analyses are consistent.

Locating and linking to medical care HIV-positive persons without a history of care: Findings from the California Bridge Project

The California Bridge Project was developed to actively locate out-of-treatment HIV-positive persons and link them to HIV care and treatment services. Three hundred and twenty five clients had never received HIV medical care. Among these clients the average time from HIV diagnosis to contact with project staff was one-and-a-half years. Nearly three-fourth of clients were persons of color and almost half were men who have sex with men (MSM). The project was effective in linking 29.2% of clients to medical care. First contact to linkage required an average of 15.4 client contacts. During these encounters project staff assessed clients' risk behaviors and physical and mental health status and addressed real or perceived barriers to care. Latinos were over two times (adjusted OR=2.33, p=0.034) and African Americans over three times (adjusted OR=3.69, p=0.002) more likely than whites to be linked to medical care. Success in linking persons of color to medical care may have been because the majority of project staff were persons of color who were purposely hired with characteristics similar to those of the target population. The Bridge Project demonstrates the challenges faced in attempting to link HIV-positive persons from underserved and marginalized populations to medical care.

Structural Tests of Precast, Prestressed Concrete Deck Panels for California Freeway Bridges

Full-scale structural tests of five precast, prestressed concrete bridge deck panels have been carried out in simple bending to failure at the San Diego State University Structural Engineering Laboratory in San Diego, Calif. The purpose of the tests was to demonstrate to Caltrans that the precast concrete panels act compositely in flexure with a cast-in-place topping slab so that they can be used for California bridges. To ensure that the full deck width was active in resisting the applied loads, steel I-beams the width of the deck were used under the load and at the reaction points. Neoprene bearing pads were incorporated at the supports to allow free rotations at the deck ends. Each bridge deck consisted of a precast, prestressed concrete deck panel with a cast-in-place topping slab, with no reinforcing bars crossing the interface between slabs. Of particular interest was the verification that no horizontal shear slip occurred between the two slabs and that the deck acted as a fully composite member to failure. One test featured the precast concrete slab by itself, three of the tests investigated the deck behavior in positive bending with different roughening levels applied to the top of the precast concrete slab, and the fifth test was performed in negative bending (upside down). Nonlinear prediction analyses were conducted for each of the tests, including the formation and spreading of flexural cracking along the deck and plastic hinging at the critical section in the center of the span. Test results, supported by detailed structural analyses, demonstrated that there was no horizontal shear slip between the precast concrete panels and cast-in-place slabs, allowing them to respond in flexure as a single composite slab.

Analysis and Design of Critical Bridges Subjected to Blast Loads

Blast-resistant design has traditionally been considered only for essential government buildings, military structures, and petrochemical facilities. Recent terrorist threats to bridges in California and New York have demonstrated the need to evaluate the vulnerability of our transportation infrastructure. Bridge engineers, however, have not typically considered security in the design process, and most of the current state of knowledge of the design of structures subjected to blast effects is based on the performance of buildings rather than bridges. This paper summarizes the results of ongoing research to develop performance-based blast design standards tailored specifically for bridges. Based on best practices obtained from an international literature review, the paper briefly discusses the incorporation of physical security and site layout principles into the design process. It then discusses the potential effects of blast loads on bridges and provides structural design and retrofit solutions to counter these effects.