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 MIL-HDBK-1013/1A
If unprotected, vertical chases connecting adjacent floors may provide
unlimited access once an intruder is inside the chase system. Similarly,
horizontal chases (walk-throughs) may offer little impedance to movement
except that afforded by the internal equipment, piping, cable, and the
entrance door(s). Entrances to, or exits from, overhead crawl spaces may
also be made from some chases.
i) Hardening against low- and medium-level threats.
Single (or multiple) fixed grills with No. 4 (12.7 mm) or smaller bars and
with spacing of 3 inches (75 mm) or more can provide limited penetration
times against hand-held tools (see Table 34).
ii) Hardening against high-level threats. Single
(or multiple) fixed grills with No. 5 (15.9 mm) or larger bars with spacings
as shown in Table 34 provide the penetration times indicated. Another
option, if maintenance access is not required, is filling the opening with a
2-foot (0.6-m) length of steel pipe, welded together and anchored securely in
place by a welded structure on the inside (secure side) of the structure as
shown in Figure 44. The honeycomb material should be of a grade of steel
reasonably resistant to cutting with hand and thermal tools [at least
1/8-inch (1.6-mm) thick]; however, the penetration time will accrue mainly
from the length of the honeycomb and the resultant necessity for multiple
long cuts and debris removal in the relatively restricted space of the duct.
If possible, such a barrier should be located at a sharp adjacent turn in the
tunnel to further restrict the use of cutting tools. This arrangement can
also be used for tunnels with electrical lines, since maintenance personnel
can have access to both sides of the impediment (constriction), and cables
can be threaded through the relatively short constriction.
Construction can be undertaken in two ways. One
approach is to weld the steel pipes front and back at least 3 inches (75 mm)
on each end and at each point where the steel pipes intersect. No steel pipe
diameter inside the pipe should be greater than 10 inches (250 mm) to ensure
a smaller-than-man-passable opening. A second approach is to eliminate the
center steel pipe and to connect the remaining six pipes inside the tunnel
with continuous welds. However, if this approach is taken, the designer
should be careful to ensure that the area in the center, which would have
been filled by the seventh center pipe, as shown in Figure 44, is not a man-
passable opening. These constrictions should be located at attack-hardened
secure walls. The length of the constriction should force the intruder to
attack and remove each barrier separately. The confined working space and
the necessity for debris removal further add to penetration time.
c) Sleeves and trays. Sleeves are pipe penetrations 1
inch (25 mm) to 8 inches (200 mm) in diameter through walls, roofs, etc.
Trays are removable composed of a sheet-metal-covered conduit 3 inches square
(75 mm square) or larger. Sleeves and trays should penetrate security walls
at a
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