levels. Grillworks of larger size bars [No. 5 (16 mm) or greater] and
various constrictive barrier designs are provided for the high threat levels.
Very high level threats require sacrificial enclosing structures around
Designing for the Very High-Severity Threat. The use of
explosives, either in bulk or augmented with flyer plates, can be especially
effective in quickly producing holes large enough for an intruder to crawl
through. There are only two ways of hardening against such threats: (1) the
use of building sacrificial areas, and/or (2) appropriately designed barriers
using massive reinforced-concrete construction.
Sacrificial Areas. Sacrificial areas in the building can be
employed above, below, and around the critical area in the building to be
protected (see Figure 30). The walls, doors, and other features of this
sacrificial area may be damaged, but will provide a standoff region to reduce
the effectiveness of the blast on the critical area. In general, the
critical area should be low, internal to the building, and well away from
exterior walls and roof. Any type of construction of the exterior walls,
roof, etc., of the sacrificial area is acceptable. Using a lighter
construction is more desirable and may preclude the attacker from using large
quantities of explosives, reducing the damage to the building.
Barriers to Counter an Explosive Attack
Construction. The only practical barrier construction to stop
a direct explosion is the use of massive reinforced concrete 18 to 48 inches
(0.46 to 1.2 m) in thickness. Because of structural considerations, such
thick cross sections normally will be limited to the walls in the area
protecting the only critical resource. Roofs and doors require the use of
sacrificial areas and foyers as shown in Figure 30.
Penetration Times. Thick, heavily reinforced concrete walls
can provide significant penetration delays. While power, hand, and thermal
tools are impractical for removing the large masses of concrete thick walls
contain, they can be used to cut and remove the reinforcing material after it
is exposed by the explosives.
a) Explosive threats. While explosives can produce large
holes, even in thick concrete walls, they do not remove the reinforcing
material. The shock waves produced by an explosion propagate throughout the
concrete, resulting in internal fragmentation and spalling (breaking off) of
the inner and outer surfaces. The pressure of the explosion forces the
fragmented concrete out of the wall, and a relatively clean hole results.
Cutting and removing the reinforcing material for a crawl hole contributes to
most of the delay.