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Practical Lessons from the Loma Prieta Earthquake (1994)

Chapter: 6. Highway Bridges

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Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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6
Highway Bridges

James E. Roberts

INTRODUCTION

The California State Department of Transportation (CALTRANS) owns and maintains over 12,000 bridges with spans over 20 feet. There are an equal number in the city and county systems. CALTRANS maintains the condition data for all of these and some 6,000 other highway structures such as culverts (with spans under 20 feet), pumping plants, tunnels, tubes, highway patrol inspection facilities, maintenance stations, toll plazas, and other transportation-related structures. Structural details and the current condition data are maintained in the Department Bridge Maintenance files as part of the National Bridge Inventory System required by Congress and administered by the Federal Highway Administration.

These data are updated and submitted annually to the Federal Highway Administration and are the basis upon which some of the federal gas tax funds are allocated and returned to the states. The maintenance, rehabilitation, and replacement needs for bridges are prorated against the total national needs. Only this year (1993) has seismic retrofitting been accepted as an eligible item for use of federal funds, because it was assumed by most other states to be only a California problem. After much lobbying by CALTRANS, the new Federal Intermodal Surface Transportation Efficiency Act of 1992 provides for seismic retrofit to be eligible for federal bridge funds.

Immediately after the February 9, 1971, San Fernando earthquake, CALTRANS began a comprehensive upgrading of their Bridge Seismic Design Specifications and Construction Details. CALTRANS's bridge design specifications

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

were modified to correct the identified deficiencies for application on new bridge designs. After this work was completed, the Applied Technology Council completed project ATC-6, which became the basis for a similar seismic-design specification for bridges, which was adopted by the American Association of State Highway and Transportation Officials as the national standard in 1983. Existing structures, however, proved to be a substantially more challenging problem. Research was undertaken in the United States and overseas (in New Zealand and Japan) to improve analytical techniques and to provide basic data on the strengths and deformation characteristics of lateral-load resisting systems for bridges. CALTRANS identified the vulnerable elements of existing bridges and began a statewide seismic-retrofit program for bridges to systematically reinforce the older, non-ductile bridges.

The initial phase of the CALTRANS Bridge Seismic Retrofit Program involved installation of hinge and joint restrainers to prevent deck joints from separating. Separation was the major cause of bridge collapse during the San Fernando earthquake and was judged by CALTRANS engineers and other investigators to be the highest risk to the traveling public. Included in this phase was the installation of devices to fasten the superstructure elements to the substructure in order to prevent those superstructure elements from falling off their supports. This phase was essentially completed in 1989 after approximately 1,260 bridges on the state highway system had been retrofitted at a cost of over $55 million. Funding for this program competed with other highway safety programs, which were arguably more critical in terms of statistical support. Consequently, the bridge seismic retrofit program was allocated only $4 million annually.

While the hinge and joint restrainers performed well, shear failure of columns on the I-605/I-5 separation bridge in Los Angeles during the moderate Whittier earthquake of October 1, 1987, reemphasized the inadequacies of pre-1971 column designs. Even though there was no collapse, the extensive damage resulted in plans for basic research into practical methods of retrofitting bridge columns on the existing pre-1971, non-ductile bridges. That research program had already been initiated in early 1987 at the University of California (UC), San Diego, and the Whittier earthquake merely speeded its approval and execution. Funding levels for seismic-retrofit-program implementation were increased fourfold after the Whittier earthquake to an annual level of $16 million. Even at that level, it would require some 100 years to complete the retrofitting program that is currently identified.

The Loma Prieta earthquake of October 17, 1989, again proved the reliability of hinge and joint restrainers, but the tragic loss of life at the Cypress Street Viaduct on I-880 in Oakland emphasized the necessity to immediately accelerate the column-retrofit phase of the seismic-retrofit program for bridges with a higher funding level for both research and implementation. Other structures in the earthquake-affected counties performed well, suffering the expected column

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

damage without collapse. With the exception of a single outrigger column-cap joint confinement detail, those bridges using the post-1972 design specifications and confinement details performed well. Damage to long, multiple-level bridges showed the need to consider more carefully longitudinal resisting systems, because earthquake forces cannot be carried into abutments and approach embankments as they can on shorter bridges. After the Loma Prieta earthquake caused 44 fatalities on the state highway system, capital funding for seismic retrofitting was increased to $300 million per year. At the same time, seismic-research funding for bridges was increased from $0.5 million annually to $5.0 million annually with an initial $8.0 million allocation from the special State Emergency Earthquake Recovery legislation of November 1989, Senate Bill 36X (SB 36X). Using the special research funding provided in SB 36X, the department engaged additional research teams and facilities to assist in this massive program.

Much research has been conducted by both U.S. and foreign researchers into the causes of damage in the Loma Prieta earthquake, and much of that research is contained in the references cited in this paper. Most of the research papers can be obtained from the National Information Service for Earthquake Engineering, Earthquake Engineering Research Center at UC Berkeley. The Earthquake Engineering Research Center, located at the Richmond, California, Field Station, has been designated as the national repository for information on the Loma Prieta earthquake. There are over 175 documents on file at the repository relating to bridge aspects of the Loma Prieta earthquake. Additional research papers and project reports can be obtained from the CALTRANS Division of Structures, Sacramento, California; the Department of Applied Mechanics and Engineering Science, UC San Diego; and the National Center for Earthquake Engineering Research, State University of New York at Buffalo. The National Center for Earthquake Engineering Research has a data base search service known as QUAKLINE.

PERFORMANCE OF PRIOR RESEARCH RESULTS

Much had been learned about bridge performance in previous earthquakes (e.g., the 1971 San Fernando and 1987 Whittier earthquakes), and only budgetary constraints prevented CALTRANS from executing seismic retrofit of older bridges at a more rapid pace. It is important, however, to observe and discuss the performance of the new seismic-design criteria that had been utilized on bridges designed after 1972 and those seismic retrofit devices that had been installed prior to the Loma Prieta event. Hinge-Joint Restraining Devices

As previously stated, the initial phase of CALTRANS' Bridge Seismic Retrofit Program involved installation of hinge and joint restrainers to prevent deck joints from separating. This was identified as the major cause of bridge collapse during the San Fernando earthquake (LeBeau et al., 1971) and in 1972 was

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

judged by CALTRANS engineers to be the highest risk to the traveling public. Included in this phase was the installation of devices to fasten the superstructure elements to the substructure in order to resist vertical accelerations and also to prevent superstructure elements from falling off their supports. This phase was essentially completed in 1989 after approximately 1,260 bridges on the state highway system had been retrofitted at a cost of over $55 million. Research and testing of the restrainers were conducted at UC Los Angeles, by Selna and Malvar (1987). These joint restrainer systems have performed well in subsequent earthquakes, including the 1987 Whittier event (Priestley et al., 1991c), the 1989 Loma Prieta event (Mellon et al., 1993), the 1992 Cape Mendocino event (Yashinsky, 1992a), and the three most recent southern California events of 1992.

In the eight counties that were declared disaster areas after the Loma Prieta earthquake, there are approximately 350 bridges that had been retrofitted with hinge joint restrainers. There was no observed failure of any of these restrainers. CALTRANS staff engineers agree that there would have been collapse of bridge spans due to the spans falling off their supports without the installation of restrainers. Maragakis and Saiidi (1991) of the University of Nevada at Reno and Yashinsky (1990) of the CALTRANS Office of Earthquake Engineering have published papers evaluating the performance of these restrainer details. The University of Nevada at Reno was awarded a CALTRANS research project (Project R-12) to test the performance of hinge and joint-cable restrainers for bridges under dynamic loading.

Properly Confined Column Reinforcement

Most columns designed since 1971 contain a slight increase in the main-column vertical reinforcing steel and a major increase in confinement and shear reinforcing steel over the pre-1971 designs. All new columns, regardless of geometric shape, are reinforced with one or a series of spiral-wound interlocking circular cages. The typical transverse reinforcement detail now consists of #6 (.75-inch-diameter) hoops or continuous spiral at approximately 3-inch pitch over the full column height. This provides approximately eight times the confinement and shear reinforcing steel in columns than what was used in the pre-1972 non-ductile designs. All main-column reinforcing is continuous into the footings and superstructure. Splices are mostly welded or mechanical, both in the main and transverse reinforcing. Transverse-reinforcing steel is designed to produce a ductile column by confining the plastic hinge areas at the top and bottom of columns. The use of grade 60, A 706 reinforcing steel in bridges has recently been specified on a few projects on a trial basis.

In the eight counties declared disaster areas after the Loma Prieta earthquake, there are approximately 800 bridges designed after 1972 using the newly revised seismic-design criteria and confinement details. With the exception of the one outrigger beam-column joint damage on the I-980 southbound connector

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

in Oakland, there was no documented damage to any of these 800 post-1972-designed bridges (Mellon et al., 1993).

Acceleration Response Spectra For Alluvium And Dense Foundation Materials

CALTRANS developed a series of acceleration response spectra for alluvium and average soils after the 1971 San Fernando earthquake, and these spectra were accurate for prediction of the dynamic response of those types of foundation materials. Professor Harry Bolton Seed of UC Berkeley was instrumental in the development of these design spectra. Those bridges situated on average foundation materials and designed using these spectra performed well in the Loma Prieta event.

Base-Isolated Girder Systems

Although only one bridge in the affected counties was base isolated, it did perform well during the Loma Prieta event (Mellon et al., 1993). The bridge was the Sierra Point Overhead, which was designed prior to 1972 for lateral-force requirements of only 0.06 g. It was subjected to lateral forces of approximately 0.18 g during the Loma Prieta earthquake and showed no signs of distress. It should be noted, however, that the CALTRANS design procedure is to force seismic loads into the abutments so the back wall must fail prior to the base-isolation bearings being engaged.

PROBLEMS WITH EXISTING CRITERIA, DETAILS, AND PRACTICE

A discussion of the problems encountered in highway bridge performance during the Loma Prieta earthquake will explain the need for research in the area of structural response in moderate and major earthquakes.

Older Bridges Designed For Pre-1972 Seismic Forces And Design Criteria

The major causes of bridge damage in the Loma Prieta earthquake were the criteria and details for which they were originally designed. There were over 4,000 bridges on the combined state, county, and city systems in the eight counties that were declared disaster areas after the earthquake. Only 100 of those bridges were damaged in the earthquake, and only 25 sustained what can be termed major damage, as reported in the Post Earthquake Investigation Team (PEQIT) Report (Mellon et al., 1993). Only one of the 800 bridges in the counties that had been designed after 1972, using the newer seismic forces and details, suffered damage as described in the PEQIT Report (Mellon et al., 1993).

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

While the Loma Prieta earthquake was, admittedly, a moderate earthquake, the bridge performance was generally what had been expected by bridge designers. Most of the research that has been commissioned since the earthquake is aimed at developing better assurance that bridges will withstand a major earthquake without collapse or major damage and that the transportation system can remain essentially functional after a major seismic event.

Seismic Performance Criteria Required

The Governor's Board of Inquiry hearings brought out the fact that there was no formal documented policy on the required seismic performance of bridges in the CALTRANS Design Specifications and Criteria (Housner, 1990). These specifications are utilized by many other public agencies, and, therefore, it is critical that a formal performance criteria be adopted.

Dynamic Response Of Deep, Soft Foundations

The effect of the dynamic response of deep, soft soils in the structure foundations also proved to be a contributing factor to the collapse of the Cypress Viaduct and must be analyzed and included in future design procedures, especially for long, tall structures with relatively high periods of vibration. The effect of incoherence in the foundation response is also an important factor in the design of very long structures such as the San Francisco-Oakland Bay Bridge and the mile-long freeway viaducts. Mitchell (1992), Bolt (1991, 1992), Der Kiureghian (1991), Der Kiureghian and Neyenhofer (1992), Zafir et al. (1990), and Tamura and Shah (1991) have published research papers on this subject.

Column-Footing Interaction

Investigation of damage at the Cypress Street Viaduct in Oakland subsequent to the Loma Prieta event revealed a deficiency in many pre-1972-designed bridge footings. Some of these footings suffered joint shear failures that caused structure settlement. These footings were typically designed for vertical loads and only a 0.06 g lateral force. Subsequent investigation and research by Seible and Priestley of UC San Diego revealed a potential for failures due to lack of reinforcing steel in the top to resist lateral moments. Their conclusions, based on analysis and tests, did show a need for a top mat of reinforcing steel (Seible et al., 1992a).

