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|Federal Highway Administration > Publications > Public Roads > Vol. 73 · No. 6 > Hazard Mitigation R&D Series: Article 1: Taking a Key Role in Reducing Disaster Risks|
Publication Number: FHWA-HRT-10-004
Hazard Mitigation R&D Series: Article 1: Taking a Key Role in Reducing Disaster Risks
by Sheila Rimal Duwadi
FHWA's hazard mitigation R&D program tackles the challenges of providing safer highway bridges and other transportation infrastructure.
Each year the United States and the global community experience numerous natural and human-induced hazards that often turn into disasters and cause suffering, disrupt lives, and induce economic damage. According to the United Nations Environment Programme, the number of people killed in disasters worldwide during the 1990s averaged 75,252 annually.
The economic impact too is staggering. In 2005, the U.N. reported that damages caused by natural and human-induced hazards globally topped $150 billion. The Federal Emergency Management Agency estimates that Hurricane Katrina alone inflicted $75 billion in damages and was responsible for 1,200 reported deaths.
Natural hazards often have significant impacts on transportation infrastructure. Examples of natural hazards that might affect highways and bridges include coastal inundation, earthquakes, floods, hurricanes, landslides, tornados, tsunamis, volcanoes, wildfires, and winter storms. Not all these events are likely to occur in all parts of the United States, but natural hazards -- unlike human-induced events -- have a high probability of affecting large geographic areas and therefore a significant number of highways and bridges simultaneously, thus impacting more lives.
For example, in 2002 a barge struck the I-40 Bridge crossing the Arkansas River in Webbers Falls, OK, disrupting a major east-west transportation corridor. In 2005 Katrina's 30-foot (9-meter) storm surge in Mississippi destroyed about 90 percent of all highway bridges located within 0.5 mile (0.8 kilometer) of the coast, even washing away large structures like the Bay St. Louis and Biloxi Bay bridges.
Human-induced events include arson, technological hazards, cyber attacks, and terrorism. Events that can affect highway bridges are the hazards of fire, collision, overloads, blast, and others.
Negative impacts on transportation from natural and human-induced events are only one side of the story. Bridges and highways play a critical role in reducing fatalities and economic damages in times of crisis by providing evacuation routes and access for response and recovery teams conducting rescues and humanitarian assistance. If roads and bridges are flooded or damaged, a manageable event can quickly turn into a disaster because people are unable to go in and out of the affected area. Having access to alternate means of travel such as ferries, transit lines, and other modes is desirable but in many parts of the country these alternate modes are unavailable, leaving the highway system as the only reasonable route. Because of the need to keep the highway network open and passable, the Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center (TFHRC) is researching solutions to reduce the impacts of extreme events.
Recognizing that most hazards cannot be prevented, the White House's National Science and Technology Council established the Subcommittee on Disaster Reduction (SDR). The subcommittee is charged with prioritizing Federal investments in science and technology to enhance the Nation's disaster resilience. To accomplish this goal, the subcommittee crafted a 10-year strategy identifying six "grand challenges" to enhance community resilience and thus create a more disaster-resilient Nation. The U.S. Department of Transportation (USDOT) and FHWA support this initiative and, together with other SDR member agencies, carry out research and development (R&D) to address the six grand challenges.
The following overview of FHWA's hazard mitigation R&D program describes the threats to highway bridges and their vulnerabilities, and demonstrates how the program is reducing risks for the Nation's transportation infrastructure. Subsequent articles in future issues of Public Roads will detail ongoing R&D related to four specific hazards: flooding and scour, wind, earthquakes, and terrorism.
The highway system in the United States includes approximately 4 million miles (6.4 million kilometers) of roads, including 47,000 miles (75,600 kilometers) of interstates and 117,000 miles (188,300 kilometers) of other National Highway System roads, plus approximately 600,000 highway bridges and 366 tunnels. Certain links in this network -- that is, bridges on essential routes -- are critical in that their incapacitation would cause great physical and economic disruption. The vulnerability of bridges depends on their design, location, and material properties.
Highway bridges are engineered to carry specific loading needs that have evolved over time. Over the years, bridge engineers have drawn on individual experience combined with scientific and engineering principles to develop designs that address specific situations. With each new event, engineers re-evaluate the parameters, modifying codes and standards accordingly.
The load that a structure experiences and the load path vary, based on its design type and the hazard event. Bridge foundations, columns, and pier caps are critical when addressing seismic loadings because earthquake forces are generated from the ground up. On the other hand, long-span, slender bridges are more susceptible to wind loadings, making aerodynamic stability the critical factor for those structural types. As with seismic loadings, substructures are vulnerable when dealing with scour. The Nation has thousands of highway bridges with unknown foundations, which increase uncertainties about vulnerability to scour.
