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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-06-072
Date: March 2006

Multiyear Plan for Bridge and Tunnel Security Research, Development, and Deployment

Appendix A Research Needs From Needs Assessment

Risk and Vulnerability Assessment

  1. Because the ways in which a terrorist could potentially attack a bridge are numerous and highly varied, and the possibilities are limitless if the bounds of reason and probability are not applied, we must first have a threat definition. The results of a decade of effort by our country, as well as others, in defining the most probable terrorist threats against military structures, embassies, etc., can be used as a starting point for defining threats against bridges.
  2. Cost-benefit and risk-assessment methodologies must be developed to economically address all terrorist threats against bridges, as there are many different bridge types and different degrees of damage depending on the bridge type.
  3. Develop methodology to conduct consequence analysis (e.g., how to assess the possible consequences of a truck bomb exploding near a critical member on the bridge).
  4. Vulnerability prediction tools: Once the loadings, damage mechanisms, and residual strengths of bridge elements are better understood, these results should be incorporated into new and/or existing vulnerability prediction tools.
  5. It is proposed that a research effort be performed to establish guidelines for evaluating the vulnerability of transportation tunnels to terrorist threats. The guidelines will be developed from the latest state of the technology with regard to tunnel structural damage from explosions, the propagation of blast pressures and thermal effects in tunnel systems, and fire or blast control and mitigation techniques for underground facilities. The guidelines will be provided in a user-friendly, AT-Planner type computer format, and will allow users to:
  6. Determine which tunnels are structurally vulnerable to terrorist attacks and which are not.
  7. Threat vs. risk definition: Prior to any detailed efforts to define specific bridge vulnerabilities, specific terrorist threats and the probability of occurrence (i.e., risk) for each threat must be defined. Definitions of the most probable threats are required in terms of type, size, and location on the bridge. Threat types considered should, at a minimum, include vehicle or boat bombs, hand-carried bombs (e.g., briefcase, etc.), precision cutting charges (i.e., shaped charges), kinetic energy threats such as vehicle or airplane impact, and fire. The size of the threat can range from a hand-cartable weight all the way up to that carried in a tractor-trailer vehicle.
  8. Guidelines for evaluating credible threats (e.g., is a truck bomb probable/credible?, is a shaped charge probable/credible?, is a ship/barge impact probable/credible?, etc.).

Design and Analysis

  1. Impact dampening designs.
  2. Bridge types and design features less prone to damage from terrorist attack.
  3. Develop more nonlinear inelastic design approaches, taking advantage of structural ductility.
  4. Better protection of piers against impact and/or blast loadings.
  5. Use of more continuous structures.
  6. Use of redundancy in structures.
  7. Deterrent effects of layered countermeasures.
  8. Additional knowledge and understanding of the influence of member geometry on local performance under blast conditions.
  9. Additional knowledge and understanding of the influence of the structural system on global performance under blast conditions.
  10. Although for large suspension cables the likelihood is low that the cable could be severed or even lose enough capacity to cause bridge collapse, the cable size (which is related to bridge size) below which this would be a very serious problem needs to be defined. The vulnerability of the smaller diameter intermediate cable is also unknown. Another concern for cable hangers is the number of successive hangers that would need to be removed in order to induce a spontaneous "unzipping" of the remaining hangers because of load redistribution.
  11. Development of guides for proper protection of end anchorages of suspension cables.
  12. Simple threat vs. required standoff distances (i.e., vulnerability curves) are required to aid engineers in mitigating the threat of bomb blasts on structures both above and below the decks of bridges. Vulnerability curves should also be developed for the typical truss elements of a through truss or through arch bridge.
  13. The intermediate supports (piers and bents) for any type of bridge span could be vulnerable to blast loadings (vehicle or boat bombs). The smaller column portion of bents will be the most vulnerable to lateral loadings from adjacent blasts. Again, vulnerability curves need to be developed for typical piers to allow engineers to design appropriate standoff devices to mitigate these threats.
  14. In the specific area of bridges, the U.S. Army Corps of Engineers' Engineer Research and Development Center (ERDC) recently developed a computer code, entitled Bridge Analysis System (BAS), for smart targeting of bridges with precision-guided, air-to-surface weapons. The BAS development effort included a thorough search of international literature in the areas of weapon effects against bridges and structural response of bridges subjected to blast and fragment loadings. A large amount of bridge attack/damage data was also collected from recent U.S. military actions, including Iraq, Bosnia, and Serbia. A key part of the BAS is a weapon effects database, which is weapon specific and predicts the level of damage imparted to the bridge structural elements based on the weapon's impact conditions and the location on the bridge. The database was developed at the ERDC using dynamic finite element (FE) analyses of steel and reinforced-concrete structural elements (beams and girders) impacted by a combination of blast and fragment loadings. The methodology developed for the definition and application of combined blast and fragment loadings on FE models was very innovative and represents the state of the art in this area.
  15. Simplified analytical techniques are insufficient to properly study the structural vulnerability of cable-supported towers. Detailed analyses should be accomplished using hydrocodes to predict the complex blast loadings, coupled with FE models of the towers with all in situ loadings present.
  16. Development of guidelines for sizing members to enhance bridge performance under blast conditions.
  17. Design of concrete structures using fracture mechanics principles rather than static loading criteria.
  18. Determination of design safety factors appropriate for dynamic and blast loading applications.
  19. Better protection of the bearings, shoes, etc., of suspension and cable-stayedbridges.
  20. The application of systems engineering and the availability of advanced technologies have made it clear that earthquake hazard mitigation effects should be considered in combination with other natural and manmade disasters. By the same logic, highway systems are interconnected with other modes of transportation. It may be useful for FHWA to point out that advanced technologies from FHWA can be extended to facilitate the intermodal transportation needs of the public.
  21. Damage and strength reduction to generic structural elements: Many of the technology shortfalls involve explosive loadings on key structural elements of specific bridge types (e.g., decks, girders, piers, etc.). Many of these elements are similar in nature and carefully planned studies of generic elements can address many bridge types at once. These studies should encompass a carefully planned combination of simplified analyses, detailed analyses, and actual testing. The residual load capacity of the damaged elements should also be studied in a similar manner. As controlled explosive tests on bridge elements have been almost non-existent in the past, testing should be a priority, even if only done on a limited basis.
  22. Structural loadings from terrorist threats: The loadings from blast-type threats must be defined in terms of airblast magnitudes and durations, and fragment densities and velocities. Kinetic energy impactors must be defined in terms of mass, velocity, and impact locations. These definitions may be accomplished through a combination of full- and reduced-scale field tests and analytical modeling. Existing predictive computer codes for military weapons can then be modified to include terrorist weapons.
  23. Vulnerability of specific bridge types: As the research progresses, it will probably become apparent that some bridge types, such as truss bridges, need to be studied as a complete structural system rather than as individual structural components. These studies will include detailed analytical calculations using the results from tasks 2 and 3 above and may involve limited field tests of actual bridge structures. As field tests of entire bridge structures will be very costly, these will only be done as a last resort to analytical modeling.
  24. Identify critical locations for possible placement of explosive charges.
  25. Determine the potential extent and type of damage as a function of the tunnel design and the explosive charge size.
  26. Determine the airblast and vehicle damage levels that would occur at any point in the tunnel as a function of the threat (charge size and location).
  27. Identify possible protection methods to reduce casualties/damage.