Inadequate Column-Confinement Reinforcement

Other than the Cypress Viaduct failure, column damage was limited to a few critical bents on the Embarcadero Freeway Viaduct, the Terminal Separation

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Structure, the Central Freeway Viaduct, and the Southern Freeway Viaduct (Route 280) (Mellon et al., 1993). Generally, those damaged bents were located in areas over deep, soft soil and bay mud. The damage on the Central Viaduct was located in a few bends on the northern end between Oak and Turk streets. This was the only damage to portions of a structure that was not constructed over deep, soft soils. These structures were closed almost immediately with the exception of that portion of the Central Viaduct south of Oak Street, where there was no sign of damage. Temporary splice beams were installed on those columns of the Central Viaduct where column hinge joints had been located in the original design. This splice was intended to keep the joint from separating in a future seismic event until a more permanent retrofit detail with new columns could be installed. A report of the analysis and recommendations was prepared for CALTRANS by Seible and Priestley (1991).

The most spectacular damage and that which was closest to collapse occurred in the vicinity of Innis Street on the Southern Freeway Viaduct, Interstate 280. The shorter of two columns supporting long outrigger bents failed in joint shear near the lower deck level. This occurred at only four bent locations on the structure, however. While the damage was minimal, there was obvious concern for the integrity of these pre-1972 design, non-ductile, reinforced-concrete structures. They had all been designed in the late 1950s to early 1960s for lateral forces of 0.06 g, using details of the period that we now know were weak and provided insufficient confinement, especially at beam-column joints. All the damaged areas were shored up with heavy-timber falsework to reinforce them during aftershocks and possible future seismic events until permanent repairs could be made. Since the duration of the Loma Prieta earthquake was relatively short and the magnitude moderate, it was prudent to close the structures to public traffic until they could be retrofitted to current seismic standards. This damage has been reported in Cooper and Van de Pol (1991), Elsesser and Whittaker (1991), Fenves (1992), Miranda and Bertero (1991), Moehle et al. (1991), Priestley and Seible (1990), Seible and Priestley (1991), and Thewalt and Stojadinovic (1992).

Inadequate Beam-Column Joint Reinforcement

Research and analysis conducted subsequent to the Loma Prieta earthquake have shown conclusively that the lightly reinforced column-pedestal detail unique to the Cypress structure was the main cause of the total collapse. Immediately following the earthquake, the Structural Engineering Department at UC Berkeley requested that CALTRANS's Division of Structures salvage a section of the damaged Cypress Street Viaduct that had not collapsed for the purpose of conducting a series of seismic performance experiments. UC Berkeley has published reports of the experiments that were to determine the fundamental period of the complex structure under low-intensity seismic loading. Additional tests

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

were performed to measure the actual lateral resistance of the framing system up to yield. Experiments on several proposed column-retrofit details were also conducted on this structure. Bollo et al. (1990), Miranda and Bertero (1991), Mahin and Moehle (1990; Mahin, 1991), Kay (1991), and Jones and Schroeder (1991) have published results of this analysis and research.

Inadequate Torsional Reinforcement

The China Basin Viaduct suffered bent outrigger damage at two locations in the vicinity of the Sixth Street northbound off ramp. That ramp was closed to traffic, but the mainline structure was kept open since it was a single-level structure on multiple-column bents, and no damage was observed at any other location. Emergency contracts were awarded to shore up the damaged bent and to effect a complete replacement of both the damaged bents and also the supporting columns at one bent location. This work was completed while mainline traffic was allowed to continue operating on the structure. Subsequent analysis of the outrigger performance indicated shear failure and a potential for combined torsion and shear failure, as reported by Moehle (1992; Moehle et al., 1991).

Pedestrian-Bridge Performance

During the vulnerability screening and seismic analysis of bridges subsequent to the Loma Prieta event, the CALTRANS Office of Earthquake Engineering staff has noted that the class of single-column-supported pedestrian bridges have universally required seismic-retrofit strengthening. They are generally lightweight and very narrow, offering little lateral stiffness, thus rendering them especially vulnerable to seismically induced lateral forces.

Steel-Bridge Performance

The documented failures of structural steel bridges during the earthquake were few, and they can be attributed to the date that those bridges were designed. The anchor-bolt failures on the San Mateo-Hayward Bridge, the San Francisco-Oakland Bay Bridge, and the viaducts on the east end of the Bay Bridge were major failures, but they were repaired in short order, especially the anchor bolts on the San Mateo-Hayward Bridge and on the East Bay Distribution Structure. Subsequent investigation revealed a large number of structural steel columns on the San Francisco Skyway portion of Interstate 80 and US 101 that must be strengthened to prevent collapse because of the low seismic lateral forces for which they were designed. There was no evidence of structural steel-girder failure on any of the bridges in the affected counties.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Importance Factor Applied To Critical Bridges

The Governor's Board of Inquiry, in its report of June 1, 1990, recommended that ''The Department of Transportation should adopt a seismic safety policy for transportation structures that assures that transportation structures are seismically safe and that important transportation structures maintain their function after earthquakes'' (Housner, 1990). During hearings conducted by the Board of Inquiry, the board discussed this issue with CALTRANS engineers. There had never been a distinction between bridges relative to their importance to the state or local community.

As a direct result of the one-month loss of the San Francisco-Oakland Bay Bridge during the Loma Prieta earthquake, it has been recommended that major transportation structures be designed to remain essentially elastic for higher seismic-force levels and longer shaking periods to reduce the damage to a non-structural type. To accomplish this goal, a new "importance factor" was introduced into the design and retrofit performance criteria. This represents a major change in the seismic-design criteria for bridges and also represents the introduction of a subjective factor that will be based on judgment more than engineering principles.

RESEARCH IN SEISMIC RESPONSE OF BRIDGES

As a result of the problems discussed above and in response to further direction by the Governor's Board of Inquiry, which recommended that "The Department of Transportation should fund a continuing program of basic and problem-focused research on earthquake engineering issues pertinent to CALTRANS responsibilities" (Housner, 1990), and the Governor's Executive Order D-86-90, June 2, 1990, the Department of Transportation immediately accelerated its Bridge Seismic Research Program with the funding of 23 major research projects at a total cost of over $8 million. This research was "problem focused" on those areas that proved to be vulnerable during the recent earthquakes. Most of the research involved half-size model testing of bridge components and joint details. Seismic-retrofit details to strengthen existing bridges were developed and proven with this research program and their good performance in three recent earthquakes in California in April and June 1992, prove the validity of the ductile design and retrofit approach adopted by CALTRANS.

After the Loma Prieta earthquake caused 44 fatalities on the state highway system, capital funding for seismic retrofitting was increased from $4 million to $300 million per year. At the same time, seismic research funding for bridges was increased from $0.5 million annually to $5 million annually with an initial $8 million allocation from the special State Emergency Earthquake Recovery legislation of November 1989, Senate Bill 36X.

Research has been conducted, and is currently underway, at the UC San

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Diego, to test and confirm the validity of several proposed design solutions for seismic retrofitting of existing single-column-bent substructure elements on bridges. Since the Loma Prieta earthquake, additional research has been and is currently being conducted at the University of California at Berkeley, San Diego, Irvine, and Davis to develop and test retrofit techniques for multiple-column bents and double-level structures, including abutment and footing details.

Development of Vulnerability-Analysis Algorithm

In order to set priorities for more than 24,000 bridges in the state for order of seismic-retrofit upgrading, CALTRANS engineering staff developed a risk-analysis procedure and adjusted it over the next three years as more information became available. Identification of bridges likely to sustain damage during an earthquake was an essential first step in the Single-Column phase of the Bridge Seismic Retrofit Program, which had begun just prior to the Loma Prieta earthquake. What can be classified as a level-one risk analysis was employed as the framework of the process that led to a consensus list of risk prioritized bridges. This risk analysis procedure was later utilized to also prioritize the multiple-column-supported bridges, but the single-column-supported bridges were deemed more vulnerable, based on experiences during the 1971 San Fernando earthquake.

A conventional risk analysis produces a probability of failure or survival. This probability is derived from a relationship between the load and resistance sides of a design equation. Not only is an approximate value for the absolute risk determined, but relative risks can be obtained by comparing the determined risks of a number of structures. Such analyses generally require vast collections of data to define statistical distributions for all, or at least the most important, elements of some form of analysis, design, or decision equations. The acquisition of this information can be costly if obtainable at all. Basically, what is done is to execute an analysis, evaluate both sides of the relevant design equation, and define and evaluate a failure or survival function. All of the calculations are carried out, taking into account the statistical distribution of every equation component designated as a variable throughout the entire procedure.

To avoid such a large, time-consuming investment in resources and to obtain results that could be applied quickly as part of the Single Column phase of the retrofit program, an alternative was recognized. What can be called a level-one risk-analysis procedure was used. The difference between a conventional and level-one risk analysis is that in a level-one analysis judgments take the place of massive data-supported statistical distributions.

The level-one risk-analysis procedure used can be summarized by the following steps:

  1. Identify major faults with high event probabilities (priority-one faults). This step was carried out by consulting the California Division of Mines and

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Geology and recent U.S. Geological Survey studies. A team of seismologists and engineers identified faults believed to be the sources of future significant seismic events. Selection criteria included location, geologic age, time of last displacement (late quarternary and younger), and length of fault (10-km minimum). Each fault recognized in step 1 was evaluated for style, length, dip, and area of faulting in order to estimate potential earthquake magnitude. Known faults were placed in one of three categories: minor (ignored for the purposes of this project), priority two (mapped and evaluated but unused for this project), or priority one (mapped, evaluated, and recognized as immediately threatening).

  1. Develop attenuation relationships at faults identified in step 1. An aver-age-attenuation model was developed by Mualchin and Jones (1992) of the California Division of Mines and Geology to be used throughout the state. It is the average of several published models.

  2. Define the minimum ground acceleration capable of causing severe damage to bridge structures. The critical (i.e., damage-causing) level of ground acceleration was determined by performing nonlinear analyses on a typical, highly susceptible structure (single-column connector ramp) under varying maximum ground-acceleration loads. The lowest maximum ground acceleration that demanded the columns provide a ductility ratio of 1.3 was defined as the critical level of ground acceleration. The level of ground acceleration determined in this study was 0.5 g.

  3. Identify all the bridges within high risk zones defined by the attenuation model of step 2 and the critical acceleration boundary of step 3. The shortest distance from every bridge in California to every priority-one fault was calculated. Each distance was compared with the distance from each respective level of magnitude fault to a 0.5 g decrement acceleration boundary. If the distance from the fault to the bridge was less than the distance from the fault to the 0.5 g boundary, the bridge was determined to lie in the high-risk zone and was added to the screening list for prioritization. The prioritization procedure is described below.

    The CALTRANS Division of Structures has developed a computerized data base that has the coordinates of all 24,000 state, county, and city bridges stored. CALTRANS can produce a map of the entire state or any portion of the state showing the bridges, the major faults, and an overlay of the combinations. These maps can be viewed on the computer screen or printed for use by designers in screening to identify high-risk bridges. The procedure is quite simple using the computer data base to locate all highway bridges on the state system, locate all earthquake faults, then determine those structures that are in a high-risk zone.

  4. Prioritize the threatened bridges by summing weighted-bridge structural and transportation characteristic scores. This step constitutes the process used to prioritize the bridges within the high-risk zones to establish the order of bridges to be investigated for retrofitting. It is in this step that a risk value is assigned to each bridge.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

A specifically selected subset of structural and transportation characteristics of seismically threatened bridges was drawn from the CALTRANS structures computer database. Those characteristics were

  • ground acceleration;

  • route type (major or minor);

  • average daily traffic;

  • column design (single- or multiple-column bents);

  • confinement details of column (relates to age);

  • length of bridge;

  • skew of bridge; and

  • availability of detour.

Normalized pre-weight characteristic scores from 0.0 to 1.0 were assigned based on the information stored in the data base for each bridge. Scores close to 1.0 represent high-risk structural characteristics or high cost of lost transportation services. The pre-weight scores were multiplied by prioritization weights. Post-weight scores were summed to produce the assigned prioritization risk value.