A bridge of any size, length, or shape, over water, can be susceptible to flooding. Wave-induced forces can lift a span off its bearings, as happened during Hurricane Katrina with New Orleans' I-10 bridge over Lake Pontchartrain. Because of the waves, 38 spans fell off their bearings and 170 others misaligned on the eastbound bridge, and 303 spans misaligned and 26 others were lost on the westbound bridge.
In addition to design, location is critical to vulnerability. For example, bridges over navigable waterways are vulnerable to vessel collisions, which impart high-energy loading at the point of contact. In recognition of this vulnerability, and also as a result of actual incidents, measures are used in heavily trafficked rivers to protect piers in navigation channels against ship and barge impacts. Most bridge failures that have occurred due to collision were the result of errant vessels hitting unprotected piers. In March 2009, a towing vessel pushing eight barges slammed into a bridge pier in Biloxi, MS, causing a section of the bridge to collapse into Biloxi Bay. Fortunately, there were no vehicles on the bridge, which normally carries 35,000 average daily traffic.
As with design and location, the strength and durability of bridge materials are critical. Concrete and steel are the major materials used for bridge construction. Other less commonly used materials are wood, aluminum, and fiber-reinforced polymers. Increasingly, higher strength and more durable steel and concrete are used on newer bridges. Researchers have yet to explore the behavior of materials under hazard loadings, especially the newer materials.
All structural materials used today are subject to damage by fire because heat can change the properties of any material. Recently, the National Cooperative Highway Research Program initiated efforts to assess bridge fire risk and to develop guidelines for bridge owners. Blasts produce high-intensity loadings in a timeframe of milliseconds, and the impact of explosive loadings on material performance needs to be studied further. The threat level of fire and blast is uncertain in terms of magnitude and probability of occurrence, so developing and implementing design and retrofit solutions presents a major challenge. Nevertheless, in recent years fire has damaged high-profile structures, such as the MacArthur Maze in the San Francisco Bay area on April 29, 2007.
FHWA Hazard Mitigation R&D Program: Focus Areas
In addition to the six grand challenges, SDR identified 14 implementation plans for major hazards. Although various programs within FHWA's offices deal with those SDR issues and hazards that affect transportation, the focus of the current R&D effort within the Office of Infrastructure R&D at TFHRC is on addressing the grand challenges for the following hazards as they affect bridges:
Although the SDR document does not consider security and terrorism issues, the FHWA program does address this hazard.
Flooding and Scour, Coastal Inundation
Flooding and scour are the leading cause of bridge failures in the United States. About 83 percent of the structures listed in the National Bridge Inventory cross waterways.
The failures of the Schoharie Creek Bridge in New York State in 1987 and the Hatchie River Bridge in Tennessee in 1989 led bridge engineers to an increased focus on flooding and scour issues, with advancements in foundation and pier design. With recent hurricanes, focus has expanded to include inundation of coastal bridges, damage due to wave forces, and tidal flow scour.
The Nation's coastal counties contain more than half of the U.S. population but only 13 to 17 percent of the total land area. These areas are prone to coastal hazards such as permanent inundation, temporary flooding, hurricanes, and tsunamis.
The hydraulics research program, conducted largely at the J. Sterling Jones Hydraulics Research Laboratory at TFHRC, carries out studies to advance understanding of the effects of flooding, scour, and coastal inundation on bridges to ensure that those structures are reliable and sustain minimal or no damage during extreme hydrodynamic events. Over the years, the hydraulics laboratory has developed, tested, and implemented multiple countermeasures to protect bridge piers from scour and superstructures from flooding and inundation.
The laboratory consists of a physical and a numerical modeling facility. The physical facility features a tilting flume, a force-balance flume, a wave tank, particle image velocimetry testing stations, and a culvert test facility. The numerical modeling facility includes systems to conduct simulations based on computational fluid dynamics. Researchers can use these numerical simulations in concert with physical experiments to address questions regarding coastal, inland, and environmental hydraulics with an emphasis on bridge scour.
Some of the studies in the hydraulics research roadmap include the following:
The laboratory's capabilities include testing the hydraulic properties of submerged bridges and highway drainage structures. Researchers can use the laboratory's equipment to solve stream stability problems, develop design standards for bridges in high-flood-risk areas, and contribute to design standards for scour around piers and submerged decks.
The aerodynamic stability of bridges began receiving attention after the collapse of the Tacoma Narrows Bridge near Tacoma, WA, in 1940. Since then, the wind engineering field has evolved and matured while addressing the many issues associated with the interaction of wind and the built environment.
FHWA's aerodynamics research program aims to advance understanding of wind effects on transportation structures to ensure they maintain a high level of performance in normal wind conditions and are reliable and sustain minimal or no damage during extreme wind events. The researchers at TFHRC's Aerodynamics Laboratory, through wind tunnel modeling and field testing and monitoring of actual structures, have developed and implemented a number of measures to address wind/structure interaction and control bridge vibrations caused by wind. As new designs evolve, new issues arise, such as deck vibrations and wind- and rain-induced vibrations of bridge cables, that require further study.
