James C. Ray
US Army Engineer Research and Development Center
ABSTRACT
Since September 11, 2001 (9–11), a new threat has been added to the list of all others that endanger our nation’s bridges: Terrorists! Prior to 9–11, bridge engi-neers would have considered it ridiculous to lump terrorism amongst the more common natural threats such as earthquakes, wind, and flood. Yet, national and international events since that time have proven otherwise.
This paper provides a glimpse of the current state-of-the-art for terrorist threat mitigation for bridges and how it has evolved from almost nothing since 9–11.
The science is still very much in its infancy and has far to go before it reaches the level of maturity and definition desired by engineers endeavoring to address the threat in their bridge designs and retrofits. While it is very sad that this science even has to exist, it is exciting for engineers and scientists looking to make a difference in the world that there is still so much room for innovation and development. As shown in the paper, we are still looking for that silver bullet that will effectively and economically mitigate all terrorist threats.
Most importantly, the author hopes to inspire engi-neers and scientists to find and pursue their niche within this science so that they can make important and innovative contributions, and ultimately achieve what we all desire, to make this world a better- and safer place for all.
So, are bridges really threatened by terrorists?
Immediately following 9–11, there were credible threats made against several major bridges within the United States, causing them to close for short periods of time and to deploy military personnel for their protection. Other countries, such as England, have experienced direct terrorist attacks against their bridges over the years (Williamson, in press). And as shown in Figures 1 and 2, terrorists fully recognize bridges as viable targets. Thus, the security of trans-portation infrastructure against terrorist attacks is now an important issue for engineers.
There are unlimited possibilities as to the types of terrorist threats that could be brought against bridge structures (AASHTO 2003). This paper discusses threats in the following basic categories: (1) Vehicle-Borne Improvised Explosive Devices (VBIEDs): These
Figure 1. Bridge destroyed terrorists.
Figure 2. Bridge with piers destroyed by terrorists.
include explosive-packed land-based vehicles that would be deployed against components reachable by land and water-based vehicles that would be deployed against any components reachable by water. (2) Hand-Emplaced Improvised Explosive Devices (HEIEDs):
These include improvised explosive devices that while not as large as VBIEDs, can be placed in direct-or near-contact with a structural member and cause severe localized cutting- and breaching type damage due to its close proximity. (3) Non-Explosive Cut-ting Devices: These include any non-explosive devices such as saws, grinders, and torches that can be used to cut/sever structural members. (4) IntentionalVehicular Impact: Like VBIEDs, these include both land-borne and water-borne vehicles, depending on the location of the component of concern. (5) Fire: Fire of size and duration can cause structural members to lose both
their stiffness and strength. Thus, a “pool fire” from such as a ruptured tanker truck on the deck of a bridge, adjacent to key components or in the water adjacent to piers or towers is of concern.
The “science” for mitigation of these threats is dis-cussed in terms of the following sub-areas: mitigation prioritization; vulnerability assessments; and threat mitigation. Detailed discussions of these items are pro-vided in the paper, but a brief overview is propro-vided below.
Mitigation Prioritization: Since 9–11, most bridge owners have completed the prioritization of their infrastructure between individual nodes (i.e. bridges, tunnels, etc.) and have begun or are ready to begin mit-igation efforts on their highest priority nodes. At this point, the question once again arises: Where do we start? Based upon the myriad of terrorist threats that could be brought to bare as well as the large number of vulnerable structural components on any given bridge, there are almost unlimited mitigation measures that can be deployed on a given bridge.Yet, there are always limited resources. A rational and consistent means is required to assess and compare individual structural component criticality and the effectiveness of varied mitigation measures throughout an individual bridge.
Thus, once again a prioritization is required, this time at the individual structure level.
Instead of prioritizing among a group of structures, the owner must now prioritize among the individual components on a given structure to determine which are at highest risk and most in need of mitigation efforts. The need in this case is to compare individual bridge components based on their specific importance and vulnerabilities. Since most of the high-priority bridges are major structures with potentially massive replacement costs and economic effects if lost, impor-tance should be primarily based on a component’s contribution to overall structural stability; i.e. if the component is sufficiently damaged, the bridge will totally collapse. However, other factors such as com-ponent replacement or repair costs can also factor in.
Component vulnerability will be a function of the spe-cific threat type and size used against the component, the likelihood of such a threat, and the component’s resistance to the threat. There are many risk assessment methodologies that can be used for this purpose. This paper provides an overview of a “component level risk assessment” methodology developed by Ray (2007) specifically for this purpose.
Vulnerability Assessment: In order to prioritize miti-gation measures on a given structure, the vulnera-bilities of important structural components to each threat must be understood. Since damage from ter-rorist threats is generally very localized (Fig. 3), vulnerability assessments must be accomplished at the individual component level.