Determined risk values are not to be considered exact. Due to the approximations inherent in the judgments adopted, the risks are no more accurate than the judgments themselves. The exact risk is not important. Prioritization-list qualification is determined by fault proximity and empirical attenuation data and not so much judgment. Therefore, a relatively high level of confidence is associated with the completeness of the list of threatened bridges. Relative risk is important, because it establishes the order of bridges to be investigated in detail for possible need of retrofit by designers. The risk analysis offers consistency in applying the judgments adopted to all bridges in the state.

A number of assumptions were made in the process of developing the prioritized list of seismically threatened bridges. This is typical of most engineering projects. These assumptions are based on what is believed to be the best engineering judgment available. It seems reasonable to pursue verification of these assumptions some time in the future. Two steps seem obvious: (1) monitoring the results of the design department's retrofit analyses and (2) executing a higher-level risk analysis.

Important features of this first step are the ease and cost with which it could be carried out and the data base that could be developed, highlighting bridge characteristics that are associated with structures in need of retrofit. This database will be utilized to confirm the assumptions made in the Single Column phase of the retrofit program. The same database will serve as part of the statistical support of a future conventional risk analysis as suggested in the second step. The additional accuracy inherent in a higher-order risk analysis will serve to verify previous assumptions, provide very good approximations of actual structural risk, and develop or evaluate postulated scenarios for emergency responses. It is reasonable to analyze only selected structures at this level. A manual screen-

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

ing process was used here, which included review of "as-built" plans by at least three engineers to identify bridges with column details that appeared to need upgrading.

After evaluating the results of the 1989 Loma Prieta earthquake, CALTRANS modified the risk analysis algorithm by adjusting the weights of the original characteristics and adding to the list. The additional characteristics are

  • soil type;

  • hinges (type and number);

  • exposure (combination of length and average daily traffic);

  • height;

  • abutment type; and

  • type of facility crossed.

Even though additional characteristics were added and weights were adjusted, the post-weight scores were still summed to arrive at the prioritization risk factor. The initial vulnerability priority lists for locally and state-owned bridges were produced by this technique, and retrofit projects were designed and built.

CALTRANS's Seismic Advisory Board reviewed the initial risk algorithm and suggested revisions to the procedure. During 1992, advances were made in the procedures employed by CALTRANS to prioritize bridges for seismic retrofit, and a new, more accurate algorithm was developed. The most significant improvement to the prioritization procedure is the employment of the multi-attribute decision theory. This prioritization scheme incorporates the information previously developed and utilizes the important extension to a multiplicative formulation.

This multi-attribute decision procedure assigns a priority rating to each bridge, enabling CALTRANS to more accurately decide which structures are more vulnerable to seismic activity in their current state. The prioritization rating is based on a two-level approach that separates out seismic hazard from impact and structural vulnerability characteristics. Each of these three criteria (hazard, impact, and structural vulnerability) depends on a set of attributes that have direct impact on the performance and potential losses of a bridge. Each of the criteria and attributes have assigned weights to show their relative importance. Consistent with previous work, a global utility function is developed for each attribute.

This new procedure provides a systematic framework for treating preferences and values in the prioritization decision process. The hierarchical nature of this procedure has the distinct advantage of being able to consider seismicity prior to assessing impact and structural vulnerability. If seismic hazard is low or non-existent, then the values of impact and structural vulnerability are not important, and the overall post-weight score will be low, because the latter two are added, but the sum of those two are multiplied by the hazard rating. This newly

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

developed prioritization procedure is defensible and theoretically sound. It has been approved by the Seismic Advisory Board.

Other research efforts in conjunction with the prioritization procedure involve a sensitivity study that was performed on bridge prioritization algorithms from several states. Each procedure was reviewed in order to investigate whether or not California was neglecting any important principles. One hundred California bridges were selected as a sample population, and each bridge was independently evaluated by each of the algorithms. The 100 bridges were selected to represent California bridges with respect to the variables of the various algorithms. California, Missouri, Nevada, Washington, and Illinois have thus far participated in the sensitivity study. Gilbert (1993), Maroney and Gates (1990), and Sheng and Gilbert (1991) have reported on this research.

The final significant improvement to the prioritization procedure is the formal introduction of varying levels of seismicity. A preliminary seismic activity map for the state of California has been developed in order to incorporate seismic activity into the new prioritization procedure. In late 1992, the remaining bridges on the first vulnerability priority list were reevaluated using the new algorithm, and a significant number of bridges changed places on the priority list, but there were no obvious trends.

Figure 6-1 and Table 6-1 show the new algorithm and the weighting percentages for the various factors.

Design engineers have been assigned the final task of verifying or discrediting the prioritized bridges' need for retrofitting and then, if necessary, developing retrofit contract plans. Verification of the need for retrofit is necessary due to the possibility of prioritized bridges already being capable of withstanding the maximum credible earthquake. This will be the case when judgments made in the prioritization process prove to be conservative. Emphasis is being placed on evaluation of the total bridge during this phase, and in almost all instances a dynamic analysis is necessary to make the final judgment on whether to retrofit or not.

Establishment of Seismic Performance Criteria

Working with the new Seismic Advisory Board, CALTRANS has recently adopted bridge seismic performance criteria for design of all new bridges. These criteria are also applied to seismic retrofitting design for older bridges where they are practical. In some cases, it is more prudent to replace rather than retrofit a structure, because of age or high retrofit costs. Table 6-2 contains the newly adopted seismic performance criteria for the California State Highway System.

Development of Acceleration Response Spectra For Deep Soft Bay Muds

The damage patterns experienced during the Loma Prieta earthquake of October 17, 1989, reemphasized the importance of the influence of soil and founda-

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-1 Multi-attribute decision procedure—an algorithm to establish priority ratings for bridges in California.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

TABLE 6-1 Multi-Attribute Decision Procedure— Weighting Percentages for Various Criteria

HAZARD CRITERIA

 

 

Hazard Attributes

Hazard Weights

 

Soil Conditions

33%

 

Peak Rock Acceleration

38%

 

Seismic Duration

29%

IMPACT CRITERIA

 

 

Impact Attributes

Impact Weights

 

ADT on Structure

28%

 

ADT Under/Over Structure

12%

 

Detour Length

14%

 

Leased Air Space (Residential, Office)

15%

 

Leased Air Space (Parking, Storage)

7%

 

RTE Type on Bridge

7%

 

Critical Utility

10%

 

Facility Crossed

7%

VULNERABILITY CRITERIA

 

 

Vulnerability Attributes

Vulnerability Weights

 

Year Designed (Const.)

25%

 

Hinges (Drop Type Failure)

16.5%

 

Outriggers, Shared Col

22%

 

Bent Redundancy

16.5%

 

Skew

12%

 

Abutment Type

8%

tion response on the seismic performance of structures. Two phenomena that were highlighted were widespread soil liquefaction and the variation of recorded ground motion with local site conditions. Most structural damage occurred to bridges and buildings that had been constructed over soft bay muds. The major damage at Watsonville, Santa Cruz, the South of Market district and the Marina District in San Francisco, and along the bay shores of Oakland is similar to damage patterns that occurred in Mexico City in 1985. The Loma Prieta event has been called the geotechnical engineer's earthquake for that reason.

The damage to state highway bridges at Watsonville and along both the east and west shorelines of the San Francisco Bay followed that pattern as shown in the following figures. Figures 6-2 and 6-3 show the south and north sections of the Cypress Street Viaduct, respectively—the site of 42 fatalities. The south section was damaged severely but did not collapse as did the north section. Several independent investigations have concluded that a major cause of the collapse of the north section was the seismic response of the soft soils and bay mud underlying that section. As seen in Figure 6-4, the original shoreline of 100

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

TABLE 6-2 Seismic Performance Criteria for the Design and Evaluation of Bridges

Ground Motion at Site

Minimum Performance Level

Important Bridge Performance Level

Functional Evaluation

Immediate Service Level Repairable Damage

Immediate Service Level Minimal Damage

Safety Evaluation

Limited Service Level Significant Damage

Immediate Service Level Repairable Damage

DEFINITIONS

Immediate Service Level: Full access to normal traffic available almost immediately.

Limited Service Level: Limited access (reduced lanes, light emergency traffic) possible within days. Full service restorable within months.

Minimal Damage: Essentially elastic performance.

Repairable Damage: Damage that can be repaired with a minimum risk of losing functionality.

Significant Damage: A minimum risk of collapse, but damage that would require closure for repair.

Important Bridge (One or more of the following items present):

 

• Bridge required to provide secondary life safety (Example: access to an emergency facility)

 

• Time for restoration of functionality after closure creates a major economic impact

 

• Bridge formally designated as critical by a local emergency plan

Safety Evaluation Ground Motion (Up to two methods of defining ground motions may be used):

 

Deterministically assessed ground motions from the maximum earthquake as defined by the Division of Mines and Geology Open-File Report 92-1 (1992)

 

Probabilistically assessed ground motions with a long return period (approximately 1,000-2,000 years)

 

For important bridges, both methods shall be given consideration, however, the probabilistic evaluation shall be reviewed by a CALTRANS approved consensus group. For all other bridges, the motions shall be based only on the deterministic evaluation. In the future, the role of the two methods for other bridges shall be reviewed by a CALTRANS approved consensus group.

Functional Evaluation Ground Motion:

 

Probabilistically assessed ground motions that have a 40% probability of occurring during the useful life of the bridge. The determination of this event shall be reviewed by a CALTRANS approved consensus group. A separate Functional Evaluation is required only for Important Bridges. All other bridges are only required to meet specified design requirements to assure Minimum Functional Performance Level compliance.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-2 South section of Cypress Street Viaduct.

years ago crosses the Cypress Viaduct near 14th Street. This is very near the transition between the collapsed section and the section that did not collapse. While the actual failure mode was a weak column pedestal and the low level of seismic acceleration specified in the California bridge-design specifications in use in the early 1950s, the amplified soft soil response contributed to the complete collapse of the north portion of the viaduct. Figures 6-5 through 6-8 show the results of soft-bay-mud response on the Struve Slough Bridge near Watsonville, where the ground movement of the soft muds caused the piling to shear off below the bent caps. Figure 6-9 shows the 100-year-old shoreline on the west bay side of San Francisco Bay; most bridge damage occurred to those structures that were built along the bay shore over the deep, soft muds.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-3 North section of Cypress Street Viaduct.

FIGURE 6-4 Present and 1880 shoreline near Cypress Structure.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-5 Collapsed Struve Slough Bridge.

FIGURE 6-6 Evidence of large ground movement.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-7 Piling sheared at girder softies.

FIGURE 6-8 Piling penetrated six-inch deck between girder stems.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-9 Present and 1880 shoreline on San Francisco side of bay.

CALTRANS developed a series of design acceleration response spectra for alluvium and normal soils after the 1971 San Fernando earthquake, but these spectra were not accurate for prediction of the dynamic response of softer soils and muds. After the Loma Prieta event, CALTRANS engaged Professors John Lysmer and Raymond B. Seed at UC Berkeley to help develop a set of similar design spectra for deep, soft, cohesionless soils and mud. Professor Seed has presented a draft report to CALTRANS, which expects to have the spectra available in 1993 (Lysmer et al., 1991). (Professor Raymond Seed's father, Professor Harry Bolton Seed, was instrumental in the development of CALTRANS's original design spectra for normal soils and alluvium.) Professor Seed et al. (1992) have concluded that the deep, soft muds amplify the bedrock ground motions by factors of two to three and that amplification of the longer-period components was especially pronounced, resulting in surface motions that are more damaging to the taller, longer period structures. Seismic ground motions have been predicted in the deep, soft soils with an analytical procedure, and the predictions have been confirmed with the actual recorded motions at four sites around the San Francisco Bay.

In addition to developing the new set of design response spectra for deep, soft soils, CALTRANS has also initiated a program to identify and map the soft soil sites in the state. While CALTRANS intends to develop a set of generic design response spectra for several representative deep, soft soil site conditions

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

for use on the more standard and smaller bridges, it will continue to concentrate on the development of site-specific response analyses for major structures at soft soil sites, based upon the recommendation of Professor Seed and other advisors. Figure 6-10 is an example of a site-specific response spectra for a deep, soft-bay-mud site. The researchers have shown that the analysis techniques and the computer programs currently available can reliably predict the response of deep, soft soils and therefore justify site-specific analyses. Papers describing this research have been published by Jakura (1992). CALTRANS Research Project R-7 was initiated to address this area in depth.