The Aerodynamics Laboratory is the only wind tunnel facility in the United States that is dedicated solely to the study of wind effects on transportation structures. It houses a large, low-speed wind tunnel and a small-scale smoke tunnel for flow visualization.
Ongoing research at the Aerodynamics Laboratory includes the following:
The laboratory has taken part in the design of stay cable damping systems; evaluation of performance problems and design of retrofits; and static, dynamic, and aerodynamic analysis of long-span bridges. In addition, the TFHRC researchers participate in a Wind Hazard Reduction Working Group that coordinates Federal research on wind hazards.
According to a U.S. Geological Survey circular, "Requirement for an Advanced National Seismic System," 75 million Americans in 39 States face significant risk from earthquakes. FHWA has been involved in seismic research since the aftermath of the 1971 San Fernando earthquake in California. However, it was the 1989 Loma Prieta earthquake that put a national emphasis on this hazard. Seismic research has led to numerous advances in the understanding of earthquake-resistant design, construction, and retrofit of highway bridges.
The focus of earthquake-resistant designs includes foundations, columns, and pier caps, as these represent critical components for this hazard type. To ensure superstructure stability during an earthquake, the FHWA seismic program has supported the development and implementation of improved bearing systems, restrainers (cables that hold the superstructure components in place), and shear keys (short stubs built into the abutment seats to keep the girders from sliding off). Understanding the effects of ground motion on bridges has increased as well through the use of sophisticated finite element analysis and comparisons with post-earthquake field observations from seismic events in the United States and abroad.
Ongoing studies include the following:
FHWA's seismic research program focuses on experimental testing and physical modeling, plus analysis and numerical modeling. The TFHRC researchers collaborate with a number of international partners to advance seismic technology and are active in the National Earthquake Hazards Reduction Program coordinating activities with other Federal agencies to reduce earthquake hazards.
Security and Human-Induced Events
Designing for security became an issue after the events of September 11, 2001. Shortly afterwards, a blue ribbon panel convened by FHWA and the American Association of State Highway and Transportation Officials (AASHTO) developed recommendations on ways to address this new hazard. In a parallel effort, FHWA conducted outreach to stakeholders, including convening a workshop to identify research gaps and needs. This effort was the basis of a plan described in the FHWA publication Multiyear Plan for Bridge and Tunnel Security Research, Development, and Deployment (FHWA-HRT-06-072).
In conjunction with State DOTs and other infrastructure owners, plus the U.S. Department of Homeland Security, FHWA has been collaborating with the U.S. Army Corps of Engineers on a number of efforts to address security measures. Researchers have concentrated on developing protective systems for vulnerable components of bridges and developing design aids such as computer programs. Installing barriers, fences, gates, lighting, and cameras also can reduce vulnerability, but alone they are not always effective against all threats. The focus to date of FHWA's security research program has been to develop innovative solutions for retrofitting existing structures.
Research conducted at TFHRC is helping build a resilient transportation system -- one that functions in normal times and during and after hazard events. The emphasis is on the engineering aspects of building disaster resiliency into the transportation infrastructure for ease in response and recovery.
Researchers are developing workable solutions for several hazards that can affect the system. Each event imparts loads on a structure of different magnitude, direction, and location, so one solution will not always satisfy all hazard requirements. Designing bridges for reduced vulnerability to multiple hazards, however desirable, might not always be practical. A solution for one hazard is not necessarily applicable to another and in some cases actually can increase the bridge's structural vulnerability to that other hazard. For instance, increasing the robustness of a structure to withstand blast loadings may negatively affect its performance during wind or seismic events. Similarly, solutions for one system are not necessarily applicable to another system; that is, a solution to protect a building may not be effective for a bridge.
The R&D effort underway focuses on single hazards, develops solutions, and then ensures the solutions are compatible with other hazards before implementation.
By addressing the SDR's grand challenges, USDOT, FHWA, and other SDR member agencies aim to develop a disaster-resilient America.
"A more disaster-resilient America recognizes and understands the relevant hazards, so communities at risk know when a disaster is imminent," says David Applegate, chair of SDR. "We need to be able to warn the right people in the right place at the right time. We need to ensure services are still accessible after an event. If communities are better prepared, we can minimize the risks of property loss and death, and reduce disruptions to life and the economy."
Sheila Rimal Duwadi, P.E., is a team leader in FHWA's Office of Infrastructure R&D at TFHRC, responsible for R&D of bridge technologies and methodologies for extreme events and specialty materials. She represents USDOT on the SDR and chairs the American Society of Civil Engineers' (ASCE) Structural Engineering Institute's Bridge Technical Activities Committee. She was the past associate editor for ASCE's Journal of Bridge Engineering. Duwadi is a registered professional engineer in Virginia.
For more information, contact Sheila Duwadi at 202-493-3106 or firstname.lastname@example.org.
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