One layer of 3/4-inch (19-mm) plywood sandwiched between two layers of this
steel provides 14 minutes of penetration time. Adding layers of 90-pound
(200-kg) gravel finish roofing paper between the plywood and steel further
increases the penetration time to about 20 minutes (Table 38).
Alternatively, additional delay time can be obtained by the use of steel
grating as shown in Figure 49. This grating is available in standard 2- by
10-foot (0.6- by 3-m) panels and can be affixed to the walls. They can be
assembled in a variety of configurations of any length [in 2-foot (0.6-m)
increments] as described in MIL-HDBK-1013/4. Additional penetration times
offered against high-severity threats are approximately 2 minutes (minimum)
for the riveted and 1 minute (minimum) for the welded grading.
Masonry Wall Construction
a) Representative existing masonry construction. Masonry
walls are typically constructed of one or more of the following materials:
concrete masonry unit (CMU), brick, structural tile, or stone. Unreinforced
masonry wall construction typically consists of CMU, brick, structural tile,
stone, or a combination of these materials. CMU may range from 4 to 12
inches (100 to 300 mm) thick and may be left hollow or grouted solid.
Single-wye brick generally comes in widths of 4 to 12 inches (100 to 300 mm).
Structural clay tile will typically range from 4 to 8 inches (100 to 200 mm)
wide, and stone will usually vary between 6 and 24 inches (150 and 600 mm).
As for combinations of these materials, brick or CMU may range from 8 to 16
inches (200 to 400 mm) with masonry ties every second CMU course. Structural
clay tile on CMU may be found in widths from 6 to 16 inches (150 to 400 mm)
with masonry ties every second course. Brick of structural clay tile may
vary from 8 to 12 inches (200 to 300 mm) with ties every sixth brick course.
Finally, stone-on-CMU may range from 6 to 16 inches (150 to 400 mm) with ties
every second CMU course. Reinforced CMU may vary from 6 to 12 inches (150 to
300 mm) wide, grouted solid with reinforcing ranging from No. 4 rebar (12.7
mm) at 32 inches (800 mm) on-center horizontally and 16 inches (400 mm)
on-center vertically, to No. 5 (16 mm) at 16 inches (400 mm) on-center
horizontally and No. 8 (25 mm) at 8 inches (200 mm) on-center vertically.
Brick-on-stone, double-wye ranges from 10 to 16 inches (250 to 400 mm) thick,
grouted solid with No. 6 (19-mm) rebar at 12 inches (300 mm) on-center
horizontally and No. 9 (29 mm) at 12 inches (300 mm) on-center vertically.
Reinforced CMU with 4-inch (100-mm) stone or brick veneer varies from 10 to
16 inches (250 to 400 mm) wide, grouted solid, with reinforcing ranging from
No. 4 (12.7-mm) rebar at 32 inches (800 mm) on-center horizontally and at 16
inches (400 mm) on-center vertically, to No. 5 (16-mm) rebar at 8 inches (200
mm) on-center horizontally and No. 8 (25 mm) at 6 inches (150 mm) on-center
b) Penetration times for existing masonry construction.
Conventional masonry walls provide only limited hardness against forced-entry
attacks. They typically offer penetration times ranging from less than 1.5
minutes for hollow masonry to 2 to 5 minutes for solid masonry against
medium-severity level attacks (see Figure 31 and Table 21 in par. 5.5.3).
For thick solid walls with significant reinforcing, higher penetration times