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|Federal Highway Administration > Publications > Public Roads > Vol. 74 · No. 2 > Earthquake!|
Publication Number: FHWA-HRT-10-006
by Wen-Huei (Phillip) Yen
FHWA is conducting research to help mitigate the impacts of seismic events on transportation infrastructure. The results are promising.
The public relies on highways for the safe transport of goods and people across the country. Because roads serve as critical lifelines in the delivery of basic daily needs, they need to function even in the face of adverse weather and natural hazards. From 1993-1996, the United States spent an average of approximately $250 million per week responding to the impacts of natural disasters, with earthquakes, hurricanes, and floods being the major causes of monetary losses. At times, earthquakes can top the list. One of the most costly natural disasters in the United States between the late 1980s and late 1990s was California's Northridge Earthquake of 1994, which resulted in a total of $20 billion in infrastructure damages.
An earthquake is a sudden ground motion or trembling caused by an abrupt release of accumulated strains acting on the tectonic plates that comprise the Earth's crust. Earthquakes often trigger other devastating events such as landslides, fires, and lateral spreads (displacements of sloping ground, primarily due to soil liquefaction during earthquakes). In addition to destroying buildings, earthquakes can damage bridges, tunnels, pavements, and other components of highway infrastructure. If an earthquake occurs in an ocean, it can trigger a tsunami that can devastate coastal roads and bridges.
Relatively speaking, the probability of large, destructive earthquakes is much lower than hurricanes and floods. Nevertheless, an earthquake can, without warning, ravage an enormous area in less than 2 minutes through ground shaking, surface fault rupture (displacement due to the movement of tectonic plates), and ground failures (landslides, liquefaction, and lateral spreads).
The loss of life and extensive property damage inflicted by the 1989 Loma Prieta and 1994 Northridge earthquakes emphasized the need to minimize seismic risks to the U.S. highway system. Seismic research projects conducted by the Federal Highway Administration (FHWA) are developing mitigation approaches to reduce those risks, including a method for assessing seismic risks and various structural designs and retrofitting measures.
"Since 1992, FHWA has conducted a series of comprehensive seismic research studies targeting retrofitting, design, and risk analysis issues for bridges," says Jorge E. Pagán-Ortiz, director of FHWA's Office of Infrastructure Research & Development. "FHWA's seismic research has produced a number of nationally applicable seismic retrofitting manuals and design and risk analysis tools."
What follows is the story of that research.
Early Earthquake Mitigation Research
First, a look at the early research. FHWA initiated its earthquake investigations after the 1964 Prince William Sound Earthquake in Alaska. FHWA's follow up focused on how bridge engineers could learn from the Alaska earthquake in terms of geotechnical issues such as soil properties.
Then, following the poor performance of bridges during the San Fernando Earthquake in 1971, FHWA and the California Department of Transportation (Caltrans) began exhaustive studies of the seismic performance of bridges. FHWA and Caltrans invested $3 million in basic research to develop national guidelines for bridge seismic design. The study evaluated the criteria used at the time for seismic design, reviewed findings from seismic research for potential use in a new specification, updated guidelines for seismic design, and evaluated the impact of those guidelines on construction and costs.
In 1981, FHWA and Caltrans completed the guidelines, which the American Association of State Highway and Transportation Officials (AASHTO) adopted in 1983 as its Guide Specification for Seismic Design of Highway Bridges. This specification became a national standard in 1992, following the Loma Prieta Earthquake.
Significant Earthquake Damages in the United States, 1964-1994
Sources: Stover and Coffman, 1993; FEMA 1994.
The design philosophy underlying this specification was to prevent collapse of any span or part of a span during large earthquakes. In small to moderate seismic events, the code's intent was for bridges to resist seismic loads without significant damage to structural components. Under this code, the design earthquake had a 475-year return period, which represents not greater than a 10 percent probability of an earthquake occurring during a bridge design life of 50 years.
ISTEA and the Seismic Research Program
FHWA's role in earthquake research did not end with the adoption of this 1992 standard. The agency renewed its commitment to mitigating effects on highway structures by establishing a seismic research program, as called for in the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991. The studies were conducted for FHWA under a contract at the National Center for Earthquake Engineering Research, later renamed as the Multidisciplinary Center for Earthquake Engineering Research (MCEER).
