8.1 Initial Costs
The initial construction cost of protection has two components: fixed and variable. Fixed costs include such items as security hardware and space requirements. These costs do not depend on the level of an attack; that is, it costs the same to keep a truck away from a building whether the truck contains 500 or 5000 lbs. of TNT. Blast protection, on the other hand, is a variable cost. It depends on the threat level, which is a function of the explosive charge weight and the stand-off distance. Building designers have no control over the amount of explosives used, but are able to define a stand-off distance by providing a secured perimeter.
The optimal stand-off distance is determined by defining the total cost of protection as the sum of the cost of protection (construction cost) and the cost of stand-off (land cost). These two costs are considered as a function of the stand-off for a given explosive charge weight. The cost of protection is assumed to be proportional to the peak pressure at the building envelope, and the cost of land is a function of the square of the stand-off distance. The optimal stand-off is the one that minimizes the sum of these costs.
If additional land is not available to move the secured perimeter farther from the building, the required floor area of the building can be distributed among additional floors. As the number of floors is increased, the footprint decreases, providing an increased stand-off distance. Balancing the increasing cost of the structure (due to the added floors) and the corresponding decrease in protection cost (due to added stand-off), it is possible to find the optimal number of floors to minimize the cost of protection.
These methods for establishing an optimum stand-off distance are generally used for the maximum credible explosive charge. If the cost of protection for this charge weight is not within the budgetary constraints, then the design charge weight must be modified. A study can be conducted to determine the largest explosive yield and corresponding level of protection that can be incorporated into the building, given the available budget.
Though it is difficult to assign costs to various upgrade measures because they vary based on the site specific design, some generalizations can be made (see Figure 8-1). Below is a list of enhancements arranged in order from least expensive to most expensive.
- Hardening of unsecured areas
- Measures to prevent progressive collapse
- Exterior window and wall enhancements