For the major bridges crossing the bays from San Diego in the south to Antioch at the extreme northeast end of the San Francisco Bay and estuary, CALTRANS has engaged consultants to conduct site-specific complete hazard analyses using a probabilistic approach to define several levels of design earthquakes for bridge seismic-design purposes. Of the ten bridge sites in this category, these hazard studies have been completed for the San Francisco-Oakland Bay Bridge and the two bridge sites in the Carquinez Straits. Geomatrix Consultants presented their final report on this study to CALTRANS in February 1993 (Geomatrix Consultants, Inc., 1992a). Consultants have been selected for the remaining hazard studies and results will be available at various times throughout 1993.

FIGURE 6-10 Typical design-response spectra for deep, soft mud.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Establishment Of Procedures For Out-Of-Phase And Non-Uniform Foundation Response

Other geotechnical factors that contributed to structural damage in the Loma Prieta earthquake were non-uniform and out-of-phase responses of the foundation materials and their affect on longer structures such as the 1.5-mile-long viaducts and the 4.5-mile-long San Francisco-Oakland Bay Bridge. Geomatrix Consultants and International Civil Engineering Consultants were commissioned to conduct a coherence study for the west bay spans of the San Francisco-Oakland Bay Bridge. That study has been completed, and the report is now available (Geomatrix Consultants, Inc., 1992b). Additional research in this area has been reported by Sitar and Salgado (1992) and Eidinger and Abrahamson (1991).

Foundation Design For Liquifiable Soil Sites

While soil liquefaction was not a contributor to bridge damage, it was apparent near several major structures in the east bay and must be considered and dealt with in future seismic design for bridges. Results of studies by Lysmer et al. (1991), Idriss (1990, 1991), and other investigators clearly indicate the shortcomings of the current provisions for dealing with the influence of deep, soft foundations on structure response.

Staff seismologists, engineering geologists, and geotechnical engineers at CALTRANS have identified and mapped the deep, soft sites throughout the state, and this information has been entered into the Geographic Information System and Bridge Data Base.

Professor Geoffrey Martin at the University of Southern California has helped develop design-mitigation procedures for these liquifiable sites (Lam et al., 1991; Lam and Martin, 1991). For one site north of San Diego, CALTRANS is using 10-foot-diameter stone columns as a foundation-stiffening technique to stiffen the soil around the immediate vicinity of the bridge piers. Mitchell (1992) has published research results on ground treatment for seismic stability of bridge foundations. Research Project R-22 will also address this issue.

Pier Foundation And Abutment Soil-Structure Interaction

The response of foundation materials was a key area of needed research and the subject of several projects. In addition to the work being done by in-house staff, we have engaged researchers at both UC Davis and the University of Southern California to study the dynamic response of bridge abutments and the soil structure interaction characteristics to be used in future bridge designs. Research Project R-19 at the University of Southern California is designed to develop improved seismic design and retrofit procedures for bridge abutments. The UC Davis study (Research Project R-20) is intended to test the soil-structure

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

interaction and help develop reliable soil—spring constants for advanced analysis and design of this important interface under dynamic loading. Maroney et al. (1991, 1993) have published interim results of those tests. Final results of that research will be available in late 1993. Another project at UC Davis is testing the performance of bridge-foundation piles that were removed from the collapsed Struve Slough Bridge near Watsonville and retrofitted to current criteria (Research Project R-5).

CALTRANS, the city of Los Angeles, and the county of Los Angeles are supporting research at the Southern California Earthquake Research Center to determine foundation-response characteristics for the southern California area. This work will be conducted by researchers from the University of Southern California and the California Institute of Technology.

Soon after the demolition work was completed at the Cypress Street Viaduct in Oakland, CALTRANS began a series of foundation lateral-load tests. Jack Abcarius (1991a, b) describes the lateral-load tests that have been conducted on foundations at both a soft site and a hard site on the Cypress Viaduct to determine soil resistance and assist in modeling soil-spring constants for dynamic analyses. CALTRANS is also conducting a series of lateral-load tests on large-diameter piles in soft soils (CALTRANS Division of Structures, 1990) and a series of pullout capacity tests on tension piles in soft soils, where additional uplift capacity on the piles in single-column bent footings is needed to resist overturning moments. Results of these two sets of tests will be available in late spring, 1993. The test results will be used in the foundation design for replacement structures in the San Francisco area and for design of retrofitting details for the foundations of many bridges in the soft soils of the San Francisco Bay area and elsewhere in California. Mason (1993), Sweet (1993), and Wilson and Tan (1990; Wilson, 1993) have published papers on research in this area. Other research in this area is funded by Research Projects R-21 and R-27.

The University of Nevada at Reno has an ongoing research program to measure the dynamic response characteristics of a full-sized bridge on the California State Highway System near the Imperial fault east of El Centro, California. Crouse (1992), Price et al. (1992), Wilson and Tan (1990), and Gates (1993) have published several research reports on this work.

Ductile Column Design

Bridge columns designed prior to the 1971 San Fernando earthquake typically contain very little transverse reinforcement. A common detail for both circular and rectangular columns consisted of #4 (.5-inch diameter) transverse peripheral hoops at 12-inch to 18-inch centers, regardless of column size and area of main reinforcement. Also, it was common practice to extend short lengths of dowel or tails from the footing-reinforcing steel out of the footing and lap splice them with the main column-reinforcing steel cage at that point. As a

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

consequence of these details, the ultimate curvature capable of being developed within the potential plastic region is limited by the strain at which the cover concrete starts to spall. The result is flexural failure resulting from inadequate ductility capacity or shear failure due to lack of adequate shear reinforcement. Tests conducted at UC San Diego confirm this theory (Priestley et al., 1991b). Several bridges suffered column-shear failures due to the elastic design philosophy under which they were designed prior to 1971.

Columns designed since 1971 contain a slight increase in the main-column-reinforcing steel and a major increase in confinement and shear-reinforcing steel over the pre-1971 designs. All new columns, regardless of geometric shape, are reinforced with one or a series of spiral-wound interlocking circular cages. The typical transverse shear and confinement-reinforcement detail now consists of #6 (.75-inch-diameter) hoops or continuous spiral at 3-inch pitch for the full column height. This provides approximately eight times the confinement and shear-reinforcing steel in columns than what was used in the pre-1971 non-ductile designs. All main-column reinforcing is continuous into the footings and superstructure. Splices are mostly welded or mechanical, both in the main and transverse reinforcing. Transverse-reinforcing steel is designed to produce a ductile column by confining the plastic hinge areas at the top and bottom of columns. The use of grade 60, A 706 reinforcing steel in bridges has recently been specified on a few projects on a trial basis and will probably become common practice by the end of 1993.

Research has been conducted to confirm the design criteria that were adopted after the 1971 San Fernando earthquake. Priestley and Seible (1990; Seible et al., 1992b) and Moehle and Aschheim (1992) have published research reports on this subject.

Retrofit-Strengthening Procedures For Existing Non-Ductile Columns

The largest number of large-scale tests have been conducted to confirm the calculated ductile performance of older, non-ductile bridge columns that have been strengthened by application of structural steel plate, pre-stressed strand, and fiberglass-composite jackets to provide the confinement necessary to ensure ductile performance. Research Projects R-l, R-4, R-8, and R-14 were commissioned to produce solutions to this problem. Since the spring of 1987, the researchers at UC San Diego have completed 30 sets of tests on bridge-column models. Priestley, Seible, and others at UC San Diego have published numerous research reports on this work.

UC San Diego was chosen for the testing because they had a new structural engineering laboratory with a five-story strong wall and other necessary equipment to conduct the tests. More important, the principal investigator, Dr. M.J. Nigel Priestley, had recently moved from Canterbury University in Christchurch, New Zealand, where he had been testing smaller-scale models of bridge columns

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

using a steel-jacket concept for exterior confinement of older, non-ductile columns. With that combination of research background and the testing facility, CALTRANS awarded contracts to UC San Diego early in 1987 and had completed several tests by the time the Whittier earthquake of October 1987 again showed the weakness of pre-1971 column designs. The test series was accelerated after the Whittier earthquake, and by the time of the October 1989 Loma Prieta disaster CALTRANS had enough test results from Dr. Priestley's work that plans were on the shelf ready to be advertised for the first column-retrofit projects. Those first projects were advertised within two months of the Loma Prieta event, and work was started in early 1990 and completed by the end of that year. Figure 6-11 is a completed column-jacket-retrofit detail, the end result of this research.

Work at UC San Diego consisted of half-scale model testing of the various single-column bent retrofit techniques. Theoretical calculations and research work previously conducted in New Zealand by Dr. Priestley showed that enclos-

FIGURE 6-11 Steel jacket retrofit detail on older concrete column.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

ing the columns in steel casings could significantly increase their shear strength and ductility by providing the additional confinement at the plastic hinge areas. A series of tests have been completed on round columns with outstanding results. Based on this work, a series of contracts for bridge-column retrofit were advertised in January 1990, and work is currently underway in the Los Angeles basin and the San Francisco Bay area. A second series of tests was begun in February 1990 on rectangular single-column bents, and the results were available within months. Both series of tests include models of the prototype columns with the pre-1971 reinforcing details without retrofitting, retrofitted columns using the steel-shell confinement, and a post-damage-retrofitted column using the steel shell to determine whether a non-retrofitted damaged column can be salvaged after an earthquake. Typical displacement ductility factors on retro-fitted undamaged columns are 6 to 8. On the post-damage-retrofitted column, a ductility factor of 2 was achieved. Even though displacement ductility factors of 6 to 8 have been common in these first tests, the analysis procedure is based on moment ductility demand no greater than 4. Priestley et al. (1991a, b) have published several reports on this research.

Beam-Column Joint Confinement

Major advances have been made in the area of beam-column joint confinement that are based on the results of research at both UC Berkeley and UC San Diego. The performance and design criteria and structural details developed for the I-480 Terminal Separation Interchange and the I-880 replacement structures reflect the results of this research and were reported by Cooper (1992). Research is continuing at both institutions to refine further the design details to ensure ductile performance of these joints. Large one-half-scale models of the most critical three-way longitudinal edge beam, transverse bent cap, column-joint detail were built and tested at both UC San Diego and UC Berkeley as a major element of the joint and column testing program. Additional large-scale-model testing programs are planned for the more complex multilevel beam-column details commonly used in major highway interchange structures. Mahin and Moehle of UC Berkeley and Seible of UC San Diego have published reports of their research in this area (Mahin et al., 1992; Mahin, 1991; Moehle et al., 1991; and Seible et al., 1993).

The most elaborate and expensive tests conducted to date have been the half-scale and third-scale models of the proposed retrofit details for the double-deck viaduct structures in San Francisco. Models, using two different seismic retrofit techniques, were tested at both UC San Diego (half scale) and UC Berkeley (third scale) in order to obtain the performance characteristics of the two different details so CALTRANS's designers and consultants could make the final design and construction decisions and get the construction contracts started in 1992. Both models were nearly 50 ft long by 20 ft wide and 20 ft high. They

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

included one-half of each span adjacent to the joint and half the column above and below the joint. Half-width of the superstructure was also modeled to recreate the actual contributing dead load and stiffness of the elements that frame into these columns. The difference in the concepts is in the connection to the edge of deck of the original structure. The UC Berkeley concept, funded in Research Project R-10, uses an integral edge-beam detail that is necessary on curved alignments, such as the Central Viaduct (US 101) in downtown San Francisco and the Alemany Interchange on US 101 in south San Francisco. This research-and-testing program was reported by Mahin et al. (1992). The UC San Diego concept is an independent edge beam that will be used on straight alignments, primarily the I-280 Southern Freeway Viaduct north of the Alemany Interchange with US 101. This research was reported by Priestley and Seible (1990).

Thirteen dynamic actuators were required to dynamically excite the UC San Diego model in all three directions. The beginning of plastic-hinge formation at the top of the lower column occurred at design ductilities, exactly where the design intended to force its location. After a ductility of 8 had been achieved, the plastic-hinge zone began to lose its cover, but the core concrete remained intact because of the heavy confinement reinforcement. Therefore, the structure should remain standing and in operational service. This level of damage is easily repaired after a seismic event without closure to public traffic. Each of these test series represents a cost of $500,000, but they are used to proof test the intended retrofitting scheme for structures totaling more than one mile in length with a replacement cost of over $200 million. Mahin and Moehle of UC Berkeley and Priestley and Seible of UC San Diego have published results of their work (Mahin et al., 1992; Mahin, 1991; Moehle, 1992; Priestley and Seible, 1990). Figure 6-12 shows the model at UC San Diego prior to testing; Figure 6-13 shows the UC Berkeley retrofit detail in the actual field installation.