Under ISTEA, Congress funded the research with more than $14.25 million between 1991 and 1997. The program covered all major highway system components (bridges, tunnels, embankments, retaining structures, and pavements).
Approximately 65 percent of the Nation's 600,000 highway bridges were constructed prior to 1971, with little or no consideration given to seismic resistance. In recognition of that situation, the FHWA seismic research program initiated two comprehensive studies. In the fall of 1992, the program began studying the seismic retrofitting of existing bridges and highway structures, and in spring 1993 began studying the seismic design of new bridges.
The first product of this research, Seismic Retrofitting Manual for Highway Bridges (FHWA-RD-94-052), appeared in 1995 and summarized lessons learned from more than 20 years of earthquake engineering research and implementation, and provided procedures for evaluating and upgrading the seismic resistance of existing bridges.
In 1999 the program published Impact Assessment of Selected MCEER Highway Project Research on the Seismic Design of Highway Structures (MCEER-99-0009), which became the major documentation used to develop recommendations for the seismic design of new bridges. In 2006 FHWA issued the final products of this research, Seismic Retrofitting Manual of Highway Structures-Part I (Bridges) (FHWA-HRT-06-032) and Part II (Retaining Structures, Slopes, Tunnels, Culverts, and Roadways) (FHWA-HRT-05-067).
These recommended seismic design specifications, proposed in 2001 under the National Cooperative Highway Research Program (NCHRP) 12-49 project, Comprehensive Specification for the Seismic Design of Highway Bridges, were performance-based. The major difference between them and the 1992 design code was that they had a two-level design criterion. The higher level was based on a 2,500-year return period, and the lower on a 100-year period. The new seismic retrofitting manuals are also performance-based and based on a two-level design criterion, but a 1,000-year return period for the high level and 100 years for the lower level.
Seismic Research Under TEA-21
While the researchers were finishing their work under ISTEA by developing the 1999 design recommendations, in 1998 FHWA launched a congressionally mandated seismic research program under the Transportation Equity Act for the 21st Century (TEA-21), funded by another $12 million, to study seismic vulnerability. In cooperation with MCEER, FHWA conducted a series of studies to develop tools for evaluating and assessing the social costs and impacts of earthquakes on the U.S. highway system. The goal was to reduce the likelihood of damage to existing and future highway structures caused by moderate to significant seismic events.
The main tasks undertaken within this program were the following:
Under TEA-21, FHWA worked with NCHRP in 2001 to develop new seismic design specifications, NCHRP 12-49. AASHTO then reviewed and revised the new design specifications and adopted them in 2007. However, publication was delayed until 2009, when they were published as the AASHTO Guide Specifications for LRFD [Load and Resistance Factor Design] Seismic Bridge Design, 1st edition. The NCHRP 12-49 specification was developed from the 1999 recommendations. The 2007 specification is a one-level design criterion for a 1,000-year return period.
Under the TEA-21 seismic research program, FHWA developed a software package called REDARS: Risks from Earthquake DAmage to Roadway Systems to estimate the loss of highway system capacity due to earthquakes. The tool helps bridge owners estimate how earthquake damages affect post-earthquake traffic flows and enables them to consider those effects during pre-earthquake planning and prioritizations, and in post-earthquake responses, such as rescue and management of damage investigations. The seismic research program released REDARS in 2006.
Also in 2006, the program published the Seismic Retrofitting Guidelines for Complex Steel Truss Highway Bridges(MCEER-06-SP05), which particularly addresses truss bridges that are more than 500 feet (152 meters) long. The guidelines use a performance-based seismic retrofit philosophy, focus on superstructure retrofit, and provide case studies. A Seismic Isolation of Highway Bridges (MCEER-06-SP07) manual also was published in 2006. It presents the principles of isolation for bridges, develops step-by-step methods of analysis, explains material and design issues for elastomeric and sliding isolators, and provides detailed examples of their application to standard highway bridges. The manual is a supplement to the Guide Specifications for Seismic Isolation Design published by AASHTO in 1999.
SAFETEA-LU Seismic Research
In 2005, Congress passed the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU). Under the legislation, FHWA oversaw $12.5 million in seismic research to work with the bridge engineering community and enhance the earthquake resistance of U.S. highway bridges. The two recipients of this congressional earmark research were MCEER and the University of Nevada, Reno.
