Risk Analysis for Explosive Attacks on Highway Bridges

Terrorism presents a real threat to all aspects of society. The terrorist attacks of September 11, 2001 have shown how devastating a successfully implemented attack can be to the United States. Homeland security has become a priority, one that government officials and civilians alike cannot take lightly.

In the past few years, the vulnerability of transportation infrastructure to terrorism has become evident. The casual observer might think that since most of the U.S. highway system has a natural redundancy, it is not susceptible to an attack. However, this is not the case. Transportation facilities are attractive targets for terrorists because they are easily accessible, and an attack could have considerable impact on human lives and economic activity. This is especially true for transportation assets such as bridges, which carry traffic through highway network meeting points, and where alternate routes are not available because of geographic constraints.

Recent terrorist threats have targeted the Golden Gate Bridge and the Brooklyn Bridge. Bridges such as these not only provide transportation connecting two regions, but they also serve as national landmarks. A successful attack would disrupt regional traffic and have severe economic consequences, not to mention cause a blow to the United States’ morale. It is imperative that homeland security procedures incorporate bridges, in order to prevent terrorist attacks, as well as minimize the impact if one should occur.

In recent years, studies have been conducted to evaluate bridge vulnerabilities to terrorism. While an attack’s probability of occurrence is very small, difficult to quantify and filled with uncertainty, the consequences in most cases would be devastating. Many studies have suggested that analyzing the consequences of such an attack on a bridge is an area of necessary future research. Only by having accurate knowledge of these consequences can risk management strategies be developed.

Risk is defined as the potential of loss to a system due to the likelihood of an event and its consequences. In the homeland security field, Risk (R) is the multi-dimensional product of occurrence probability (L) and consequences given occurrence (C) and the vulnerability measure (V), which assesses the likelihood of an attack’s success, given its

R = C V L

The process of risk assessment answers three basic questions:

  1. What could occur?
  2. How likely is it to occur?
  3. What are the consequences if it occurs?

This study proposes a method for evaluating the effects of a terrorist attack on highway bridges. The approach uses probability distributions to characterize the parameters of the vehicle-bound explosives attack scenario: magnitude and location. Applying Monte Carlo simulation, attack scenarios are randomly generated and the equivalent static loads are applied to the bridge’s structural model. Next, nonlinear static analysis is conducted using commercially available structural analysis software.

Performance criteria, established in performance-based seismic design guidelines, are
used to measure the structural damage to the bridge. Finally, the method estimates the costs associated with three consequences: structural damage, casualties, and bridge downtime. Probability distributions account for uncertainty in the consequences. Case studies demonstrate the proposed approach with three functional bridge types of an existing long-span bridge: prestressed concrete beam, continuous steel plate girder, and deck cantilever truss. The case studies analyze attack scenarios to conventional explosives in a vehicle traveling on the bridge.

Knowing the effects of a terrorist attack is beneficial to bridge owners, who have limited funding for security-related issues. The bridge owner can use the proposed method to analyze one critical bridge or multiple bridges in their inventory. Examining the structural responses and consequences of an attack on different functional bridge types can shed light on which bridges are most vulnerable. The results of the proposed method can be valuable when bridge owners must decide among risk management or mitigation strategies.

Prestressed Concrete Bridge under Explosive Attack

Figure 1 – Prestressed Concrete Bridge under Explosive Attack

Steel Girder Bridge under Explosive Attack

Figure 2 – Steel Girder Bridge under Explosive Attack

Steel Truss Bridge under Explosive Attack

Figure 3 – Steel Truss Bridge under Explosive Attack

Formation of plastic hinges on the center of mid-span of a Suspension Bridge for Blast Load Step 1 of Progressive Analysis

Figure 4 – Formation of plastic hinges on the center of mid-span of a Suspension Bridge for Blast Load Step 1 of Progressive Analysis