Outrigger Bent Cap Performance Under Combined Bending, Shear, And Torsion

The Earthquake Engineering Research Center at the Richmond Field Station of the UC Berkeley began a series of tests on the performance of outrigger joints in 1991. They tested a model of the outrigger joint of the I-980 structure that was damaged in the Loma Prieta earthquake by using the identical prototype details. They replicated very closely the actual field damage, then tested the retrofitted joint as CALTRANS had redesigned it and confirmed the validity of the redesigned joint details. Based on these tests, column-cap joint details have been improved, column transverse reinforcement is typically continued up through the joint regions, and the joints are further confined for shear and torsion resistance. The details for these joints usually require 1 percent to 3 percent confinement reinforcing steel. Thewalt and Stojadinovic (1992) of UC Berkeley have reported on this research.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

FIGURE 6-12 Half-scale model of edge beam retrofit in test stand.

FIGURE 6-13 Edge beam/joint detail on Interstate Route 280 in San Francisco.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Seismic Performance Of Tall, Slender Pier Walls

UC Irvine has a series of research projects and large-scale model tests to investigate the performance of tall bridge pier walls under dynamic loading (Research Project R-13). They are also investigating the performance and shear strength of column-hinge-pin details (Research Project R-3). Haroum and Shepard of UC Irvine have reported the interim results of this research.

Performance Of Column-Footing Joints Under Dynamic Loading

Both UC Berkeley and UC San Diego will be conducting tests on the column-footing joint details as other work is completed. Results of previous column-retrofit tests indicate that pre-1971 footing details need substantial improvement. These improvements have already been made in the retrofit contract plans, but the research is necessary to confirm theoretical calculations of performance and to continually improve details. Seible et al. (1992a, b) of UC San Diego recently published a report on this work.

Bond Development Length Of Large-Diameter Reinforcing Steel Bars Under Dynamic Loading

Peer Review Panel concerns about the adequacy of the American Association of State Highway and Transportation Officials' (AASHTO) design code provisions for bond development length for number 14 and number 18 reinforcing steel bars, which are typically used in large bridge columns in California, prompted additional research testing. Full-size elements of the column-cap connection were built and tested at UC San Diego, and the results were reported by Scible et al. (1993) of UC San Diego in late 1992. The Earthquake Engineering Research Center of UC Berkeley recently completed the first in a series of tests on the performance of column-footing joint details. They also conducted tests of the bond development length as a function of concrete cover. Results of that test were reported by Filippou and Cheng (1992). Full-size tests of these details have been conducted because of concerns regarding scaling of such large bars. These tests will confirm the adequacy of current provisions or recommend changes to be made to the current code requirements at the AASHTO bridge engineer's meeting in May 1993.

Nonlinear Analysis Procedures For Reinforced-Concrete Members Subject To Dynamic Loads

This is an area where much additional research is needed to develop the technology to a state that it is a practical tool for analyzing complex structures. Engineers need to understand when nonlinear performance is important and when

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

to spend the large resources needed to conduct a meaningful nonlinear analysis. Proper design and detailing cannot be effective without the correct performance characteristics. Professor Graham Powell of UC Berkeley was commissioned to provide guidelines for effective use of nonlinear structural analysis for bridge structures. This project provides guidelines and training to the CALTRANS engineers in the use of the new nonlinear analysis programs recently installed. Powell (1992a, b) has discussed this research in reports to CALTRANS. Filippou (1992) of UC Berkeley, Goudreau (1991) of Lawrence Livermore National Laboratories, and Yashinsky (1992b) of CALTRANS have also published reports on this subject.

Review And Revise Existing Seismic Design Specifications And Details

One of the research projects that was funded is the Applied Technology Council ATC-32 Project (CALTRANS Research Project R-11) to review and revise the entire CALTRANS Bridge Seismic Design Specifications. That project is in its second year, and the specific areas that need revision have been identified and work plans have been approved for the various sub-contractors to complete the project. Several Structural Engineers Association of California members and prominent university professors and seismic researchers are members of the Project Engineering Panel, which meets semi-annually to provide guidance to the project consultant and sub-contractors. Work should be complete by the end of 1993.

Base-Isolation Systems

Research on performance of base-isolation systems has been conducted at UC Berkeley and at the National Center for Earthquake Engineering Research in Buffalo, New York. Kelly et al. (1991) of UC Berkeley, Mayes (1992) of Dynamic Isolation Systems, and Constantinou of the National Center for Earthquake Engineering Research have reported on research in this area.

Tension-Pile Capacity Tests

CALTRANS is currently conducting a series of tension and pullout tests for eight types of piles furnished by their manufacturers. The purpose of the research program is to determine the pullout capacity of each pile type in deep, soft muds overlying rock near Interstate 280 in San Francisco. Design values will be established for each tension-pile type and included in plans for retrofitting and new construction where overturning moments require tension piles. Mason (1993) of CALTRANS, the project manager, has reported on the test program and will publish results in the fall of 1993.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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PRACTICAL LESSONS APPLICABLE TO OTHER AREAS AND STATES

A great deal of the published research that resulted from the Loma Prieta earthquake served the useful purpose of validating many of the conclusions that the bridge-engineering community had derived empirically. There were too many researchers and too much money spent on analysis of the failures at the San Francisco-Oakland Bay Bridge and the Cypress Street Viaduct. Both these structures were designed many years prior to the development of the modern seismic design specifications for bridges. Both were designed for lateral-force requirements of 0.06 g to 0.10 g and could not be expected to withstand the seismic forces that even a moderate earthquake such as Loma Prieta produced. It is a tribute to uncalculated redundancy and better than specified material strengths that these structures and other older highway structures performed as well as they did during the Loma Prieta event. This reasonable performance of older bridges in a moderate earthquake is significant for the rest of the United States, because that knowledge can assist their engineers in designing an appropriate seismic-retrofit program for their structures. While there is a necessary concern about the ''Big One'' in California, especially for the performance of important structures, it must be noted that many structures that can be bypassed need not be designed or retrofitted to the highest standards. It is also important to note that there will be many moderate earthquakes that will not produce the damage associated with a maximum event. These are the earthquake levels that should be addressed first in a multi-phased retrofit-strengthening program, based on the limited resources that will be available. Cost-benefit analysis of proposed retrofit details is essential to measure and ensure the effectiveness of a program.

Nevertheless, the screening processes and prioritization procedures that have been developed in California can have immediate application to other areas. Many excellent design details have been developed, through research and model testing, to guarantee ductile performance, and these can be used elsewhere without "reinventing the wheel." The excellent work that has been accomplished in foundation response and soil-structure interaction during a seismic event represents significant improvement in the state of the art and can be directly applicable in other parts of the country.

Good Emergency-Response Plan

From the experience CALTRANS has gained over the years responding to natural disasters, and the results of its response to the Loma Prieta disaster, it is clear that a sound, well-rehearsed emergency-response plan is a must for any agency charged with public safety and mobility. Because of the emergency exercises that have been conducted by the various state and local agencies and

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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the contracting industry, CALTRANS was able to mobilize workmen, equipment, and materials almost instantaneously to repair, shore up, and reopen vital transportation facilities. Roberts (1990) and others of CALTRANS have published papers on this subject. Every public agency should have a plan prepared and exercised so personnel know exactly where they are to report and what their responsibilities are in the event of a natural disaster. Given that an earthquake is over in seconds, there is no time to plan after the event. Response must be based on prior planning and practice exercises. The plan must include redundant routes and an assessment of important roads and structures needed for immediate recovery.

Vulnerability-Prioritization Procedure

Given the large number of bridges that most states and major counties own, there is a need for a procedure to assess the relative vulnerability of each structure and prepare a priority list for retrofit work. The most critical bridges must be reinforced and retro fitted first. There will be insufficient funds to upgrade all the bridges to the current criteria, so some trade-offs must be made between cost of retrofitting versus cost of damage repair when and if an individual bridge is damaged by an earthquake. The question that must be considered is "How much do we spend on insurance now to reduce future damage?" The great deal of research that has gone into development of a vulnerability-prioritization procedure relieves other agencies from that task. The currently used algorithms can always be improved, but there is in place today an adequate procedure that has been tested statistically with other states.

Prioritization Of Phases Of Retrofit Work

Retrofitting older bridges can be an expensive undertaking. CALTRANS has learned from the experiences before and after the Loma Prieta event that there are a number of measures that can significantly improve the performance of bridges for a nominal investment. It has analyzed causes of bridge failures in the large earthquakes over the past 20 years in California and has observed and analyzed the performance of bridge-retrofit details and drawn some important conclusions.

First, CALTRANS prioritized the bridges using the vulnerability prioritization procedure that is discussed above. This prioritization procedure ensures that the most vulnerable bridges are scheduled for seismic retrofitting first. This uses the limited funds where they will do the most good for the overall system.

Second, it determined that the unrestrained hinge and abutment joints were the cause of many failures in earlier earthquakes. They had been restrained with engineered joint-restrainer details prior to the Loma Prieta event, and they performed very satisfactorily. In the later Cape Mendocino and Landers earth-

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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quakes of 1992, these joint restrainers again performed well. CALTRANS had spent approximately $55 million to tie joints and superstructures together on 1,260 bridges prior to the Loma Prieta earthquake.

Third, it determined, again from analysis of actual damage in the two previous earthquakes, that the single-column supports were the next most vulnerable detail on bridges. CALTRANS began the retrofitting of single columns immediately after the Loma Prieta event and to date has spent approximately $120 million to retrofit 262 bridges that are supported with single-column bents.

Fourth, it determined that multiple-column bents will sustain major damage but, in most cases, will not collapse. There is a large amount of uncalculated ductility, even in a non-ductile design. There are multiple-load paths and multiple joints to resist lateral forces, and there is not the potential for overturning that is inherent in bridges with single-column bents. CALTRANS determined that these bridges were the third priority for retrofit and is now working on the design and construction to complete this third phase of the seismic retrofit program. It will spend another $1,300 million to complete the seismic retrofitting on the remaining 550 bridges that are supported on multiple-column bents to bring them up to the current seismic-safety criteria.

Fifth, CALTRANS knows that when this program is completed there will be many bridges left in the system that will sustain damage and even require closure in the event of a maximum credible earthquake. It will continue working its way down the vulnerability-priority list and retrofit those bridges to reduce future damage. At this stage of the program, CALTRANS will have met the seismic safety criteria, however, and future seismic-retrofit projects will again compete with other highway safety projects for limited funds.

Response Of Deep, Soft Soils

One of the most important lessons learned from the Loma Prieta earthquake and the research that has followed is the importance of calculating the seismic response of soft mud foundation sites for use in design of a bridge. Geotechnical engineers and geologists have been sounding the warnings for some time, especially after the 1985 Mexico City earthquake. It required the loss of life in the Loma Prieta earthquake to spring the funding for research into foundation response in softer, cohesionless soils and bay muds. That funding has, generally, been made available, and much valuable research has been completed with additional work to follow in the development of techniques to deal with soft and liquifiable soils. Much research has been and will continue to be completed in the area of foundations and soil-structure interaction. The bridge community is now equipped with the tools to design the appropriate structures in any foundation soil condition. Other areas of the country should learn from the foundation problems experienced in the Loma Prieta event, identify similar foundation problem areas in their jurisdictions, and build on the excellent research that has been

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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completed since the Loma Prieta event to analyze the response of their structures to these varying foundation conditions.

Ductile-Design Details

The amount of research in ductile-design details is second only to the amount of research that has been completed in foundation response. The continuation of research in this area will be aimed at reducing the conservatism that permeates ductile column and joint design in California today. Prior to the Loma Prieta earthquake, there had been little or no research into the performance of very large column-cap joints and confinement details for large joints. Satisfactory performance of those bridges that had been designed for ductile performance during the earthquake gives the bridge-design community reassurance that details can be developed to guarantee ductile column and joint performance in a major seismic event. Recent research results show that the amount of joint reinforcement that has been designed into these details since Loma Prieta can be significantly reduced. Additional research will be funded to corroborate those initial conclusions and design details will be modified accordingly.