Also, SAFETEA-LU mandated a technology exchange and transfer task, which FHWA conducted through a series of bridge engineering workshops and conferences held nationally and internationally. The meetings involved exchange of technical information and performance of cooperative studies.
One of these technical exchange programs is a panel on wind and seismic effects under the U.S./Japan Cooperative Program in Natural Resources. The outcomes of this succession of programs held over the past four decades include greater understanding in three areas: assessing seismic vulnerability of specific locations, geotechnical hazards, and infrastructure vulnerability. Building on this increased body of knowledge, FHWA currently is developing improved seismic designs for new and retrofitted bridges, plus instrumentation to monitor performance.
Assessing Seismic Vulnerability: Hazard Maps
To design a bridge to resist earthquakes, understanding the seismic vulnerability or earthquake intensity of the bridge's location is essential. This vulnerability usually is described as seismic hazard. The U.S. Geological Survey (USGS) publishes National Seismic Hazard Maps that display various probability levels of earthquake ground motions across the United States. The seismic provisions of building codes, insurance rate structures, risk assessments, and other public policy provisions commonly apply probability levels based on the hazard maps.
A 2003 update of the maps incorporates new findings on earthquake ground shaking, faults, and seismicity (that is, how prone a region is to earthquakes). USGS derived the new maps for a grid of sites across the United States by calculating seismic hazard curves that describe the frequency of exceeding a set of ground motions. Currently, the new seismic design and retrofitting criteria for bridges use a 1,000-year return period for a given level of earthquake, which represents not greater than a 7 percent probability of an earthquake occurring during a bridge design life of 75 years. USGS and AASHTO issued the updated maps and computer software for obtaining seismic hazards by entering ZIP Codes or longitude and latitude coordinates.
Assessing Geotechnical Hazards
Another factor in designing and retrofitting highway bridges is the geotechnical hazards that an earthquake can trigger, such as soil liquefaction and settlement, slope failure (landslides and rockfalls), surface fault ruptures, tsunamis, and flooding. Assessing geotechnical hazards is a two-part procedure. First, engineers conduct a quick screening evaluation, generally using information available from field reconnaissance.
If various criteria are satisfied, they consider the risk to be low and require no further evaluations. If a hazard cannot be screened out, they conduct more detailed and rigorous evaluations, which usually require obtaining additional data to assess the hazard and its consequences.
Assessing Infrastructure Vulnerability
To assess the seismic vulnerability of the U.S. bridge inventory, researchers often use an indices method to determine a bridge's seismic rating. The method involves assessing the structural vulnerability of the bridge, the seismic and geotechnical hazards of the site, the socioeconomic factors affecting the structure's importance, and other issues such as the structural redundancy with the bridge and nonseismic structural issues. Through this method, researchers arrive at a final, ordered determination of the retrofitting priority of individual bridges and, ultimately, for the Nation's entire infrastructure inventory.
The rating system has two parts: the quantitative part, which produces a seismic rating ("bridge rank") based on structural vulnerability and site hazard; and the qualitative part, which modifies the rank in a subjective way that accounts for importance, network redundancy, nonseismic deficiencies, remaining useful life, and similar issues to arrive at an overall priority index.
Mitigation Design Of New Bridges
Based on advanced seismic research and experience with destructive earthquakes, AASHTO and FHWA have improved seismic designs for new bridges. The results include design details that directly affect bridge performance under increased loadings due to earthquakes.
"The performance of U.S. highway bridges in recent large earthquakes has shown that the current state of the art has saved many bridges from collapse by preventing unseating of the superstructure or shear failure of the columns," says FHWA's Pagán-Ortiz.
The fundamental design objective of current seismic specifications in small to moderate events is to resist seismic loads within an elastic range without significant damage to structural components. The objective in large earthquakes is that no span, or part of a span, should collapse. The specifications consider limited damage to be acceptable in these circumstances, provided it is confined to flexural hinging (that is, hinging that allows an angle to be adjusted while remaining in place) in pier columns. This is to allow steel rebars to yield and absorb earthquake excitation energy while not rupturing and leading to collapse. Further, damage above ground is preferable so that it is visible in sections of the bridge that are accessible for inspection and repair.
