Featuring developments in Federal highway policies, programs, and research and technology.
|This magazine is an archived publication and may contain dated technical, contact, and link information.|
|Federal Highway Administration > Publications > Public Roads > Vol. 74 · No. 6 > Hazard Mitigation R&D Series: Article 5—Securing the Nation's Bridges|
Publication Number: FHWA-HRT-11-004
Hazard Mitigation R&D Series: Article 5—Securing the Nation's Bridges
by Sheila Rimal Duwadi and Eric Munley
FHWA is conducting R&D to protect transportation infrastructure from human-induced threats.
Securing the Nation's critical infrastructure, including bridges and tunnels, emerged as a major issue after the terrorist attacks on September 11, 2001. Shortly thereafter, President George W. Bush began issuing a series of directives pertaining to homeland security. In December 2003, "Homeland Security Presidential Directive (HSPD) 7: Critical Infrastructure Identification, Prioritization, and Protection" established a national policy for Federal agencies to identify and prioritize critical infrastructure and to protect it from terrorist attacks. HSPD 7 listed the transportation system as a critical infrastructure sector and a lifeline essential to the Nation's mobility and economy. Further, HSPD 7 states that "the [U.S.] Department of Transportation and the Department [of Homeland Security] will collaborate on all matters relating to transportation security and transportation infrastructure protection."
The highway network provides for the continued movement of people and goods. Bridges and tunnels are crucial links in the system, and therefore their protection is an essential component of any security plan.
As noted in the National Bridge Inventory, the United States is home to approximately 600,000 bridges on public roads. Before September 11, these structures, like many other public properties, generally were not registered on security watch lists. However, after September 11, the Federal Highway Administration (FHWA), State departments of transportation (DOTs), and other bridge owners began to look closely at the vulnerabilities of the public properties they share responsibility for protecting.
FHWA, over the years, has in-vested extensive research dollars into providing safer highway infrastructure to protect against earthquakes, floods, hurricanes, traffic and construction incidents, and structural collapses. In many cases, FHWA started these hazard-related R&D programs in response to specific events. For example, the collapse of the Tacoma Narrows Bridge in 1940, caused by wind-induced oscillation, led FHWA's predecessor, the Bureau of Public Roads, to initiate a Federal program of research on bridge aerodynamics. But terrorism presents a unique challenge in that, to date, no bridges in the United States have been attacked by ter-rorists. However, authorities have apprehended individuals with ties to known terrorist groups scouting and reporting on major bridges, offering further reason for FHWA to take proactive steps to improve security.
Although there are only a handful of different bridge types, each structure is designed to fit its specific location and circumstances. That means each has its own distinct vulnerabilities, depending on the design type, location, and materials used to construct it. Understanding these vulnerabilities is key to finding workable solutions for protection. Why a structure might fail depends on the structure type and the nature of the forces it must resist. Fire, blast, earthquake, flooding or scour, wind events, impact loadings, fatigue, fracture, and corrosion each represent a situation or event that could cause a bridge to fail. Unfortunately, there is no singular solution to make a structure immune to all conditions.
For natural hazards, such as earthquakes and hurricanes, scientists have identified regions of the country with statistically higher probabilities for exposure and determined where to concentrate mitigation efforts. But for human-induced hazards, the question is where to start? Should efforts be concentrated on individual landmark structures or structures on major economic corridors? Should the focus be on older structures or on developing new designs for bridges that have yet to be built? Transportation and security agencies further do not want counterterrorism strategies publicized, therefore development and deployment are seldom discussed openly, making sharing of information another challenge.
If economic disruption is a terrorist goal, highway corridors that move goods and services through rural parts of the country could be potential targets as well, especially those with major bridges in areas with few practical detours. But is it reasonable to expect that every bridge and tunnel—all 600,000 of them—should be designed or retrofitted on the chance that such an event might occur there? The easy answer would be "yes," if this were a small physical problem. However, bridge and tunnel protection is anything but.
