Quantcast Retrofit options for high-severity threats

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o  Precast walls.
These are constructed elsewhere and
shipped to the site.
The thickness of typical precast or tilt-up walls may be as low as
3-1/2 inches (90 mm) to as high as 12 inches (300 mm).  A cast-in-place wall
typically begins at 4 inches (100 mm) and may reach as high as 30 inches (760
mm).  The corresponding reinforcement may be as low as a single layer of No.
3 (9.7-mm) steel bars at 12-inch (300-mm) spacing each way, for the
3-1/2-inch (90-mm) or 4-inch (100-mm) wall, to as high as No. 8 (25-mm) bars
at 3 inches (75 mm) each way at each face for the 12-inch (300-mm) wall.
Concrete with compressive strengths between 3,000 and 6,000 psi [20.7 and
41.4 megaPascal (MPa)] and steel rebar with tensile strengths between 40,000
and 60,000 psi (276 and 414 MPa) are typically used.
b)  Penetration times for existing reinforced concrete
construction.  For conventional concrete materials, the penetration times
range from about 2 minutes to more than 60 minutes.  At the time this
handbook was written (1992), no data for hand, power, and thermal tool
attacks on concrete walls exceeding 12 inches (300 mm) in thickness were
available.  One can expect, though, that these thicker walls will exceed 40
to 45 minutes to penetrate, with a 30-inch (760-mm) wall taking much more
than an hour. For walls up to 12 inches (300 mm) thick, Figure 33 and Table
22 (par. 5.5.3) can be used to estimate penetration times for various
thicknesses and rebar combinations.  It should be noted that the data point
on Curve C in Figure 33 with a penetration time of about 15 minutes for
8-inch (200-mm) reinforced concrete walls is the expected penetration time of
the 8-inch (200-mm) reinforced concrete wall construction mandated for
Category II AA&E storage facilities by DOD 5100.76-M.
c)  Retrofit options for medium-severity level threats.
Note in Figure 33 that reinforced concrete 6 inches (152 mm) thick with the
"B" rebar combination shown in Table 22 achieves minimum penetration times up
to about 7.5 minutes for medium-severity level threats.  If the concrete
thickness and rebar combination is outside the medium threat region shown in
Figure 33, using hand-held attack tools alone is not practical.  In this
case, the existing wall provides adequate security.  If the concrete
thickness is at least 6 inches (152 mm) and the rebar combinations are at or
within the medium threat level region of Figure 33 and more delay time is
required, about 14 additional minutes can be gained by attaching a 9-gauge
(3.8-mm) flattened expanded steel grate to the interior of the wall.
d)  Retrofit options for high-severity threats.  If the
required delay time is greater than that achievable by the existing
construction, this penetration delay time can be doubled from the values
shown in Figure 33 by simply fixing a 10-gauge (3.4-mm) sheet steel (ASTM
A589) to the interior surface of the wall using lag screws or bolts.  If this
is still not adequate, the steel-ply options summarized in Table 38 and
Figures 46 through 48 or the riveted or welded grating shown in Figure 49 can
be used.


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