Under current specifications, the seismic performance objective is no collapse based on a one-level rather than a two-level design approach. The current one-level design criterion is based on a 1,000-year return period event with not greater than a 7 percent probability of occurring during a bridge's 75-year design life. As an operational objective, bridge designers may use a higher, two-level performance criterion, but only with authorization from the bridge owners. Current specifications, however, do not provide guidance beyond the one-level approach.
Seismic Retrofitting Of Existing Bridges
Retrofitting is the most common method of mitigating risks; in some cases, however, the cost might be so prohibitive that abandoning the bridge (total or partial closure with restricted access) or replacing it altogether with a new structure may be favored. Alternatively, doing nothing and accepting the consequences of damage is also a possible option. The decision to retrofit, abandon, replace, or do nothing requires careful evaluation of the importance of the bridge and its degree of vulnerability. Limited resources generally require that deficient bridges be prioritized, with important bridges in high-risk areas being retrofitted first.
Bridges constructed prior to 1971 in particular need to be retrofitted, based on seismicity and structural types. Toward this end, FHWA issued several publications, including Seismic Retrofitting Guidelines for Highway Bridges (FHWA-RD-83-007) in 1983 and Seismic Design and Retrofit Manual for Highway Bridges (FHWA-IP-87-6) in 1987. In 1995, FHWA updated these manuals with current knowledge and practical technology in the Seismic Retrofitting Manual for Highway Bridges (FHWA-RD-94-052), mentioned earlier.
Then, also as mentioned earlier, FHWA published Seismic Retrofitting Manual of Highway Structures-Part I and Part II. This two-volume manual contains the following procedures for evaluating and upgrading the seismic resistance of existing highway bridges:
The manual does not prescribe rigid requirements as to when and how bridges are to be retrofitted. The decision to retrofit depends on a number of factors, several of which are outside the engineering realm. These other factors include, but are not limited to, the availability of funding and a number of political, social, and economic issues. A bridge may be exempt from retrofitting if it is located in a seismic zone with very little ground motion or has limited remaining useful life. Temporary bridges and those closed to traffic also may be exempt if they are not crossing a major national highway (lifeline system) or defense highway.
Recognizing the earthquake vulnerabilities of highway bridges constructed prior to 1971, many State departments of transportation, including California, Illinois, Missouri, Oregon, Tennessee, and Washington, initiated and performed retrofitting funded by FHWA to increase seismic safety. Many retrofits involve hinge seat extensions, which enlarge the size of the hinges that connect sections of bridge decks, or installation of a restrainer to link superstructures (decks) together and help prevent them from separating during severe ground movement. Some single columns were retrofitted with a steel casing to increase the earthquake resistance (ductility) to prevent collapse. FHWA's new seismic retrofitting manuals provide details on this retrofitting process.
Performance Monitoring In Missouri
The seismically active New Madrid Fault region in Missouri and adjacent States requires a hazard mitigation program that addresses the possibility of strong shaking of structures and the potential for ground failures in the vicinity of bridges. Designers of the cable-stayed Bill Emerson Memorial Bridge in Cape Girardeau, MO, which is within the New Madrid Fault region, had to take into account the possibility of a strong earthquake (magnitude 7.5 or greater) occurring during the design life of the bridge.
To capture data on strong ground motions, FHWA worked with the Missouri Department of Transportation (MoDOT) and USGS to complete a seismic instrumentation plan for the bridge before the start of construction. To assess differential motions at the piers along the total bridge span of 3,956 feet (1,206 meters), the instrumentation includes 84 accelerometers attached to the pier foundations and superstructure (caissons, tower, and deck). In addition to recording events at the site, the system can broadcast the data to outside users. This real-time seismic monitoring system can support signal transmission via the Internet from combinations of one-dimensional and three-dimensional accelerometers to recorders at the site.
Synchronized systemwide timing of the accelerometers can ensure time-variant response recording at one location in the bridge relative to other locations. Real-time streaming of the data will facilitate remote maintenance and data acquisition and retrieval capabilities. The bridge owner, researchers, and engineers now are able to use the response data to assess the bridge performance; check design parameters, including comparison of dynamic characteristics with actual responses; and improve the design of similar bridges in the future.
By appropriate configuration of the streamed data, the researchers also can use the instrumentation as a health monitoring tool to serve as an early warning system for defects or unexpected behaviors, and to assess damage to the bridge. The need to monitor the response of bridges in real time or near real time usually arises when information on rapid responses is required, such as during homeland security emergencies.
