Three primary categories of bomb threats should be addressed: improvised explosive devices (IEDs); vehicle-borne improvised explosive devices (VBIEDs); and walk-in suicide bombers wearing explosive vests. The input of a qualified blast engineer should be sought when designing these countermeasures.
In all of these cases, a primary consideration is collapse mitigation. During the Oklahoma City bombing in 1995, half of the building crumbled within minutes of the detonation of the 5,000-pound truck bomb and caused an estimated 80 percent of the fatalities. Beams, girders, and columns should be carefully designed to prevent a sudden progressive collapse.
As a simplified example of this type of structural failure, the loss of an exterior column should not result in the upper levels immediately giving way. The building connections can be engineered so that even after a major explosion within the facility, the structure will survive long enough to allow occupants to evacuate.
The facility should be hardened at vulnerable, critical, and high-risk locations, especially areas where security and life-safety systems are vulnerable. In the security command center and throughout the hotel, electronic systems supporting security operations should be designed to survive a detonation. Adequate sources of emergency electrical power should also be incorporated into the design.
Designers should consider the use of pressure-release walls to direct blast overpressures away from areas where there would normally be a lot of people. (Overpressure is the pressure caused by the explosion that is over and above the normal atmospheric pressure. That is what constitutes the bomb blast or shock wave. At a certain level, it can shatter glass. At another level, it can cause catastrophic organ failure and instant death.) They should also carefully consider where emergency assembly areas will be outside of the hotel following either an interior or exterior blast. These areas may be the planned location for a delayed secondary detonation, either by another bomb or by a suicide bomber—a recent evolution of terrorist tactics. To help defend against this, all possible post-primary incident assembly points should be away from areas with a lot of glass.
The building’s windows, frames, and walls can be designed to withstand higher PSI (pounds per square inch) loads on the basis of a blast analysis. The amount of glass in exposed facades should be reduced or the orientation of glazed areas changed to minimize the effects of blast overpressures.
Glass shards cause the majority of injuries during a bombing. Laminated glass is the preferred glazing to reduce shards. It should be used for all glazing, especially at vulnerable locations, such as external walls near streets where a truck bomb could be parked. This type of special glass consists of layers of glass held together by interlayers of sticky materials like polyvinyl butyral.
Another way to mitigate the risk of shards is to cover windows on the inside with window treatments, such as blast curtains. Film appliqués can also be applied to prevent or minimize flying shards by retaining the shards on the film. This is, however, the most expensive option because the film must be periodically replaced due to normal wear and tear.
Setback. Outside, there should be as great a distance as possible between the facility and the likely positioning of VBIEDs, because the overpressure or blast shock wave gets weaker as it travels. Known as “setback” or “stand-off,” the importance of this distance to mitigate the effects of explosions cannot be overstated, but it is equally important to acknowledge its limitations.