Prevention, Detection, and Surveillance

  1. Threat reduction/mitigation measures: As the research progresses, the true vulnerability of specific bridges and bridge elements will become apparent. This will allow for the development of threat reduction/mitigation measures such as standoff devices, intrusion prevention doors, fragment protection panels for beams and cables, blast-resistant design detailing, etc.
  2. Consider intrusion detection monitoring for major structures.
  3. Detection and warning systems to prevent dangerous cargo from getting into tunnels.
  4. Classify major structures and coordinate with the Department of Defense to monitor by satellite.
  5. Identify bridge surveillance and security technologies.
  6. Use of global information system (GIS) technology to safeguard critical structures, record current road network conditions, etc.
  7. Global positioning system (GPS)-based systems: Increased reliance on GPS-based systems for communication with many transportation systems in the United States could compromise traveler safety in the event of signal disruption. That is the conclusion of a study by the Volpe Transportation Center in a report entitled Vulnerability Assessment of the Transportation Infrastructure Relying on the Global Positioning System (www.navcen.uscg.gov/news/FinalReport-v4.6.pdf). GPS technology can be adversely affected by atmospheric effects, signal blockages from structures, interference from other signals, and deliberate disruptions. Although of primary concern to the aviation industry, other modes of transportation are increasingly relying on GPS technology for everything from tracking to traffic management. The report recommends that the State DOTs create an awareness of GPS vulnerabilities, improve their backup systems, and install monitoring systems to warn users of interference with GPS signals.
  8. Means of protecting traffic control systems from physical and cyber attacks.
  9. Real-time chemical sensors.
  10. Neutralizing agents and robots that can test areas and perform decontamination.
  11. Deterrent effects of tactics to create uncertainty ("curtains of mystery").
  12. Explosive detection systems able to detect a wider range of materials.
  13. Means to network and combine sensors into "sensor fusion."
  14. Standoff and accurate field sensors with low rates of false alarms.
  15. Development of guidelines for restricting access to critical bridge members.
  16. Determine when and how to take mitigative measures - surveillance and intervention (e.g., physical barrier coming up), hardening critical members, limiting truck operations along critical lanes, etc.
  17. Provide guidelines for the design of surveillance systems, including the kind of system, location, etc.
  18. Detection of dirty bombs in tunnels through the use of sensors. Determine if there is a justifiable need or a practical method for detecting dirty bombs on a continuous basis.
  19. Monitoring of structural performance to detect problems and prevent the occurrence of critical situations.
  20. Development of an active safety system for critical transportation facilities that could stop a suspect vehicle traveling on a structure.
  21. Use of remote-sensing, space-based observation systems to assist in a variety of ways in improving transportation security. Identifying and reducing vulnerabilities through the use of remote-sensing technologies would help security professionals protect our vast transportation system. Our recommendations to DOT call for establishing interoperability standards for remote-sensing transportation information, which will then be used by security officials at the Federal, State, and local levels.
  22. Remote sensing is the process of employing electronic cameras or other types of sensors to image a subject or sense its presence and composition at a distance from the subject. A digital camera and a radar detector are both simple forms of remote-sensing technologies. Sensors may be mounted on satellites, aircraft, or on ground-based platforms. For example, the Landsat satellite system, developed by NASA, has been collecting remotely sensed images of the Earth for more than 30 years. A report is available based on a workshop convened at George Washington University by the National Consortium for Safety, Hazards, and Disaster Assessment for Transportation Lifelines (NCRST-H) in order to effectively assist transportation officials to meet the threat of terrorist activities throughout the country. In addition to identifying potential applications for the technology, major barriers were also identified, alerting experts to the need for additional tools to scrutinize these weaknesses.

