When Al Qaeda is in hiding, its yellow streak becomes an escalating threat.
Highlighted in training manuals everywhere are the goals and missions of the terrorist group, and one particular message sent a chilling warning to the U.S. bridge community. It stressed the importance of “gathering information about the enemy and blasting and destroying bridges leading into and out of cities.”
The half-line of intelligence came through bold and clear for the Federal Highway Administration (FHWA) and the American Association of State Highway & Transportation Officials (AASHTO). The two joined together to form the Blue Ribbon Panel (BRP) consisting of bridge and tunnel experts from professional practice, academia, federal and state agencies, and toll authorities, which convened to examine bridge and tunnel security and to develop strategies and practices for deterring, disrupting and mitigating potential attacks.
Their report, released in September of 2003, did not attempt to dodge any truths.
“Among the 600,000 bridges in the United States, preliminary studies indicate that there are approximately 1,000 where substantial casualties, economic disruption and other societal ramifications would result from isolated attacks,” the report stated. “Additionally, the U.S. transportation system includes 337 highway tunnels and 211 transit tunnels; many are located beneath bodies of water, and many have limited alternative routes due to geographic constraints. The BRP recommends prioritization of these bridges and tunnel assets, followed by risk assessment as a guide for allocating federal and state funds to address security concerns, and then implementation of cost-effective operational security measures and engineering design standards to reduce the vulnerability of high priority bridges and tunnels to terrorist attacks.”
The BRP also documented the cost of such a terrorist strike. The ordinary cost of construction to replace a major long-span bridge or tunnel on a busy interstate highway corridor in the U.S. may be $1.75 billion. The panel of experts, however, noted that reconstruction following major earthquakes suggests expediting replacement can double the cost of construction. Furthermore, during the five years estimated for reconstruction the socioeconomic loss to the region resulting from losing as many as 14 interstate highway lanes for an extended period is many times the replacement of the facility.
Several recommendations came out of the BRP report. This article focuses on two areas—planning, design and engineering and countermeasures.

Blue Ribbon’s red alert

In order to prepare for a terrorist event, the prioritization process becomes essential. The BRP believes a “process is necessary for prioritizing all bridges and tunnels with respect to their vulnerability in terms of their criticality of the ability to deter, deny, detect, delay and defend against terrorist attacks.” The panel suggests the development of an assessment model to serve as a framework for evaluating alternatives for thwarting an attack.
Agencies already have prioritization methods in place. AASHTO’s version uses a set of critical asset factors to identify assets that are important to achieving an agency’s mission. It assesses the vulnerability of these critical assets to terrorist attack based on target attractiveness, accessibility and expected damage.
The Transportation Security Administration determines relative risk as a function of relative target attractiveness, relative likelihood and vulnerability.
According to the BRP, a large number of bridges and tunnels lends itself to a two-tier approach: prioritization and risk assessment. Prioritization is usually done in two steps. The data-driven approach ranks bridges by using common criteria based off data supplied by the National Bridge Inventory. The second step of prioritization calls on owners and operators familiar with specific characteristics of the facilities and the services they provide to come up with additional data.
This first-tier ranking is based on the following:

• Potential for mass casualty based on the average daily traffic and associated peak occupancies;
• Criticality to emergency evacuation and response to emergencies;
• Military or defense mobilization;
• Alternative routes with adequate available capacity;
• Potential for extensive media exposure and public reaction (symbolic value);
• Mixed-use bridges and tunnels where highway and rail are co-located;
• Potential for collateral damage, including collateral property and utilities;
• Maximum single span length as it relates to the time required to replace the facility;
• Commercial vehicle vs. passenger vehicle mix and volume as a surrogate for economic impact;
• Bridge or tunnel dimensions;
• Significance of revenue streams associated with the facility; and
• Bridges and tunnels at international border crossings.

The second tier, risk assessment of high priority bridges, is based on the first tier to determine vulnerabilities and evaluate countermeasures to discourage an attack and mitigate damages. According to the BRP, the risk (R) is determined following an approach similar to the one used for seismic retrofit (R=OxVxI).

Occurrence (O)

The occurrence factor is hazard oriented and will change with the nature of the hazard. It approximates the likelihood that terrorists will attack the asset and includes target attractiveness, level of security, access to the site, publicity if attacked and the number of prior threats. Input into the factor typically comes from the law enforcement and intelligence communities.

Vulnerability (V)

Vulnerability is an indication of how much the facility or population would be damaged or destroyed based on the structural response to a particular hazard. It serves as a measure of expected damage, outcome of the event, expected casualties and loss of use. Input usually comes from engineering analysis and expertise.

Importance (I)

Importance is a characteristic of the facility. It is an indication of consequences to the region or nation in the event the bridge or tunnel is damaged or destroyed. Input into this factor usually comes from owners, operators and users.