Soil-Structure Interaction

There had been a significant amount of work in soil-structure interaction completed overseas before the Loma Prieta event, and much additional research in this area has been completed or commissioned since the earthquake. This information is available and is important in the seismic retrofitting of older bridges, because very few of these older structures were designed with any consideration for the soil-structure interaction. Significant reduction in structural-member forces can be achieved by considering the effects of the foundation resistance on the total structure response. The limited research in this area has given designers some analytical tools for modeling soil—spring constants for both piling and pile caps/footings. For new design, it is essential to consider the effects of foundation soil-structure interaction in the modeling of the system for an accurate dynamic-response analysis. Typically, the abutments can be designed to resist a large percentage of the longitudinal earthquake forces in most bridges of shorter and moderate lengths. For any bridge, a savings in column retrofitting, and reinforcing steel in new designs, can be achieved by proper consideration of the foundation-structure interaction.

Response And Retrofitting Of Structural Steel Bridges

Despite the fact that structural steel is ductile, bridges that have been designed by the pre-1972 seismic specifications must be evaluated for the seismic forces expected at the site based on earthquake magnitudes as they are known

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

today. Typically, structural steel superstructures that had been tied to their substructures with joint and hinge restrainer systems performed well as discussed in the section on steel-bridge performance earlier in this paper. However, many elevated viaducts and some smaller structures supported on structural-steel columns that were designed prior to 1972 and that will require major retrofit strengthening for them to resist modern earthquake forces over a long period of shaking have been identified. One weak link is the older rocker bearings, which will probably roll over during an earthquake. These can be replaced with modern neoprene, teflon, pot and base-isolation bearings to ensure better performance in an earthquake. Structural-steel columns can be strengthened easily to increase their toughness and ability to withstand a long period of dynamic input.

Nonlinear Analysis Procedures

Prior to the Loma Prieta event, there was little use of nonlinear analysis in the design of bridges. In order to correctly analyze bridge performance in a major earthquake of long duration, the use of nonlinear analysis techniques is mandatory. Ample research has been completed in this area to give designers the necessary tools to conduct reasonable nonlinear analyses and design structures that will perform in a ductile manner during a major earthquake with long duration. Additional work in this area will continue to improve the expertise of, and build confidence in, the bridge designers.

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Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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GENERAL REFERENCES

Ang, A.H.-S., W. Kim, and S. Kim. 1993. Damage Estimation of Existing Bridge Structures. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.

Astaneh-Asl, A. 1991. Behavior of Major Steel Bridges During October 17, 1989 Earthquake, Abstract Only. Structures Congress "91", American Society of Civil Engineers, New York.

Astaneh-Asl, A. 1992. Seismic Behavior and Retrofit of the San Francisco-Oakland Bay Bridge. Structures Congress "92", Compact Papers, American Society of Civil Engineers, New York.

Astaneh-Asl, A., B. Bolt, G. Fenves, J. Lysmer, and G. Powell. 1991. Seismic Condition Assessment of the Bay Bridge. In Proceedings of the First Annual Seismic Research Workshop, California Department of Transportation, Sacramento, California.

Astaneh-Asl, A. 1992. Seismic Evaluation and Retrofit of the Bay Bridge. In Seminar Proceedings, Seismic Design and Retrofit of Bridges, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Sacramento, California.

Astaneh-Asl, A. 1992. Steel Bridge Design and Evaluation Considerations Related to Local and Member Stability. In Seminar Proceedings, Seismic Design and Retrofit of Bridges, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Sacramento, California.

Astaneh-Asl, A., and J. Shen. 1993. Rocking Behavior and Retrofit of Tall Bridge Piers. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.


Buckle, I.G. 1991. Revisions to the AASHTO Seismic Design Criteria for Bridges in the United States. Pacific Conference on Earthquake Engineering, New Zealand National Society for Earthquake Engineering, Wellington.

Buckle, I.G. 1991. Seismic Design Criteria for Highway Bridges. Third Bridge Engineering Conference, Report: Third Bridge Engineering Conference at Denver, Colorado, March, 1991. Report: Transportation Research Board Record 1290. TRB, National Research Council, Washington DC.

Buckle, I.G. 1990. The Preliminary Screening of Bridges for Seismic Retrofit. In Proceedings, Second Workshop on Bridge Engineering Research in Progress, University of Nevada, Reno.


CALTRANS. 1992. First Annual Seismic Research Workshop. Proceedings: California Department of Transportation, Sacramento, California.

Chai, Y.H., M.J. Priestley, F. Seible. 1991. Seismic Retrofit of Bridge Columns by Steel Jacketing. Third Bridge Engineering Conference at Denver, Colorado, March, 1991. Report: Transportation Research Board Record 1290. TRB, National Research Council, Washington, D.C.


Dickenson, S.E., and R.B. Seed. 1991. Correlations of Shear Wave Velocity and Engineering Properties for Soft Soil Deposits in the San Francisco Bay Region. Report No. UCB/EERC-91/xx, Earthquake Engineering Research Center, University of California, Berkeley, California.

Dickenson, S.E., R.B. Seed, and C.M. Mok. 1992. Recent Lessons Regarding Seismic Response Analyses of Soft and Deep Clay Sites. Seismic Design and Retrofit of Bridges, Seminar Proceedings, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California.


Fenves, G.L., F.C. Filippou, and D.T. Sze. 1991. Evaluation of the Dumbarton Bridge in the Loma Prieta Earthquake. In Proceedings of the First Annual Seismic Research Workshop, California Department of Transportation, Sacramento, California.


Ghose, A., R.M. Polivka, and B.A. Maroney. 1991. Evaluation of Techniques for the Seismic Modeling of Elevated Freeway Bridges. Lifeline Earthquake Engineering: Proceedings of the Third U.S. Conference, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 4, American Society of Civil Engineers, New York, August.

Gross, J.L., and S.K. Kunnath. 1992. Application of Inelastic Damage Analysis to Double-deck Highway Structures. Report: NISTIR-4857, National Institute of Standards and Technology, Gaithersburg, Maryland, June.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Idriss, I.M., and J.I. Sun. 1992. SHAKE 91 - A Computer Program for Conducting Equivalent Linear Response Analysis of Horizontally Layered Soil Deposits. Users Manual, University of California, Davis.

Imbsen, R.A., and R.A. Schamber. 1991. Training Program for Implementation of Newly Developed Guidelines for Seismic Design and Retrofitting of Highway Bridges . Third Bridge Engineering Conference at Denver, Colorado, March, 1991. Report: Transportation Research Board Record 1290. TRB, National Research Council, Washington DC. 1991.

Imbsen, et al. 1990. Seismic Design of Highway Bridges, Training Course Participant Workbook. FHWA Project, Second Printing, July.


Kasai, K., W.D. Liu, and V. Jeng. 1992. Effect of Relative Displacements Between Adjacent Bridge Segments. SMIP 92: Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data. Proceedings: Strong Motion Instrumentation Program, California Division of Mines and Geology, Sacramento, California.

Ketchum, M.A., and C.T. Seim. 1990. Golden Gate Bridge Seismic Evaluation. Prepared in Cooperation with Imbsen and Assoc., Inc. and Gerspectra, Inc. San Francisco, California, November 2.

Ketchum, M.A., and C.T. Seim. 1991. Golden Gate Bridge Seismic Retrofit Studies. Prepared in Professional Collaboration with Imbsen and Assoc., Inc. and Geospectra, Inc. San Francisco, California, July 10.


Lysmer, J., and Deng, N. 1991. Two-Dimensional Site Response Analysis. In Proceedings of the First Annual Seismic Research Workshop, California Department of Transportation, Sacramento, California.


Maroney, B., and J. Gates. 1991. Seismic Risk Identification and Prioritization in the CALTRANS Seismic Retrofit Program . In Proceedings of the 4th U.S.-Japan Workshop on Earthquake Disaster Prevention for Lifeline Systems, Los Angeles, California.

Mayes, R.L., et al. 1990. Enhancing the Seismic Performance of Toll Road Bridges. In Proceedings: 59th Annual Convention, Structural Engineers Association of California, Sacramento, California, September.

McCallen, D., and G.L. Goudreau. 1990. Post Loma Prieta Earthquake Initiative, Seismic Analysis of an Elevated Portion of the Bay Bridge Distribution System Structure. Report: UCRL-ID-104179, Lawrence Livermore National Laboratory, Livermore, California, June 15.

Moehle, J.P., S.A. Mahin, and R. Stephen. 1990. Implications of Nondestructive and Destructive Tests on the Cypress Street Viaduct Structure. May.

Moehle, J.P. Undated. San Francisco Earthquake, October 17, 1989, Evaluation of the Performance of the I-880 Cypress Viaduct. Abstract of Research in Progress.

Mualchin, L. 1990. Seismic Hazards of CALTRANS Facilities After the Loma Prieta Earthquake and the Implications for CALTRANS Seismic Programs. International Symposium on Safety of Urban Life and Facilities: Lessons Learned from the 1989 Loma Prieta Earthquake. November 1-2, 1990. Tokyo Department of Environmental Engineering, Graduate School at Nagatsuta, Tokyo Institute of Technology, Yokohama, Japan, 1990.


Niazy, A., S. Masri, and A. Abdel-Ghaffar. 1993. Analysis of the Seismic Records of a Suspension Bridge. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.


Penzien, J., W.S. Tseng, and M.S. Yang. 1991. Seismic Performance Investigation of the Hayward-BART Elevated Section Instrumented Under CSMIP. SMIP 91: Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, Preprints, Strong Motion Instrumentation Program, California Division of Mines and Geology, Sacramento, California.

Penzein, J. 1993. Seismic Design Criteria for Transportation Structures. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.

Priestley, M.J.N., and F. Seible. 1992. Performance Assessment of Damaged Bridge Bents after the Loma Prieta Earthquake. Bulletin of the New Zealand National Society for Earthquake Engineering, Volume 25, No. 1, March.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Priestley, M.J.N., and F. Seible. 1993. Assessment and Testing of Column Lap Splices for the Santa Monica Viaduct Retrofit. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.

Ramey, M.R., et al. 1991. Experimental Testing of Epoxy Injected and Steel Shell Retrofitted Sections from the Collapsed Struve Slough Bridge. In Proceedings of the First Annual Seismic Research Workshop, California Department of Transportation, Sacramento, California.

Rashid, Y.R., R.A. Dameron, and I.R. Kurkchubasche. 1992. Predictive Analysis of Outrigger Knee-Joint Hysteresis Tests: A Torsion/Flexure Tests at University of California, San Diego. Report to University of California, Berkeley, University of California, San Diego, and CALTRANS , Sacramento, California.

Roberts, J.E. 1989. Bridge Seismic Retrofit Program For California Highway System. In Proceedings: U.S.-Japan Workshop on Lifeline Earthquake Engineering, Public Works Research Institute, Tsukuba Science City, Japan, May.

Roberts, J.E. 1989. Theory of California Seismic Bridge Design And Analysis For The Beginner. California Department of Transportation, Division of Structures, Training Course for Entry Level Engineers, July.

Roberts, J.E. 1990. Recent Advances in Seismic Design and Retrofit of Highway Bridges. In Proceedings: Earthquake Engineering Research Institute Annual Meeting Palm Springs, California, May.

Roberts, J.E. 1990. Recent Advances in Seismic Design and Retrofit of Highway Bridges. Structural Engineers Association of California, Proceedings: 59th Annual Meeting, Incline Village, Nevada. September.

Roberts, J.E. 1991. Recent Advances in Seismic Design and Retrofit of Highway Bridges. Proceedings. Seismic Retrofit Workshop, University of California at San Diego, July.

Roberts, J.E. 1991. Seismic Performance of Steel Bridges. Structural Steel Fabricators Annual Meeting, Saint Louis, Missouri, September, 1991. Published in Modern Steel Construction magazine, July 1992.

Roberts. J.E. 1991. Recent Advances in Seismic Design and Retrofit of Highway Bridges. In Proceedings of the Third U.S. Conference, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 4. American Society of Civil Engineers, New York, August.

Roberts. J.E. 1991. Seismic Retrofitting of San Francisco Viaducts. In Proceedings: 60th Annual Meeting, Structural Engineers Association of California, Palm Springs, California, October.

Roberts. J.E 1992. Seismic Design of Bridge Foundations. In Proceedings, Transportation Research Board Annual Meeting, Washington, D.C., January.

Roberts. J.E. 1992. Large Scale Model Testing for Seismic Design and Retrofit, Initiating, Funding and Managing. Presented at the Earthquake Engineering Research Institute Annual Meeting, San Francisco, California, February.