For researchers at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA, in late 2001, any debate about bridge location and criticality quickly gave way to the physical problem of protecting bridge components. What are the best ways to protect them? It turns out that the means to secure bridges and tunnels against this type of threat still needs to be developed and refined. Through TFHRC, FHWA is doing just that.
Responding to the security threat required collaboration among numerous public agencies, universities, and consultants. In the research and development (R&D) arena, partners including FHWA, State DOTs, and toll bridge authorities worked together to identify issues. Together, these agencies brought expertise in bridge design, construction, and operation, and an understanding of the many issues involved in the highway transportation field.
However, these agencies quickly discovered that the civilian highway community had little experience actually designing transportation infrastructure for this level of physical security and reached out to the military. Shortly thereafter an interagency collaboration started with the U.S. Army Corps of Engineers (USACE). With its extensive expertise developed during past and current military operations, USACE provided critical background on blast effects and other means of destruction and ways to protect structures against them.
Today, the U.S. Department of Homeland Security (DHS) plays a critical role in this collaboration, stemming from its primary responsibility for ensuring the Nation's safety and security. In addition, the Transportation Research Board (TRB), through its cooperative research programs, had been addressing security issues prior to 9/11 and substantially expanded its effort after. FHWA has continued to work with TRB in this effort.
In the end, transportation infrastructure owners will be the ones responsible for implementing any research recommendations or products developed, so R&D is being closely coordinated through owner agencies and through various American Association of State Highway and Transportation Officials (AASHTO) committees.
Post-9/11 R&D Begins
Shortly after 2001, FHWA, in collaboration with AASHTO and TRB, convened a blue ribbon panel of bridge and structures experts representing Federal and State agencies and the consulting and academic communities. The panel met three times in late 2002 and early 2003 to develop strategies and practices for deterring, disrupting, and mitigating potential attacks, and reduce the vulnerability of bridges and tunnels. This collaboration resulted in the report Recommendations for Bridge and Tunnel Security (FHWA-IF-03-036), released in September 2003, which recommends policies and actions to "reduce the probability of catastrophic structural damage that could result in substantial human casualties, economic losses, and socio-political damage."
Meanwhile, in 2002, FHWA conducted a needs assessment with extensive outreach to solicit information on technology gaps from bridge owners, national laboratories, academia, consultants, and associations. This effort generated a list of research needs.
Then, in 2004, FHWA convened an R&D security workshop at TFHRC to augment the statements of research needs and broaden the basis for understanding the issues involved in developing a focused R&D program. Using the information gathered from these activities, FHWA identified the following major focus areas for R&D:
In 2006, FHWA released its Multiyear Plan for Bridge and Tunnel Security Research, Development, and Deployment (FHWA-HRT-06-072). The plan called for, first, developing means and methodologies to prevent an incident from occurring; second, protecting inhabitants and the structure if an event were to occur; third, having adequate resources to conduct a postevent assessment; and fourth, providing for repair and restoration of the structure in the most efficient manner possible.
Looking ahead, more effective detection, surveillance, and warning systems would help mitigate incidents. Improved designs and effective uses of new materials could help protect structures and ensure that unpreventable damage does not result in complete failure, which could cause major disruption to the economy and possibly numerous lives lost. After an attack, transportation agencies would need technologies, equipment, and guidelines to determine the extent of damage and the residual strength remaining in the structure. The strategic research program developed calls for innovations in protective hardware, structural systems, and rapid construction and reconstruction. These systems and practices, when integrated into existing standards, could not only counter the effects of terrorist attacks but also might offer additional protective benefits relevant to other hazards, including earthquakes, scour, wind, overloads, or collisions.
Field Evaluations Refine Threat Discussions
In addition, soon after the September 11 attacks, FHWA started conducting onsite assessments of potential targets, including bridges and tunnels. In collaboration with USACE, FHWA began evaluating these structures around the country for their ability to resist attack. In 2003, FHWA's engineering assessment teams, along with engineers and inspectors from USACE, State and city DOTs, police and fire officials, and toll authorities, began to walk, climb, and crawl on, around, and under many of the Nation's bridges and tunnels. The following year, DHS independently sponsored several assessment studies and security retrofit projects on major bridges and tunnels.
