Analysis Techniques for Terrorist Risk Assessment and Mitigation of Bridges

In 2003, the Blue Ribbon Panel on Bridge and Tunnel Security issued its recommendations for risk assessment and mitigation of bridges and tunnels. The report gives general guidelines to help decision makers improve the security of bridges and tunnels. However, the actual implementation can be highly dependent on the methods used to carry out the assessment. Variability in risk assessments may prevent limited funding from going to projects where it is needed most. The factors that determine the relative risk of a bridge include: Occurrence — the likelihood of an event taking place, whether intentional or unintentional; Vulnerability — the consequences of an event; and Importance — the tangible and intangible value of the asset. The product of weighted scores for each of these factors determines the relative level of risk. A real life example of risk assessment is presented with special attention paid toward the calculation of the weighted score for vulnerability. The threats listed in the Blue Ribbon Panel report were used to determine the consequences of various terrorist tactics of a real-life group of movable bridges over a waterway used for shipping. Potential consequences of attack included damage to bridge, downtime of bridge and waterway, human casualties, and collateral damage to surrounding area. Conventional blast resistant engineering methodology, more commonly used on buildings, was used where applicable and compared to high fidelity modeling. The damage expected on each component in the bridge was used to estimate the vulnerability scores. The Blue Ribbon Panel report discusses possible mitigation measures and how to prioritize their implementation. Mitigation measures usually focus on reducing either occurrence, though deterrence measures, or vulnerability, through hardening or increasing standoff. Both types of mitigation measures are applied to these real life examples as a model of how bridge owners can most effectively reduce terrorist risk on their assets.