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 MIL-HDBK-1013/1A
detention units American National Standards Institute/American Society for
Testing and Materials (ANSI/ASTM A 750-84) shows at least one secure design.
Duct dimensions should be kept at less than a man-passable cross section.
However, airflow capacity requirements and cost can be applied to ducts in an
effective manner due to confined working spaces and the possibility of using
multiple and widely spaced grills.
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, and with
spacings as shown in Table 34 provide the penetration times indicated.
Another option is to insert strategically placed honeycomb sections (similar
to those shown in Figure 44) to restrict passages. Although such sections
will require care in design to avoid airflow and noise problems, they are
feasible. Since duct walls are generally easy to cut through, i.e., 18- to
24-gauge (1.2- to 0.6-mm) sheet steel, the honeycomb must be strategically
located so that the intruder cannot bypass it by gaining entrance to the
crawl space or "soft" ceilings. It may be necessary to reinforce the duct
walls at some locations with high resistance materials such as
steel/polycarbonate laminates (see Table 38 and Figures 46 through 48 in par.
5.6.3). The honeycomb sections should be located, if practical, at sharp
bends in the ducting. Depending on duct size, cost, and air flow, an
alternative approach would be to replace the single duct with a double- or
triple-duct system at selected, strategic points. As previously noted, the
inclusion of required appurtenances, turning vanes, dampers, pressure plates,
or the final air distribution fixture may also add a few minutes to
penetration time. This can be further enhanced by anchoring such fixtures
securely and by using grills and bar gratings of a dimension and shape that
force the use of large and unwieldy tools (see Table 34).
b) Gravity Vents. Gravity vents vary in size from 6
inches (150 mm) to 4 by 8 feet (1,200 by 2,400 mm). Since they terminate
inside the building, gravity vents can provide direct entrance if not
properly protected. A typical barrier now used in these ducts is a 3/8-inch
(9-mm)-thick perforated steel plate welded to an 18-inch-diameter
(450-mm-diameter) pipe. A key limiting factor effecting the hardening of a
vent is the depth (i.e., volume of space) available for installing barriers.
i) Hardening against low- and medium-level threats.
If the vent is simply an aperture in a wall or roof, the problem is analogous
to hardening a window with grills or bars. Single (or multiple) fixed grills
with No. 4 (12.7 mm) or smaller bars and with spacings of 3 inches (75 mm) or
more can provide limited penetration times against hand-held tools (see
Table 34).
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