The field actions provided valuable feedback—an early focus on details—for the R&D effort. The detailed assessment and analysis of the vulnerabilities outlined earlier through the needs assessment and later by the engineering assessment teams began to quantify effects. Thus began the fine-tuning of retrofit measures and an inventory of potential restrictions on proposed countermeasures. The original focus was on vehicle-borne charges, but the engineering assessment teams found alternative attack methods to be feasible, so where possible the researchers factored those into the development of countermeasures.
The field investigations helped identify long-term issues and focus implementation. Other issues considered by the researchers included size and weight limitations on existing structures, especially older ones; material and geometric restrictions; practical restrictions imposed by construction, maintenance, and inspection; and the need to coordinate retrofit designs and hardware with those from other retrofits.
State DOTs and other infrastructure owners needed tools to help them assess vulnerability and preparedness. In terms of surveillance systems, new technologies flooded the market after September 11, but the States lacked guidance for choosing among them. DOTs also needed better information on costs, effectiveness, design, and applicability of state-of-the-art technologies for protecting structures. One of the first research studies FHWA undertook was a study to synthesize the latest surveillance technologies and security practices, and develop a protocol to assist infrastructure owners in their decisionmaking processes.
With support from the California, Kentucky, Missouri, New Hampshire, New Jersey, New Mexico, Ohio, and Texas DOTs, FHWA led this Transportation Pooled Fund effort focusing on the state of current and future surveillance and monitoring technologies available both within the United States and abroad. Researchers surveyed bridge and tunnel owners about their existing surveillance and security capabilities, including their experiences with these technologies. The research also included site visits to examine existing systems. The study produced a report, Bridge and Tunnel Security and Surveillance Technologies, and a database of available security and surveillance systems. Currently, the report is available by contacting FHWA, and the long-term plan is to make it available through the National Technical Information Service.
Although surveillance and security systems such as cameras and sensors, when skillfully used, can detect and deter terrorist activities, is it practical to monitor every truck and van that crosses every bridge and through every tunnel in the country every day? A decade after September 11, is the American public willing to pay for this level of security? This approach might well be appropriate for certain structures but not likely for all.
The next steps, therefore, included concentrating on developing design aids and retrofit measures to enable the structures to handle the extreme event loads. Because vehicle bombs are the terrorist weapon of choice worldwide and are a significant concern for highway bridges, FHWA research focused first on mitigating these blast loadings.
To this end, FHWA initiated a study to develop simple design aids, such as standardized blast response curves for bridges. The blast response, or range-to-effect, curves indicate the required threat standoff, the distance from the bomb to a critical bridge component, to prevent maximum allowable level of damage for the explosive charge size. These curves are intended to provide reasonable and conservative estimates of safe standoff distances for vehicle bombs ranging in size. They are, however, based on generic cases requiring many simplifications and assumptions. Because of their limitations, these curves are intended for use not in final design, but as a screening tool to identify those designs that are clearly safe or those that require more detailed analyses.
The study has produced three reports that are under review by FHWA: Vulnerability of Precast Pretensioned Concrete Bridge Girders Exposed to Vehicle-Borne Improvised Explosive Devices, Vulnerability of Steel Bridges Exposed to Vehicle Borne Improvised Explosive Devices, and Vulnerability Curves for Reinforced Concrete Bridge Piers Exposed to Vehicle-Borne Improvised Explosive Devices. However, due to the sensitive nature of the subject matter, distribution of the final reports will be limited to those with a need to know.
