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ii)  Hardening against high-level threats.  If
possible, the vent should be kept to less than man-passable size.
Penetrations through vents smaller than the 96-square-inch (0.06-sq-m) man-
passable opening require attacking the surrounding wall or roof to enlarge
the vent.  The vent itself may provide an advantage in such an attack since
it can eliminate the necessity of drilling a hole for introducing tools.  Any
vent, no matter how small, can provide a convenient entry for the blade of a
tool used to breach the roof or wall through which the vent passes.
Therefore, as a minimum, all vents should be hardened with massive steel
collars at the structure interface, as illustrated in Figure 45.  If the vent
must be kept at a man-passable size, and if space exists behind it, the best
approach to increase penetration time is to fill the opening with lengths of
steel pipe welded into a "honeycomb" (see Figure 44).  This causes the
intruder to have to make cuts in depth, which increases cutting time but also
seriously interferes with his use of tools.  An alternative approach is
multiple and widely spaced grilled barriers in the shaft or duct leading from
the vent (see Table 34).  This approach is only effective, however, if the
facility's mechanical layout is such that the intruder cannot cut his way out
of the duct or shaft and gain access to the facility before the grilled
barriers are reached.  Even very small vents must be protected since they can
be an easy route for introducing explosive charges.  Traps or bends at
carefully selected locations can often prevent this.
c)  Exhaust vents.  Exhaust vents through roofs and walls
are generally considered to be protected by the equipment used in conjunction
with them.  However, if the equipment is removed, the entrance is open.
Because the ductwork, damper, etc., are usually constructed of light sheet
metal, penetration can be accomplished through the use of hand tools.
Typical exhaust ducts range in size from 6 inches (150 mm) to 4 by 8 feet
(1,200 by 2,400 mm).  The discussion of hardening techniques under gravity
vents, ventilation ducts, and air distribution fixtures (above) generally
applies in the case of exhaust vents.  Possibilities for reducing vents to
less than man-passable size by using multiple honeycombs (Figure 44) should
be considered. In some cases, the exhaust system machinery itself may add to
penetration time.
Roof-Mounted Equipment.  Roof-mounted equipment, such as air-
supply fans, exhaust fans, gravity ventilators, and filter banks, are usually
welded or bolted to an equipment curb, duct system, or foundation and can be
removed with hand tools.  Openings uncovered when equipment is removed can
provide an adversary entry to the interior of the facility.  In many
installations, the removal of only eight bolts, plus the withdrawal of the
equipment, can provide access.  Although expensive, the only known way of
providing extended penetration time for man-passable openings exposed by the
removal of roof-mounted machinery is a hardened "penthouse" to house the
machinery.  Penthouse penetration time will depend on structural components,
doors, and openings used.  Specific penetration times can be estimated by the
same methods described for structures throughout this handbook.  The use of


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