Cost-effectiveness of Mitigation Strategies

Abstract

Decision-making regarding implementing measures to protect buildings from vehicle attacks is often undertaken using highly judgment-based risk processes. This paper presents a quantitative risk-cost model for using vehicle barriers to create setback distance around a new office building. The model explicitly considers both the attack probability, and the damages in the event of an attack (both target building and collateral), as well as how both of these might change as mitigation measures are implemented. The attack damages are assessed using a new empirical blast model, which adapts the estimation methods used by the U.S. Geological Survey for earthquake damages, and is based on data from three well-studied vehicle attacks. Monte Carlo simulation is used to carry the uncertainty in the inputs through to the final results. The model outputs are the mitigation costs, the attack damages, the “breakeven” attack probability (at which the benefits of the mitigation justify its costs), and the cost per statistical life saved (assuming an attack). The results suggest that this mitigation option is cost-effective only when the attack probability (for the case without the mitigation measures present) is rather high.

Background

Various works (e.g., [11,13,15,16]) examine protective design using a quantitative risk framework, but rely on highly simplified assumptions regarding the avoided damages and costs (e.g., 90% reduction in risk for a 10% increase in building construction costs). Foo et al. [14] offer a blast risk assessment method for buildings; however, their model does not account for progressive structural collapse, and many aspects of it are not overly transparent.

Previous Work

Vehicle-borne improvised devices (VBIEDs) are a favored device. Davis [1] calls them “stealth of surprising power and destructive efficiency” – the “poor man’s air force” – and notes that over a period of 25 years, VBIED attacks have occurred in at least 58 countries. However, decision-making regarding blast protection for buildings is often undertaken using highly judgment-based risk processes. First, a design basis threat (that is, size of device) is specified, and a portfolio of mitigation measures is selected. The damages with the mitigation are then assessed and, if deemed to be reasonable, the cost is examined. If either the damages or the mitigation cost are deemed to be unreasonable, the portfolio of mitigation measures is reworked. As such, the attack probability tends to be treated as binary, with the benefits and costs of the mitigation examined somewhat separately of one another [2,3,4,5,6,7,8,9,10]. The need for more risk-informed methods for blast protection – including greater consideration of uncertainties – has been widely recognized [6,9,11,12,13,14,15,16,17,18].


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