Another study, titled "Blast Specific Loading Program," adapted an existing USACE computer program to assess the effects of blast loadings on bridges. Conventional Weapons Effects (ConWep) is a software program widely used within the military engineering community to predict the effects of blast loadings from conventional weapons, including terrorist-type vehicular bombs, on buildings. The final product, the Bridge Explosive Loading (BEL) Code, uses the blast algorithms from ConWep (a low-resolution program) as well as algorithms from another USACE program, BlastX, which offers medium resolution. The new BEL Code considers three types of loadings—on decks, on vertical surfaces adjacent to decks, and on columns—and produces pressure distribution curves that designers can use to analyze different load cases.
Ongoing R&D: Steel Bridges
An ongoing transportation pooled fund study initiated in 2004, "Validation of Numerical Modeling and Analysis of Steel Bridge Towers Subjected to Blast Loadings (TPF 5-110)," has successfully provided a much better understanding of blast phenomenology associated with large explosive devices (such as truck bombs) detonated close to or almost in contact with bridges and other structures and has developed mitigation measures. Supporters of the study include FHWA; DHS; the State DOTs in California, New York, Texas, Washington, and Wisconsin; the Golden Gate Bridge, Highway, and Transportation District; and the Bay Area Toll Authority.
These tests have been the first to be conducted on bridge towers. Researchers performed a series of four progressive tests to study increasingly complex phenomenon. The first series consisted of simple structural configurations, such as flat steel sections to represent the facing of a tower. Through these tests, researchers were able to determine the effect of various charge sizes and standoff distances on these critical bridge components. The next series included more detailed cellular sections. As in the previous series, researchers conducted tests to determine the range of performance behaviors for various charge sizes and range effects. This series also tested retrofit details.
The third series of tests returned to simple configurations similar to those used in the first series and tested different types of energy-absorbing devices, such as fiber reinforced concrete panels and steel plates, to see if they would shield the structure from damage, thereby preventing collapse. The energy-absorbing devices would be sacrificial. The last series tested a more complex, closer representation of an actual cellular steel bridge tower. Researchers also tested retrofits as part of this series and performed "blind" computations using advanced computer analysis to validate the analytical models and predict performance before the actual physical testing. This research and the series of tests represent the beginning of efforts to develop countermeasures to protect these types of bridge members against blast loadings. Reports on each of the test series and an executive summary are under review by FHWA and study participants. The resulting reports will be available for controlled distribution.
FHWA and DHS now are supporting research by USACE in a followup study to further develop strategies to strengthen steel bridges. The new study adds development of countermeasures to protect several other components identified as critical in the risk analysis. The goal is to study both material combinations and retrofit designs beyond those considered in earlier research. In addition, the researchers will look at design issues including size and weight limitations on existing structures as well as practical restrictions imposed by construction and maintenance.
As in the previous research, steel bridge members will remain the primary focus. In addition to studying specimens constructed with modern steel and bolted connections, the followup research will examine specimens cut from steel members salvaged from recently demolished bridges. The purpose is to extend the range of the earlier project's results to early 20th century steels and to component and connection details characteristic of those bridges. The new study also will quantify the likely effect of wear and corrosion on blast resistance.
Remaining Gaps and Needs
The FHWA publication Multiyear Plan for Bridge and Tunnel Security Research, Development, and Deployment identified the steps needed for securing the Nation's infrastructure. The aforementioned projects represent just the beginning. With the support of DHS and USACE, FHWA and other partners in the pooled fund program will continue to advance the state of the practice and help enable State DOTs and other bridge and tunnel owners to identify the most effective ways to enhance the security of their infrastructure.
The security environment is unpredictable, and threats likely will continue to evolve over the coming years. Given the vast bridge population in this country, even narrowing down the number of structures to a smaller subset to receive increased security measures remains a formidable challenge with many variables. Developing solutions will require modeling and simulations, analyses, and experimental testing, all of which will take significant time and resources.