Roberts, J.E. 1992. Research Based Bridge Seismic Design and Retrofit Program, Criteria, Standards and Status. In Proceedings: Fifth U.S.-Japan Workshop on Earthquake Disaster Prevention for Lifeline Systems, Tsukuba Science City, Japan, October 26.

Roberts, J.E. 1992. Effect of Foundation Sod Response on Bridge Seismic Performance. Proceedings: Fifth U.S.-Japan Workshop on Earthquake Disaster Prevention for Lifeline Systems, Tsukuba Science City, Japan, October 26.

Roberts, J.E. 1992. Bridge Seismic And Other Research Needs-CALTRANS Overview. In Proceedings: Third NSF Workshop on Bridge Engineering Research in Progress, University of California at San Diego, November 15.

Roblee, C.J. 1992. Synthesis of CALTRANS. Foundation Seismic Research Program. Internal Report: Division of New Technology, Materials & Research, California Department of Transportation, Sacramento, California, November.


Saadeghvazri, M.A. 1990. Response of the Struve Slough Bridge under the Loma Prieta Earthquake. In Proceedings: Second Workshop on Bridge Engineering Research in Progress, University of Nevada at Reno.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Schnabel, P.B., J. Lysmer, and H.B. Seed. 1972. SHAKE, A Computer Program for Earthquake Response Analysis of Horizontally Layered Sites. Report: Earthquake Engineering Research Center, University of California, Berkeley.

Seim, C., and S. Rodriguez. 1993. Seismic Performance and Retrofit of the Golden Gate Bridge. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.

Sikorsky, C., N. Stubbs, and M. Richardson. Nondestructive Damage Assessment of a Bridge Using Modal Testing and Structural Reliability.

Stuart, R.J. 1991. Seismic Modeling Parametric Studies. In Proceedings of the First Annual Seismic Research Workshop, California Department of Transportation, Sacramento, California.

Thorkildsen, E. 1992. Overview of CALTRANS' Bridge Seismic Research Program. Structure Notes, No. 27, California Department of Transportation, Sacramento, California, July.


Werner, S.D., and C.E. Taylor. 1990. Seismic Risk Considerations for Transportation Systems. Recent Lifeline Seismic Risk Studies, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 1, American Society of Civil Engineers, New York, November.

Werner, S.D., S.A. Mahin, and N.C. Tsai. 1993. Compilation and Evaluation of Current Bridge Damping Data Base. Report to CALTRANS, February.

Werner, S.D., L. Katafygiotis, and J. Beck. 1993. Seismic Analysis of Meloland Road Overcrossing Using Calibrated Structural and Foundation Models. In Proceedings: ASCE Structures Congress XI, Irvine, California, April.

Whittaker, A.S., E. Elsesser. 1991. Seismic Design Criteria for Transportation Structures. Lifeline Earthquake Engineering: Proceedings of the Third U.S. Conference, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 4, American Society of Civil Engineers, New York, August.


Zelinski, R. 1990. California Highway Bridge Retrofit Strategy and Details. In Proceedings: 59th Annual Convention, Structural Engineers Association of California, Sacramento, California. September.

Zelinski, R. 1990. California Highway Bridge Retrofit Strategy and Details In Proceedings: Second Workshop on Bridge Engineering Research in Progress, University of Nevada at Reno.

Zelinski, R. 1991. San Francisco Double Deck Viaduct Retrofits. Lifeline Earthquake Engineering: Proceedings of the Third U.S. Conference, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 4, American Society of Civil Engineers, New York, August.

Zelinski, R., and A.K. Dubovik. 1991. Seismic Retrofit of Highway Bridge Structures. Lifeline Earthquake Engineering: Proceedings of the Third U.S. Conference, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 4, American Society of Civil Engineers, New York, August.

COMMISSIONED RESEARCH PROJECTS

R-1. Retrofitting of Bridge Columns, Stage I, using third to half scale model tests. UC, San Diego. Contract $400,000. Completed June 30, 1990. This is the first contract with Dr. Priestley which was initiated in early 1987. The emphasis was on single columns using steel jackets for confinement. The physical testing is complete and all reports are completed.

R-2. Guidelines for Effective use of Nonlinear Structural Analysis for Bridge Structures. UC, Berkeley. Contract $ 47,000. Completion July, 1993. Principal investigator Dr. Graham Powell. This project is for providing guidelines and training to the Department in the use of the new non-linear analysis programs recently installed. Work is underway.

R-3. Shear Strength Capacity Vs. Rotation of Column Pins at Base of Elevated Roadway Structure, using third to half scale model tests. UC, Irvine. Contract $80,000. Completion date April 30, 1993. Principal Investigator Dr. Robin Shepard. This project is necessary because of the many columns which are pinned at the base on multi-column bents. As we move into the multi-column phase of seismic retrofit, we need this information. Report will be submitted spring 1993.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

R-4. Retrofitting of Bridge Columns, Stage II, using third to half scale model tests. UC, San Diego. Contract $331,000. Completion June 30, 1991. This is the second contract with Dr. Priestley and is a continuation of the previous work using different retrofit techniques and different column types. Work is completed and reports are submitted.

R-5. Experimental Testing of Epoxy Injected Steel Shell Retrofitted Sections From Collapsed Struve Slough Bridge, using full-scale tests. UC, Davis. Contract $53,000. Completion date June 30, 1993. Principal investigator Dr. Melvin Ramey. We salvaged several broken piles from the Struve Slough Bridge at Watsonville for this test. Since we have a large number of these type bridges in the state, both state and locally owned, it is necessary to test techniques for improving their seismic performance. Work completed. Reports will be submitted spring 1993.

R-6. Evaluation of the Dumbarton Bridge in the Loma Prieta Earthquake. UC, Berkeley. Contract $50,000. Completion date June 30, 1991. Principal investigator Dr. Gregory Fenves. The Dumbarton Bridge was instrumented with strong motion instruments and the records are available. It is the only long crossing of the San Francisco Bay which was designed to modern bridge earthquake codes and criteria and this performance evaluation will be useful to the profession. Work completed. Report submitted.

R-7. Seismic Response of Deep Soil Sites in the San Francisco Bay Area. UC, Berkeley. Contract $315,000. Completion date December 31, 1992. Principal investigators Dr. John Lysmer, Dr. Raymond Seed. As a major part of the comprehensive earthquake vulnerability evaluations of important transportation structures we have a need to first determine the foundation response. This research is a direct result of problems with structures constructed on deep, soft soils during the Loma Prieta earthquake. The results of this project will be a new set of Acceleration Response Spectra (ARS curves) for deep, soft soils. Work completed. Report due spring 1993.

R-8. Retrofitting of Bridge Columns, Stage III, using third to half scale model tests. UC, San Diego. Contract $768,000. Completion July 12, 1993. This is the third contract with Dr. Priestley and is a continuation of the previous work using different retrofit techniques and different column types. This project, however, concentrated on the multiple column bridge bent configuration where the columns typically have moment connections at both top and bottom, but in many cases are pinned at the base. Work completed. Report due spring 1993.

R-9. Seismic Retrofit of Bridge Column Footings, using third to half scale model tests. UC, San Diego. Contract $374,000. Completion July 12, 1993. This is the fourth contract with Dr. Priestley and is the final contract m the series to evaluate and test the best techniques to retrofit the footings of older bridges where the original design moments introduced into the footings were much smaller than now anticipated after columns are retrofitted to carry more moment. Work underway.

R-10. Evaluation and Retrofitting of Multi-Level and Multi-Column Structures, using third to half-scale model tests. UC, Berkeley. Contract $1,900,000. Completion December 31, 1993. Principal investigator, Dr. Stephen Mahin. This research is essential for the future evaluation of older and newer multi-level structures. Interim results will be used to confirm the techniques and details being used in the current retrofit program. Work underway.

R-11. Develop Bridge Seismic Design Criteria with Higher Degree of Safety and Reliability than Provided with Current Design Procedures. Applied Technology Council. Contract $800,000. Completion date is December 15, 1993. This project is to evaluate the current CALTRANS/AASHTO bridge design criteria/code and recommend improvements and changes.

R-12. Evaluation of the Performance of Bridge Cable Restrainers During the Loma Prieta Earthquake. University of Nevada, Reno. Contract $91,000. Completion date June, 1993. Principal investigator, Dr. M. Saiidi. Work complete. Final report due spring 1993.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

R-13. Reduced Scale Tests of Pier Walls Under Cyclic Loading for Seismic Retrofit. UC, Irvine. Contract $461,000. Completion date July 1992. Principal investigator Dr. Robin Shepard. Because of the many bridge piers of this design we need test results to determine the best retrofit technique and to provide better design criteria for new pier wall designs. Work completed. Report due spring 1993.

R-14. Development of High Strength Fiber Composite Column Wrap. Fyfe Associates, Inc. Contract $73,000. Completion date December, 1991. This is an alternative method to provide confinement on existing bridge columns. It has been used in Japan for industrial smoke stacks using carbon fiber, which is ten times the cost of other fiber composites. Work complete. Report submitted.

R-15. Inspection and Data Collection on the Substructure of the Bay Bridge. UC, Berkeley. Contract $49,900. Completion date not yet negotiated. The principal investigator, Dr. Astaneh needed additional funding to complete work begun under an NSF grant. Work complete. Report included with major contract on SFOBB.

R-16. Flexural Integrity of Column/Cap Connections Using Number 18 bars. UC, San Diego. Contract $500,000. Completion date December 1992. The principal investigator, Dr. Priestley will build and test full size components to test the bond development length required for these large bars. Most large bridge columns in California utilize these bars and there is some question regarding the adequacy of the AASHTO code requirements. Work complete. Draft report submitted.

R-17. Seismic Evaluation of the San Francisco-Oakland Bay Bridge. GENESYS. Proposed contract amount $160,000. This proposal is for a rapid evaluation of the west bay spans using current technology. Has been evaluated by the Seismic Research Advisory Panel. Contract negotiations underway.

R-18. Evaluation of Earthquake-Induced Cyclic and Permanent Ground Displacements for Soil Sites. Earth Mechanics. Proposed contract amount $50,000. Still being evaluated.

R-19. Development and Implementation of Improved Seismic Design and Retrofit Procedures for Bridge Abutments. University of Southern California. Proposed contract amount $100,000. Completion date not yet determined. Still being negotiated.

R-20. Experimental Measurements of Bridge Abutment Stiffness and Strength Characteristics. UC, Davis. Contract amount $350,000. Completion date June, 1993. Principal investigator, Dr. Karl Romstad. This project will incorporate the centrifuge to test models of various combinations of bridge abutment-soil interaction. Work still underway. Report due late spring 1993.

R-21. A Simplified, Verified Procedure to Analyze Soil-Pile Structure Interaction During Earthquake Loading Conditions Using an Effective Stress Method. UC, Davis. Contract amount $25,000. Completion date December 1991. Principal investigator Dr. I.M. Idriss. Draft Report due spring 1993.

R-22. Response of Pile-Supported Bridge Elements due to Liquefaction. National Cooperative Highway Research Program (NCHRP). Contract cost will be borne by NCHRP if this project is approved. It has national application. Being evaluated by research committee.

R-23. Seismic Condition Assessment of the Bay Bridge. UC, Berkeley. Contract amount $800,000. Contract completion date June 30, 1993. Principal investigator, Dr. Abduhollah Astaneh. This project is the major effort to model and conduct an extensive dynamic analysis of the bay bridge with time history to evaluate its response to a larger earthquake such as an 8.0 on the San Andreas or a 7.3 on the Hayward fault. We have decided to begin with the San Francisco-Oakland Bay Bridge for obvious reasons. This project will include a comprehensive analysis of the foundation material response and upon completion we plan to retrofit the bridge to withstand the forces and movements recommended. The project is the beginning of the assessment of all major bay and river crossings in California. Work completed. Draft report submitted. Final report due June 1993.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

R-24.Seismic Hazard Risk Analysis for the San Francisco Bay Region. This proposed project will evaluate the seismic hazard risks for the region and provide appropriate ground acceleration input for geotechnical evaluations of specific structure sites. Geomatrix Consultants completed work. Final report delivered February 1993.

R-25. Seismic hazard risk analysis for Southern California. This proposed project will evaluate the seismic hazard risks for the region and provide appropriate ground acceleration input for geotechnical evaluations of specific structure sites. Consultant, Woodward Clyde Associates. Contract completion date, June 1993. Work completed. Draft report due June 1993.