Total bridge collapse will only occur if the locally affected structural components (i.e. column, truss member, tower wall, cable, etc.) are sufficiently dam-aged and structurally important enough to induce a progressive collapse of the entire structural system.
Figure 3. Explosively damaged girders.
The paper provides an overview of the vulnerability assessment process for terrorist threats and discusses various analytical tools that are available for this purpose.
Threat Mitigation: Physical security of any asset must essentially comprise a layered and fully inte-grated combination of four basic mitigation measures, referred to herein as the “Four D’s”: Deter, Detect, Delay, and Defend. These measures cannot be applied independently and must be employed as part of an interdependent systematic approach to a layered secu-rity perimeter around the protected asset, in this case the bridge component or a specific critical bridge com-ponent. The paper provides discussion on each of the Four D’s, but emphasis is placed on the Defense aspect (better known as “hardening”), which addresses the scenario where the attacker overcomes the denial methods and carries out the attack before a capable response occurs. Or, with the case of a vehicular-borne device (i.e. vehicle bomb), the attack can be carried out so quickly and with such force that detection and denial methods are essentially of no use. Hardening is the only viable defense for this threat.
The type of structural defense employed will of course be threat dependent. Hand-emplaced explo-sive threats, non-exploexplo-sive cutting threats, fire, etc. all require radically different defensive measures. Signif-icant advances have been made since 9–11 for protec-tion of vulnerable components from hand-emplaced explosive- and non-explosive cutting threats. How-ever, most of these technologies are proprietary and/or Classified and thus cannot be discussed in the paper.
And while there is always room for improvement, fire and vehicular impact mitigation technologies are well-evolved and require no additional discussion. Thus, the major focus of the paper is on hardening against vehi-cle bomb blast, which is a predominant threat against which hardening is required.
In sifting through the myriad of potential solutions to structural hardening, the most important thing to remember is that no matter how exotic or high-tech a proposed mitigation scheme may appear, it must
ultimately affect at least one of the three variables of Newton’s Second Law as given in equation (1).
where F= the summation of applied force and resisting force; m= mass; and a = acceleration.
The response of any structural component to a blast loading can depend upon many factors, but as shown in equation (1) it will depend primarily upon: structural mass; strain capacity and strength of the component;
proximity to the detonation (i.e. standoff); magnitude of the detonation (i.e. explosive type and weight);
and support conditions for the responding compo-nent. For points of discussion herein, equation (1) is expanded to:
where Fapplied= the applied forces (such as blast load-ing) and Fresisting= any resisting forces, such as that due to bending, shear and support reactions of the responding component.
This basic law certainly does not require review, but it is used to make several basic but important points about the role of mass and resistance in the response of a bridge component to blast. First: The greater the mass, the higher its inertial resistance to acceleration;
and likewise the greater its strength, the more it will be able to resist the applied explosive force. Con-versely, once the mass is moving, the component must have sufficient strain capacity (i.e. resistance) to over-come the momentum of the moving mass and arrest its motion. If not, the component will fail and if suf-ficient momentum remains, it will ultimately fly away as a “fragment”.
In addition to these factors, magnitude and proxi-mity of the explosion affect the applied loading and impulse imparted to a bridge component. Near-contact detonations, such as in Figures 4 and 5, produce extremely severe pressure and impulse load-ings. Because the detonation is so close-in and the pressure durations are generally so short in relation to the fundamental response mode of the component, the response is more a function of the total blast impulse and not the peak pressure.
Blast effects can be mitigated by any combination of the following basic categories, all of which affect either the load- or the response side of equation (2):
Increase Bridge Component Resistance to Load:
Strengthening of a component against blast is accom-plished in conventional ways, such as component thickening, span shortening, etc. It is almost always accompanied by an increase in the component’s mass, and as seen in Equation (2), this is always a good thing.
Mass is an important part on the resistance side of the equation and it can significantly decrease structural response and damage.
Steel has the significant benefit of high mass and many newer steels also offer high-strength and ductil-ity. While not as high in mass, concrete can offer very high strength and ductility if detailed properly.
Figure 4. First few microseconds of a detonation near the face of a target.
Figure 5. Approximately one millisecond into same deto-nation as in previous figure.
In addition, many other high-strength, ductile, and lightweight advanced materials have evolved in recent years and gained wide acceptance within the design/construction communities. However, the reader is reminded that while these materials can have appli-cation to blast mitigation, the light weight (i.e. low mass) can be detrimental in terms of inertial resistance.
Additionally, many of these high-strength materials have a relatively low strain capacity.