The following questions are among those that researchers will need to address in future studies. Will a given blast-load retrofit change the behavior of the structure? Can the structure support any extra weight, especially in the case of older bridges with severe size and weight limitations? Will the retrofit affect the structure's behavior under other loads, such as seismic loads? Will the retrofit affect the performance of mitigation measures installed to address other types of hazards? For example, how would design and installation details for a blast-load retrofit coordinate with those of an existing or planned seismic retrofit? How will the retrofit affect maintenance and serviceability, since coordinating security retrofits and maintenance is important to ensure that access for inspection is not blocked? Still other issues to consider include how to address sites with little to no standoff distance or options for lane closures and how the retrofit might affect traffic safety. Further still, could a blast-load retrofit be built into existing safety barriers? Most of these issues are of a practical nature, common to many types of retrofits. Security retrofits, however, require developing flexibility and a range in countermeasure details.
Developing protective designs for new bridges in a sense is the easier problem. But in the end, transportation owners need to consider all potential targets, old and new, and the impact of their destruction. A terrorist's choice of target may not hinge on whether it is old or new, famous or not, but simply on whether its destruction is achievable.
In summary, FHWA has identified the following areas for future research regarding bridge and tunnel security:
All these studies will require verifying and calibrating analytical predictions of the behavior of structural members and individual components when subjected to attack. Also of interest are the performance of currently used or proposed mitigation measures and the testing of recommended variations. Finally, any new retrofit or member designs will require screening of materials, as well as analysis and testing.
Developing solutions for some of engineering's most challenging problems has taken many years, sometimes decades, so expecting the same for this hazard seems reasonable. Nearly a decade has passed since the September 11 attacks, and researchers have learned much about how to protect the Nation's infrastructure, but much work remains.
When dealing with security issues, implementation is always a challenge. Ever-changing threats and the inherently low probability of an attack at any one location make decisionmaking about mitigation measures difficult. Security mitigation is an easy decision—but not an easy job—when the target and consequences are clear. With changing threats and so many potential targets, however, there are limitations on what can be done in terms of structural retrofits and designing for security. An effective security plan, therefore, may be more layered than transportation owners would care for, one that's designed to detect, delay, and deter attacks and to defend when possible. A plan for organized and rapid response and recovery may be the most appropriate and effective security action for most of the Nation's bridges and tunnels.
As underscored in the Multiyear Plan for Bridge and Tunnel Security Research, Development, and Deployment, FHWA will continue to conduct research designed to provide highway structures that are safe and reliable for all service conditions—including natural and manmade "conditions" as well. In the process of rehabilitating existing structures, to the extent possible, steps will be taken to incorporate designs and mitigation measures to secure against all potential hazards. Even after the bridge of the future delivers high performance with low maintenance, eliminating hazards, old and new, will continue to be an agency goal.
Sheila Rimal Duwadi, P.E., is a team leader in FHWA's Office of Infrastructure R&D at TFHRC. She is responsible for R&D of bridge technologies and methodologies for extreme events and specialty materials. She represents USDOT on the National Science and Technology Council's Subcommittee on Disaster Reduction and recently was appointed a member of the Executive Committee of the Technical Activities Division of the American Society of Civil Engineers' (ASCE) Structural Engineering Institute (SEI). Previously she chaired the SEI Technical Administrative Committee on Bridges and was associate editor for ASCE's Journal of Bridge Engineering. Duwadi is a registered professional engineer in Virginia.
Eric Munley, P.E., is a structural engineer in FHWA's Office of Infrastructure R&D at TFHRC. Since 2003, he has been a participant in FHWA's engineering assessment teams to evaluate security issues on field structures, and, more recently, he has been involved in the structures security research program. Previously he directed the research program to develop material, design, and construction specifications and test methods for fiber reinforced polymer composites. From 1997 to 2003, he was chairman of TRB Committee A2C07 (now AFF80) Structural Fiber Reinforced Polymers. Munley received a bachelor of science in civil engineering from the University of Connecticut and a master of engineering from Cornell University. Munley is a licensed professional engineer in Connecticut.
Page Owner: Office of Corporate Research, Technology, and Innovation Management
Scheduled Update: Archive - No Update
Technical Issues: TFHRC.WebMaster@dot.gov