iv) Riveted-steel grate. Riveted steel grate has a
minimum bearing bar size of 3/4 by 1/8 inch (19 by 3 mm) spaced 2-5/16 inches
(60 mm) on center and a maximum bearing-bar size of 2-1/2 by 3/16-inches (63
by 5 mm) spaced 3/4 inch (19 mm) on-center. The spacer bars are riveted
about 7 inches (175 mm) on-center for average installations or 3-1/2 to 4
inches (90 to 100 mm) for heavy traffic, or where wheeled equipment is used.
v) Welded-steel grate. Welded-steel grate has
minimum and maximum bearing-bar sizes of 3/4 by 1/8 inch (19 by 3 mm) and
2-1/2 by 3/16 inches (63 by 5 mm), respectively. The minimum spacing is
15/16 inch (24 mm) on-center, and the maximum spacing is 1-3/16 inch (30 mm)
on-center. Spacer bars are typically welded either 2 or 4 inches (50 or 100
vi) Expanded-steel grate. The expanded-steel grate
has a minimum diamond size of 1.33 by 5.03 inches (35 by 125 mm) and a
maximum diamond size of 1.41 by 5.33 inches (36 by 135 mm).
b) Penetration times for existing construction. Table 44
provides the penetration times for existing metal roof and floor
construction. In most cases, low- and medium-level threats are not practical
against this type of construction. Metal roofs provide only about 1.5 to 2
minutes of delay against medium- to low-level forced-entry threats. In
floors, riveted or welded expanded or steel grates provide less than 1 minute
of delay against a high-level attack.
c) Low- and medium-severity threats. With the exception
of ribbed steel, corrugated metal, and expanded steel grate, it is not
practical for a low- or medium-severity threat to attack metal roofs or
floors (see Table 44). The existing construction should be adequate. For
the above-listed exceptions, riveted, or welded steel grate can be afixed to
the underside to enhance the hardness.
d) Retrofit options for high-severity threats. The steel-
ply retrofit options shown in Figures 46 through 48 and Table 38, or the
riveted or welded steel grating shown in Figure 49 can be affixed to the
interior of the roof or underside of the roof or floor to increase the
effective delay time. See par. 18.104.22.168(1)(d) for additional information.
Door Retrofit Construction
Personnel, Vehicle, and Vault Doors. Where existing personnel,
vehicle, and vault doors do not provide adequate penetration delay, it may be
possible to replace the existing doors with new ones meeting the desired
delay requirements. Refer to par. 5.5.5 for personnel, vehicle, and vault
door options and details. Where the existing construction will not support