It is not widely publicized that setback standards and guidelines are usually based on comparatively small explosive-charge weights. The U.S. federal government restricts revealing the amount of TNT-equivalent explosives used to calculate minimum acceptable setback distance, therefore, it cannot be revealed here. It must suffice to say that it is a low figure. As a result, private-sector enterprises meeting these setback standards in good faith can be operating under a dangerous false sense of security.
The setback distances required to adequately protect against large bombs can be great. For example, a safe setback for a large truck filled with 4,000 pounds of explosives may be more than 900 meters (or slightly more than half a mile). The attack on the Islamabad Marriott involved at least one ton of explosives and large charge weights have been employed by terrorists in other attacks, such as 10,000 pounds in the attack against the Khobar Towers in Saudi Arabia in 1996.
As noted, the idea of the setback is to put the building out of the range of the explosive blast or shock wave as much as practical. The effects of explosive overpressure are calculated on the basis of the inverse of the cube of the distance apart. Because the value is cubed, the destructive effects of a blast lessen exponentially as distance increases. Therefore, every meter of setback is vitally important. This said, such massive setbacks are unrealistic for many project sites, especially in dense, urban environments.
Optimal setback is rarely achievable, but other design elements can help to mitigate the risk. All vehicle ingress and egress routes, including the delivery and loading dock, trash pickup area, and guest and visitor parking locations should be planned to minimize the effect of VBIEDs. If possible, roadways that are perpendicular to the hotel should be blocked because of the high speed an approaching vehicle could attain. A location where vehicles can be inspected should be established at a safe distance. The distance between this vehicle control point and hotel entry points should be as great as possible.
Another facet of the design should be a hardened and protected post from which armed security officers can monitor the vehicle control point without likelihood of being entirely suppressed during an attack. Yet another part of the plan might be to subtly design vehicle entry control points that “lock-in” vehicles while they are being inspected and the driver’s credentials are being examined.
Designers should avoid creating underground parking beneath the hotel. For the possible consequences, we only need look to the 1993 attack on the World Trade Center when a 1,336-pound truck bomb was detonated below the North Tower, beneath the lobby of the Vista Hotel. If an underground garage can’t be avoided, robust columns and thicker upper floor slabs in parking areas should be used and connections made more ductile. Beyond design would be policies and procedures that control access to the garage as well as monitor for suspicious activity.
None of this has to appear to guests and visitors as overwhelming security. For example, hotel staff drivers at the control point could take over arriving vehicles, including those of guests who have chauffeurs. This can be presented as a service while it simultaneously allows security to carefully inspect the vehicles and to control their movement. It also allows the hotel to control all vehicles entering underground parking. This practice was implemented by the Vista Hotel following the 1993 New York City World Trade Center bombing. Most guests welcomed the valet parking and being treated like VIPs and only a few recognized it as an antiterrorism countermeasure.
The architecture and engineering design process can mitigate the risk of armed attacks using many of the same countermeasures applied to bomb threats. There are, however, some critical additional design criteria to address this specific threat.
The property boundary should be studied to discover any feature that might enhance or hinder both terrorists and counterterrorism response forces. These features may include bodies of water; natural barriers such as boulders, hedges, and bushes; and topographic features such as hills or culverts. One high-concept idea to assist counterterrorism forces is to include secret entrance routes from the perimeter, as well as interior passages, for use in a terrorist hostage taking.