Decrease Applied Load: If sufficient resistance can-not be obtained for the component of concern, is it possible to affect the loading side of equation (2) and reduce the blast energy that is applied to the com-ponent? Blast loadings decrease very rapidly with distance from the point of detonation (US Army, 2002).
Thus, the most effective means to reduce blast load-ings is to enforce standoff. This can be accomplished via conventional means such as impact-resistant traffic barriers, etc. However, as previously discussed, this is rarely an option on bridges as this generally requires narrowing- or closure of traffic lanes, and our nation’s bridges are so heavily taxed with traffic that this is generally not an option.
Thus, the only remaining option in this category is to place a mitigation measure between the bomb and the target that has the effect of reducing the amount of explosive energy that actually makes it to the protected structural component. Numerous “energy absorbing”
concepts have been proposed and explored for this purpose as it is clear that all materials demonstrate various phase changes (an energy absorbing process) as a function of shock pressure. In addition, all mate-rials absorb energy to varying degrees as they undergo gross irrecoverable volumetric strains (i.e. crushing).
Beyond energy absorption, are there ways to completely- or partially shield the component of con-cern from the explosive energy? This can conceivably be accomplished through blast barriers that can be bro-ken into two basic categories: structural barriers and sacrificial barriers.
A structural barrier is essentially a “wall” in front of the protected component that has sufficient strength to stay in place throughout the blast event, collecting and dissipating all of the explosive energy and completely shielding the protected structure. A sacrificial barrier provides shock wave reflection and inertial resistance just like structural barriers, but has minimal structural resistance and breaks apart under the blast loading, thus minimizing support reaction forces. The exper-imental testing of various materials for blast energy absorption and shielding is discussed in detail in the paper. Materials considered included: elastomers, dilatants, porous aggregates, concretes, and water.
It is ultimately shown that energy absorption or shielding is not effective for bridge components where bomb standoffs and shield thicknesses are minimal.
Certainly the theory is sound, but unfortunately there is just not enough space to place a sufficiently thick shield with enough material to significantly affect the extremely high blast pressures from a near-contact detonation.
Load Path Redundancy: Complete mitigation of damage from very large explosive threats may not be economically or logistically possible. In many cases, it makes more sense to just limit the extent of damage to the most exposed components and ensure that there is enough redundant/residual capacity in other less exposed members to insure that the structural system as a whole can continue to function and not undergo a progressive collapse.
Layered Hardening Approach: A wide variety of hardening concepts are discussed and each has at least some validity and usefulness for specific sce-narios. There are also no concepts that just allevi-ate the extreme loadings without consequences. The
explosive energy does not go away and must be defused in some manner. Essentially, any concept only serves to re-distribute the energy; either through inertial resistance, strain energy, or momentum transfer (frag-mentation). Mitigation designers are encouraged to consider all of the strengths and weaknesses of each of the concepts and develop a layered hardening approach that capitalizes upon the strengths of each. An exam-ple of a layered hardening approach for a cellular steel column is provided.
Multi-Hazard Considerations: Terrorist threat miti-gation cannot be considered alone. In addition to terrorism, there are many other hazards that threaten a bridge, including earthquakes, wind, water, fire, weathering, etc. and a risk-based approach must be utilized to determine the relative degree of importance of each threat to a given bridge. And, as funding is always limited, the mitigation efforts must be priori-tized according to the level of risk. The paper discusses mitigation measures that can address multiple hazards, such as wrapping of reinforced concrete columns to increase both seismic and blast resistance. In addition to the beneficial multi-hazard overlaps, discussion is also given to detrimental overlaps of mitigation mea-sures. One example is: Local hardening of a structural component to increase its blast resistance may add detrimental mass and stiffness within the structural system, affecting its seismic resistance.
REFERENCES
American Association of State Highway and Transportation Officials (AASHTO). 2003. Recommendations for Bridge and Tunnel Security, prepared by the Blue Ribbon Panel on Bridge and Tunnel Security, Washington, DC.
Ray, J. 2007. Risk Based Prioritization of Terrorist Threat Mitigation Measures on Bridges. ASCE Journal of Bridge Engineering. March/April 2007: pp. 140–146.
US Army, Air Force, Navy, & the Defense Special Weapons Agency. 2002. DAHS-CWE-UFC 3-340-01, Design and Analysis of Hardened Structures to Conventional Weapons Effects, Washington, DC.
Williamson, E., Williams, D., Holland, C., Bayrak, O., Marchand, K. (in press). Blast-Resistant Highway Bridges: Design and Detailing Guidelines, Final Report to the National Cooperative Highway Research Program, Project 12–72.
Bridge Maintenance, Safety, Management and Life-Cycle Optimization – Frangopol, Sause & Kusko (eds)
© 2010 Taylor & Francis Group, London, ISBN 978-0-415-87786-2