R-26. Seismic Condition Assessments of Remaining Toll Bridges. These projects will be advertised by the RFP process and appropriate principal investigators and consultants will be selected for the 9 remaining major toll crossings on the state highway system. They are the Dumbarton Bridge, the San Mateo-Hayward Bridge, The Richmond-San Rafael Bridge, the Carquinez Bridge (1927 and 1955 structures), the Benicia-Martinez Bridge, the Antioch Bridge, the Terminal Island Suspension Bridge (Vincent Thomas Bridge). the Gerald Desmond Bridge on Terminal Island (soon to be taken into the state highway system) and the San Diego-Coronado Bridge.

R-27. Implement Advanced Soil Structure Interaction Techniques for the Analysis of Bridge Structures. Consultant, Coast Analytics, Incorporated. Contract completion April 1993. Contract amount $88,500. Work underway. Report due May 1993.

R-28. Response of Soft Soil Sites Using the Centrifuge. UC Davis. Various Site Conditions will be Tested at Varying Levels of Shaking to Augment Recordings From Recent Earthquakes and to Calibrate Analytic Procedures. Contract Amount $125,000. Contract Completion Date October, 1993. Principal Investigator Dr. I.M. Idriss.

R-29. Construction of Shaker for the Large Centrifuge. U.C. Davis. Partial Support for Construction of This Shaker for Future Research Involving Soil-Pile and Soil-Structure Interaction, Liquefaction, Site Improvement and other projects. Contract Amount $82,500. Contract Completion Date November, 1993. Principal Investigator Dr. I.M. Idriss.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

DISCUSSANTS' COMMENTS: BRIDGES

James D. Cooper, Federal Highway Administration

We owe a lot to CALTRANS for the way they reacted to the earthquake—namely, in their openness in allowing investigators to come in, examine, and quantify the damage and in their dissemination of information to researchers. This is the chief reason we learn from earthquakes.

My interest stems from what the lessons from the Loma Prieta earthquake mean nationally. For example, in California, there are 24,000 bridges; to date, 1,300-1,400 of those have been retrofitted. Nationally, there is an inventory of 577,000 bridges with as many as 75 percent of those being bridges at risk, either because of location or no design for seismic resistance, and few have been retro-fitted.

We have learned lessons from previous earthquakes—the San Fernando event being one of the most significant events for the bridge community. Lessons from previous events point to the need to define and accommodate forces, accommodate displacements, evaluate ground-motion amplification and attenuation, and identify liquefaction potential from site-specific studies or from macro analysis. We have also learned that retrofit enhances performance.

Loma Prieta was a moderate earthquake, with a short duration of strong ground motion. It produced two to three cycles of inelastic response. We need to consider carefully whether the earthquake can be used as a base for design of structures in other areas of the country, particularly those east of the Rockies, or whether eastern events will produce larger cycles of inelastic response. The latter will have significant impact on the design/retrofit philosophy.

Nevertheless, there were technical lessons learned from the Loma Prieta event:

  • Simple retrofit helps. Following the San Fernando earthquake, CALTRANS embarked on a program to identify vulnerable bridges and details and implement a simple, relatively inexpensive retrofit technique to provide displacement control across expansion joints. The relatively good performance of those bridges retrofit with hinge restrainers is testimony that large numbers of structures can be economically retrofit to enhance seismic resistance.

  • Detailing is critical. Bridges designed and constructed following the San Fernando earthquake performed relatively well. Column and column-connection details were revised to accommodate earthquake-generated shears, moments, and pull-out forces. In addition, the development of improved analytical technology was key to improved detail design.

  • Vulnerability assessment is required. Identification of hazard exposure, coupled with site and structural analyses are required to determine structural vulnerability. Only when the system as a whole is compared can a rational decision be made as to which structures—and to what extent—to retrofit.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×
Mitigation Strategy
  • Design new structures to current criteria. The increased cost associated with providing seismic resistance in newly designed and constructed bridges can vary significantly. However, typical cost increases are about 2-3 percent, a very affordable figure. If all new (or replacement) bridges were designed by the latest standards, it would take upwards of 100 years to reduce the seismic vulnerability of the highway system. In the long term, this strategy is affordable and will prevail.

  • Retrofit the most important/critical bridges. Since most retrofit is very costly, retrofit only those structures that are defined as important with simple, relatively inexpensive technology. Retrofit only those bridges that are defined as critical with the more complex (thereby costly) technology as it evolves.

  • Multidisciplinary, approach required. The Loma Prieta earthquake taught us that scientific and engineering knowledge is advancing; many public policy, legal, and financial issues remain to be resolved; public consciousness and awareness about the catastrophic consequences of a great earthquake are being raised, and emergency planning and response procedures are advancing. It is clear that a sound earthquake-hazard-mitigation strategy will require the coordinated and cooperative involvement of professionals of varied disciplines.

Initiatives Required
  • Awareness. Continue promoting technical and public awareness programs.

  • Evaluation. Complete seismic evaluation of the bridge inventory.

  • Design update. Develop philosophically consistent design criteria.

  • Retrofit criteria. Develop and adopt a rational retrofit criteria.

In conclusion, earthquake-hazard-mitigation is a long-term endeavor. As we implement new technologies, we are making a significant impact on improving the seismic performance of our highway system.

Gregory Orsolini, Parsons Deleuw, Inc.

The I-280 Southern Freeway viaduct in San Francisco is a project I am intimately familiar with, having been working on it since just days after the Loma Prieta earthquake. This is a structure on a very high acceleration site and the poorest soils in the Bay Area. We were asked to adhere to the basic philosophy of CALTRANS—to prevent collapse—but also to have a serviceability mechanism addressed. The project was to replace the columns, the joint areas, and the foundations. I'll be concentrating on the reconstruction of these joints and outriggers (some in excess of 50 ft in length).

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×
Beam-Column Joint Shear Design

Current design practice for beam-column joint shear for bridges results in heavily reinforced connections. Regarding beam-column joints in bridges, there are some unique considerations—the tendency to use some sizeable bar sizes (#14 and #18 are not uncommon) in retrofit and new construction, and to use large joint shear stirrups. This leads to complications for placement because of the consideration of the bend diameters. We are still understanding/learning about density of reinforcement in the joints.

In building these joints, we are fighting uncertainty in the construction industry about how these things are put together. As the contractors learn what we're trying to do, the learning curve should accelerate quickly. To get adequate implementation, we are interacting with contractors and inspectors on how this is actually done.

Bent Cap Outrigger Design

The original design used a column that was pinned at the top and the bottom, which reduced the lateral bending in these outrigger caps to zero. Our peer review panel asked us to increase redundancy. We did that by adding a fixed joint at the bottom, which adds lateral bending problems to the design of outriggers. That is where we got the flared configuration.

For vertical loads, we have post-tensioning in a parabolic shape but also post-tensioning following a bit of a flare. We added bolsters within the box girder to take the reaction of those large flares.

We approached the combination of vertical and lateral bending by trying to keep the strains and the pre-stressing strains to below the proportional limit (.008 strain). To consider the vertical accelerations, we multiplied the vertical dead-load by 50 percent—either increasing or decreasing the vertical load by that amount.

Use of Simplified Nonlinear Analysis Methods for Seismic Analysis

There are many options for nonlinear analysis available. I would like to encourage the use of a more simplified type of analysis, as it has a lot of benefits and is a good tool to use.

A number of useful nonlinear analysis methods are currently being used for assessing the ductile behavior of existing structures—displacement ductility, equal energy, and equal displacement concepts can be applied to bridge-retrofit work. It does not necessarily have to be a complex procedure to use nonlinear analysis.

Thank you very much.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

Nicholas F. Forell, Forell/Elsesser Engineers

After the Loma Prieta earthquake, it was determined that all of the six San Francisco double-deck freeways were in need of retrofit. Most of the freeways (the exceptions were the Alemani Viaduct and the Terminal Separation structure) were damaged and closed to traffic. These structures were built in the late 1950s and have similar construction to the Cypress Viaduct, which failed catastrophically.

Immediately after the earthquake on November 1, 1989, CALTRANS retained six major engineering firms with adequate staffing to immediately design and implement repairs and retrofits. The Governor's Board of Inquiry, in its hearings, concluded that because there was no precedent for this type of retrofit, CALTRANS should initiate a peer review process to assure compliance of the designs with the performance criteria established.

The peer review panel selected by CALTRANS consisted of six practicing engineers experienced in seismic design and four technical advisors.

The technical advisors (two professors from UC Berkeley and two from UC San Diego) were extremely important because of their in-depth knowledge of analysis and concrete design as well as their research experience. The technical advisors supplemented the technical knowledge of the panel members and the consultants.

An early and detailed establishment of the scope of work of the peer review panel is important. In this case, the panel reviewed the seismic design criteria and the geologic-hazard report, the design and performance criteria, applicable and available research results, analysis and modeling assumptions, design details, construction documents, and constructability. The panel did not perform a check on the drawings' calculations; therefore peer review cannot be considered a substitute for independent plan checks.

It is most important that the peer review panel be convened at the very beginning of the project.

Although the consultants were on board immediately after the earthquake, the peer review panel, for many reasons, could not be convened until March 1990. This resulted in wasted effort and money. Some of the work under construction had to be halted or abandoned and much of the design redone. It is therefore important to have the peer review panel functioning as the retrofit concepts are formed.

On unique and complex projects, the peer review process can be laborious. On the double-deck retrofit projects, 56 hearings were held. Yet, the peer review process proved itself invaluable and provided CALTRANS with the assurance that the retrofit design will meet the established performance criteria.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
×

John Clark, Anderson Bjornstad Kane Jacobs

I would like to address what worked. We've seen a great deal of spectacular things that failed, but it is important to address what did work—anything designed to the current code (i.e., the post-1983 guidelines that Mr. Cooper alluded to—the ATC-6 document).

The ATC-6 document was a revelation that we are dealing with a displacement phenomena. Under the old—even the post-1971 San Fernando codes—we simply doubled our forces and went about things the same way. The code we have now is a good code. I don't mean to say that it solves all our problems. The current code has brought a strong need to consider the foundation effects—the liquefaction potential. Unfortunately, in building bridges, we don't have the luxury to choose the best sites. Consideration should be given to soil-structure interaction effect—not only as it may tend to amplify things but also as it may tend to reduce the response through damping and other effects. The strength of the current code is its concentration on providing ductile details and in providing adequate seat length. Mr. Cooper made an important point about the efficacy of simple retrofits—the joint restrainers. The empirical design tools we have for that are very crude, but they work.

Seismic isolation is another very promising tool. There was not anything in the bridge field during the Loma Prieta earthquake that gave us a good test, but it is a good principle. There are things we need to learn about it yet—particularly the increased vulnerability at the joints and the means to address that.

We also need to focus our research on whether we can relax some of the confinement provisions we have in the code. That's a constructability issue more than anything else. There are tools out there that we need to get into our code—such as a reduction factor based on our percentage of axial load. We have not yet incorporated that into the code because people think they are being conservative. I would beg to differ with that.

A few brief comments on what I might call the retrofit ''philosophy'': There is a critical need to think clearly about what we are doing in codifying the retrofit process in general, because it is a very cost-benefit-sensitive issue. The need for retrofitting far outweighs the available resources. Do decision makers (legislators and the general public) know the risk involved, and are they willing to accept it? For consultants, is this codified to protect us from our legal brethren? As Greg Orsolini pointed out, we need to codify or provide more guidance on how to use the simple nonlinear techniques and to focus on system ductility as opposed to member component ductility and strength. Finally, I would ask researchers to help in the field. How do we really design for the long-duration effects? Are the attenuation relationships the same in these long-duration earthquakes? Are there frequency shifts due to more rapid attenuation of the higher frequencies? These are some future points that we critically need some help with.

Thank you.

Suggested Citation:"6. Highway Bridges." National Research Council. 1994. Practical Lessons from the Loma Prieta Earthquake. Washington, DC: The National Academies Press. doi: 10.17226/2269.
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The Loma Prieta earthquake struck the San Francisco area on October 17, 1989, causing 63 deaths and $10 billion worth of damage. This book reviews existing research on the Loma Prieta quake and draws from it practical lessons that could be applied to other earthquake-prone areas of the country. The volume contains seven keynote papers presented at a symposium on the earthquake and includes an overview written by the committee offering recommendations to improve seismic safety and earthquake awareness in parts of the country susceptible to earthquakes.

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