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.
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.
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).
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.
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:
Determine which tunnels are structurally vulnerable to terrorist attacks and which are not.
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.
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.).
Bridge types and design features less prone to damage from terrorist attack.
Develop more nonlinear inelastic design approaches, taking advantage of structural ductility.
Better protection of piers against impact and/or blast loadings.
Use of more continuous structures.
Use of redundancy in structures.
Deterrent effects of layered countermeasures.
Additional
knowledge and understanding of the influence of member geometry on local
performance under blast conditions.
Additional
knowledge and understanding of the influence of the structural system on global
performance under blast conditions.
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.
Development of guides for proper protection of end anchorages of suspension cables.
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.
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.
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.
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.
Development of guidelines for sizing members to enhance bridge performance under blast
conditions.
Design of concrete structures using fracture mechanics principles rather than static loading criteria.
Determination of design safety factors appropriate for dynamic and blast loading applications.
Better
protection of the bearings, shoes, etc., of suspension and cable-stayedbridges.
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.
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.
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.
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.
Identify critical locations for possible placement of explosive charges.
Determine the potential extent and type of damage as a function of the tunnel design and the explosive charge size.
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).
Identify possible protection methods to reduce casualties/damage.
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.
Consider intrusion detection monitoring for major structures.
Detection and warning systems to prevent dangerous cargo from getting into tunnels.
Classify major structures and coordinate with the Department of Defense to monitor by satellite.
Identify bridge surveillance and security technologies.
Use
of global information system (GIS) technology to safeguard critical structures,
record current road network conditions, etc.
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.
Means of protecting traffic control systems from physical and cyber attacks.
Real-time chemical sensors.
Neutralizing agents and robots that can test areas and perform decontamination.
Deterrent effects of tactics to create uncertainty ("curtains of mystery").
Explosive detection systems able to detect a wider range of materials.
Means to network and combine sensors into "sensor fusion."
Standoff and accurate field sensors with low rates of false alarms.
Development of guidelines for restricting access to critical bridge members.
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.
Provide guidelines for the design of surveillance systems, including the kind of system, location, etc.
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.
Monitoring of structural performance to detect problems and prevent the occurrence of critical situations.
Development of an active safety system for critical transportation facilities that could stop a suspect vehicle traveling on a structure.
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.
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.
Improved inspection techniques to assess damage to structures.
Rapid determination of structure condition to determine residual stresses in structural members.
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.).
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.
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).
Material performance.
Additional knowledge and understanding is needed in the performance of different materials under different types and magnitudes of blasts.
. Development of guidelines for material selection to enhance bridge performance under blast conditions.
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.
Determine the effects of radiation exposure on the structural properties of materials used to design our structures.
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.
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.
Integration of critical databases in a GIS format.
Institute a Highway Innovative Technology Evaluation Center (HITEC)-type evaluation program for bridge and tunnel security.
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.
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.