Post-Event Assessment

  1. Improved inspection techniques to assess damage to structures.
  2. Rapid determination of structure condition to determine residual stresses in structural members.
  3. There are very specific procedures that should be followed any time a tunnel is entered after an internal explosion. First, the air quality must be sampled to ensure that the tunnel is safe to enter without a breathing apparatus. If hazardous gas levels are still high in the tunnel, these must be used. Secondly, the tunnel must be inspected as it is entered for unstable structural damage that could result in injuries from falling debris (hardhats alone are not good enough). The Bureau of Mines has well-defined procedures for re-entries in the mining business. In the case of an explosion in a transportation tunnel, some minimal risks must be accepted in order to reach and rescue any people inside. It is expected that the Bureau of Mines guidelines would cover such rescue operations. Most fire departments should have similar procedures for entering fire- or explosion-damaged buildings. Since the initial re-entry will require the use of air quality monitors and other equipment, inexpensive handheld radiation detectors could easily be added if there is a suspicion that a dirty bomb might have been used. It is likely that those detonating a dirty bomb have the objective of getting the contaminants dispersed as widely and efficiently as possible and that structural damage is not their primary objective. Thus, the use of such a bomb to also damage a structure would probably be less likely. They would try to put it closer to population concentrations (e.g., downtown, etc.).

Repair and Restoration

  1. Rapid repair of damaged structures.
  2. Decontamination of large-scale transportation infrastructure. In FHWA-led workshops on security, the difficulty of fully decontaminating large-scale transportation infrastructure arose repeatedly. This was particularly the case with radiological events, where it may not be possible to simply "wash off" surface contamination as would be done with chemical or biological hazards. This may also be a consideration in whether additional detection is necessary to protect bridges and tunnels. In the extreme, it might be necessary to replace (in part or whole) a bridge or tunnel structure that was structurally sound, but which could not be adequately decontaminated. This area needs to be fully explored. The most complex workshop decontamination scenario was a 32-centerline kilometer (20-centerline mile) section of interstate contaminated with a persistent chemical agent. The cost and scale of decontamination was astounding. The attendees could not figure out if the soil outside the shoulders would have to be removed (and replaced) in order to avoid recontamination of the roadway. Firefighters and hazardous materials specialists have never considered anything on this scale.
  3. Rapid replacement/repair: Uninterrupted traffic flow is the most important requirement, so we have to focus on the development of efficient procedures/methods (new materials, technology).
  4. Material performance.
  5. Additional knowledge and understanding is needed in the performance of different materials under different types and magnitudes of blasts.
  6. . Development of guidelines for material selection to enhance bridge performance under blast conditions.
  7. Development of new technology to promote toughness in concrete materials (e.g., micro- and nano-fiber materials) that can inhibit crack propagation from dynamic loading and blast loadings.
  8. Determine the effects of radiation exposure on the structural properties of materials used to design our structures.
  9. The most promising area for improvement of bridge performance and longevity is new materials. We need new materials and more sophisticated electrical equipment for the future.
  10. There is a need to know what are the pressures generated in blasts of ammonium nitrate, dynamite, nitroglycerin, etc., as a function of poundage and distance (radial pressure distribution), and the millisecond duration of such pressures.

Evaluation and Training

  1. Integration of critical databases in a GIS format.
  2. Institute a Highway Innovative Technology Evaluation Center (HITEC)-type evaluation program for bridge and tunnel security.
  3. There could be a course on anti-terrorist measures given to bridge inspectors to specify the areas of vulnerability on a bridge-by-bridge basis. Immediate field action should follow, with a biennial repeat.


  1. There is a need to work with other agencies that have been involved with terrorism and the protection of military structures to transfer knowledge to civil structures.
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