not practical to conduct an upward attack on floors without power tools.
Note in Figures 33 through 35 that the minimum penetration times are
presented as a function of the thickness of the cross section and the size
and spacing of reinforcing. Different combinations of size and spacing of
the reinforcing are reflected in the family of curves identified by A, B, C,
etc. These combinations are summarized in Tables 22 and 23. In general, a
required penetration time can be achieved by providing either a thicker cross
section and by adding more reinforcing. Which is more appropriate may be
decided by structural or other considerations. Note also in Figures 33
through 35 that the penetration times are minimum values based on the proper
selection and optimal use of the attack tools. In this regard, the region
identified as the "medium-severity threat level" for a downward attack in
Figures 33 and 34 assumes only hand-powered tools and some limited
battery-powered tools are used. For these cases, the thickness and rebar
combination required is less than the "high-severity threat level" where
power and thermal tools also may be used.
Medium-Severity Threat Level
a) Roofs and ceilings. If the threat is of medium
severity and the delay time requirement is within the medium threat
cross-hatched region in Figures 33 and 34, the chart can be used to establish
the minimum penetration time.
On the other hand, if the delay time requirement is outside the
medium-severity region, a design having a thicker cross section and more
rebar is required to preclude only the practical use of hand- or
battery-powered tools. In this case, pick a cross section that is just
outside the medium-severity region to stop the threat. For example, if your
delay requirement is 10 minutes for a medium-severity threat against
reinforced concrete (Figure 33), providing something just over 6 inches (15
mm) of concrete will stop the threat.
b) Floors. Because an attacker is working against
gravity, it is not practical to attack a reinforced concrete floor from below
for a medium-severity threat using only hand-held tools. Any thickness or
rebar combination is suitable for this case.
High-Severity Threat Level
a) Roofs and ceilings. Figures 33 and 34 with Table 22
give concrete thickness and rebar combinations that will achieve a required
penetration time for both conventional and SFR concrete.
b) Floors. Figure 35 and Table 23 give concrete thickness
and rebar combinations that will achieve a required penetration time for
conventional and SFR concrete.