The recent major earthquakes in China in 2007 and Chile and Haiti in 2010 have challenged earthquake engineering disciplines around the world. The intensity of peak ground accelerations and long duration of shaking resulting from large earthquakes create greater difficulties for designing and retrofitting highway bridges. Through its seismic research program, FHWA is exchanging technical information and collaborating on research with seismically active States in the United States and with other countries, including Chile, China, Italy, Japan, Taiwan, and Turkey.
Over the past 15 years, the program has sponsored a series of conferences around the United States and bilateral workshops with other countries to promote new technology and exchange technical information. In 2009, the 25th U.S.-Japan Bridge Engineering Workshop, held in Tsukuba, Japan, marked the silver anniversary of this technology exchange and cooperation.
FHWA continues to work with MCEER, located at The State University of New York at Buffalo, and the University of Nevada, Reno. Under current legislation, two major initiatives are underway, focusing on innovative protection technologies and seismic resilience for larger earthquakes yet to come.
Developing innovative protection technologies. This initiative is to improve the seismic resistance of the U.S. highway system by developing innovative technologies, expanding their applicability, and developing cost-effective methods for implementing design and retrofitting technologies. As FHWA applies accelerated methodologies to construct new bridges and maintain existing bridges in high seismic areas, research is underway to develop more advanced design details to accommodate bridge movements due to large ground motions.
Improving seismic resilience. Life-safety (no collapse and no loss of human life) is no longer the sole requirement for success in designing a highway system capable of resisting the impacts of a major earthquake. The traveling public now expects resilience in the surface transportation infrastructure as well -- that is, rapid recovery and minimal impact on the socioeconomic fabric of modern society.
The need for resilience has led to development of the concept of performance-based seismic design. Performance measures calculated by REDARS include congestion and delay times. These measures allow system-level performance criteria to be specified for earthquakes of various sizes, such as maximum permissible traffic delay times and minimum restoration times. Thus, these measures allow resilience of a highway system to be defined and measured in quantitative terms, such as the time it takes to restore the system to pre-earthquake capacity. Accordingly, local transportation authorities can develop financial and societal incentives that will improve resilience and at the same time reduce risk to life and property.
FHWA and others have made substantial progress in this area, particularly with respect to the performance of individual components of the built environment, such as buildings and bridges. But the real potential for performance-based design comes when these concepts are applied to systems and subsystems of the infrastructure, such as transportation networks, subject to both service load conditions and extreme events.
This initiative will study the resilience of highway systems with a view to improving performance during major earthquakes. Refining the REDARS program's current loss estimation methodologies is included, along with providing a comprehensive assessment tool to measure highway resilience. Further, the project will identify factors affecting system resilience, such as damage tolerance of bridge structures and network redundancy, and will develop design aids for curved bridges and structures in near-fault regions.
The greatest difficulty in mitigating earthquake hazards is that seismic events occur without any notice and without any way of accurately predicting when they will occur, nor what their magnitude will be. Earthquakes are devastating, often resulting in a large number of deaths, injuries, and extensive infrastructure damage. These losses occur within minutes. Systematic approaches to evaluating earthquake risks, including indirect losses such as economic impacts, have become an important issue to the engineering community. Hazard mitigation methods to reduce earthquake losses require an enormous effort for development and implementation.
The Turner-Fairbank Highway Research Center has played an important role in developing guidance for seismic hazard reduction. The seismic research program is an important component of the multihazard research program within the Office of Infrastructure Research & Development, which includes wind, flooding and scour, and terrorism.
"FHWA is working closely with AASHTO and NCHRP and others to mitigate earthquake hazards and reduce losses," says Pagán-Ortiz. "These efforts to implement all practical measures to enhance the safety of the Nation's highway infrastructure and mobility of users are in a race against time with earthquakes. Fortunately, we think that the outlook is promising."
Wen-Huei (Phillip) Yen is a structural research engineer and manager of the seismic research program in FHWA's Office of Infrastructure Research & Development. He holds Ph.D. and master's degrees in civil engineering from the University of Virginia and is a registered professional engineer in Virginia.
For more information, contact Wen-Huei Yen at 202-493-3056 or email@example.com.
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