In addition to recommending a state identification and prioritization of bridges and tunnels, the BRP suggested a federal re-prioritization for federal funding based on the following:

Near-term (3-6 months)

1. FHWA determines and promotes a process for reviewing bridges and tunnels with respect to risk and vulnerability in terms of the ability to detect, deny, delay and defend against terrorist attacks. Methodologies considered should be developed and include the AASHTO Guide for Highway Vulnerability Assessment, the Texas DOT methodology, and others.

2. Using the FHWA-endorsed methodology, states should prioritize their bridges and tunnels and submit lists of their most critical structures to FHWA.

3. FHWA and AASHTO should guide the development of an immediate, near- and mid-term cost-benefit methodology based on probabilistic risk assessment for implementing countermeasures. Within the framework of this risk assessment adopted from seismic retrofit programs, consideration should be given to existing methodologies.

Mid-term (6-12 months)

1. Based on the states’ priority lists of critical bridges and tunnels, the FHWA should develop a national list.

2. States use the risk assessment methodology to develop a countermeasure plan using a cost-benefit ratio as a metric and provide costs for implementing countermeasures for each of their critical bridges and tunnels to FHWA.

Long-term (12-18 months)

1. FHWA, with help from other agencies, seeks new appropriations from Congress to develop a national bridge and tunnel countermeasure program. FHWA starts to fund bridges and tunnels of highest priority as identified by the states and other owners and operators in accordance with accepted risk assessment methodologies.

2. Non-state DOT bridge and tunnel owners begin implementing countermeasures consistent with federal security standards using appropriate funding sources.

3. FHWA and AASHTO develop and implement modifications to existing bridge and tunnel inspection programs to evaluate conformance to federal security standards.

4. States implement countermeasures when funding becomes available. One source recommends an initial sum of at least $1.5 billion to address immediate security measures.

When looking at design criteria, owners can mitigate the threat by preventing terrorists facility access; mitigate the consequence effect which would in turn lessen the effect from an attack; or apply both options. The BRP report contains the following examples of approaches to mitigate threats and consequences.

Approaches to mitigate threats:

• Establishing a secure perimeter using physical barriers;
• Inspection surveillance, detection and enforcement, closed-circuit television (CCTV);
• Visible security presence; and
• Minimize time on target.

Approaches to mitigate consequences:

• Create standoff distance from primary structural components. The BRP report says there are three basic approaches to blast-resistant design: increasing standoff distances; the structural hardening of members or higher acceptable levels of risk. Using a percentage of each strategy is suggested;

• Add design redundancy. Great redundancy among structural components will help limit collapse in the event of severe structural damage from terrorist acts; and

• Structural retrofitting and hardening priority should be assigned to critical elements that are essential to mitigate the extent of collapse. Secondary structural elements should be dealt with to minimize injury and damage.

The BRP also calls for the development of an accelerated response and recovery plan, one that maps out alternative routes and evacuation procedures.
As you can see, prevention is the key to thwarting any and all types of terrorist attacks. The BRP study outlines several countermeasure options, including measures covering planning and coordination, information control, site layout, access control and retrofit. While all five areas are important, the BRP did offer a number of retrofit options which every owner should evaluate. Keeping terrorists away from a bridge or tunnel is the first line of defense, but the stronger the structure, the less chance of catastrophic consequence if an attacker does slip through surveillance. The BRP list of retrofit options is:

• Reinforcing welds and bolted connections to ensure members reach their full plastic capacity (designed for 120% of connected member capacity to account for strength increases during high-rate straining);
• Using energy-absorbing bolts to strengthen connections and reduce deformations;
• Adding stiffeners and strengthening lateral bracing on steel members to prevent buckling;
• Designing portions of the deck to “blow out” and create a vent to reduce pressures on the support structure (possibly near abutments);
• Adding carbon fiber reinforced polymer (CFRP) wraps on concrete columns, which can be reinforced with longitudinal wraps, to enhance concrete confinement, increase bending resistance and ductility and add protection against spalling;
• Strengthening the lower portions of columns against impacts and localized blast damage by encircling them with a steel casing (connected with high-strength bolts and epoxy or a layer of grout);
• Adding lateral bracing to columns to allow them to develop plastic hinges while preventing buckling;
  Adding 360° pier protection for impacts and standoff distance;
• Restraining sections of the bridge with steel cables to reduce the chance of deck collapse at the supports, including cable supports to keep the deck from separating at the joints and hinge restrainers to hold the deck to the columns;
• Increasing the size of abutment seats and adding hinge seat extensions under expansion joints to reduce the chance of deck collapse;
• Increasing footing size;
• Wrapping the lower portions of cables on cable-stayed and suspension bridges with CFRP or other types of protective armor;
• Increasing standoff distance and reducing access to critical elements with structural modifications;
• Including reinforcing steel on top and bottom faces of girders to increase resistance to uplift forces from blasts that are in the opposite direction from those due to gravity and live loads;
• Providing system redundancy to ensure alternative load paths exist should a critical structural element fail or become heavily damaged as a result of a terrorist attack; and <
• Strengthening the deck on curved steel trapezoidal girder bridges to ensure sufficient torsional strength is provided should a portion of the deck by compromised.