missile shape factor = 0.72 for flat-nosed bodies, 0.84 for
blunt-nosed bodies, 1.00 for average bullet-nosed
(spherical end) bodies, and 1.14 for very sharp-nosed
missile weight (lb);
effective missile diameter (in.);
missile impact velocity (ft/sec).
For noncircular or irregular shaped missiles, D is computed as the equivalent
diameter of the contact area. The formula is only applicable when the target is
sufficiently thick to prevent scabbing. Scabbing would lead to more penetration
than would be predicted by the formula. The penetration depth, x, is used to
predict scabbing and perforation thickness.
The NDRC formula was derived from impact data for missile velocities greater
than 500 ft/sec or 340 mph (152 m/s) and missile diameters ranging from 1 in. to
16 in. (2.54 cm to 40.6 cm). The testing conditions included ratios of target
thickness to missile diameter equal to or greater than 3. However, because it
was based on the theory of penetration, it can be extrapolated beyond the range
of available test data. The effects of reinforcing are inherently included.
In a properly designed reinforced concrete structural member with adequate
longitudinal and transverse reinforcing steel in each direction, the spreading of
the radial cracks produced by the impact will be inhibited by the rebars, and the
amount of concrete that spalls off the front face will be limited.
The missile velocities from an aircraft crash event are typically less than
500 ft/sec or 340 mph (152 m/s ), and the missile diameters are greater than
12 in. (30.5 cm). Missiles impacting the structures are also mostly deformable.
Because of these factors, the Modified NDRC formula shall only be used to
predict local penetration of the reinforced concrete structures. For further details
on limitations, see Reference 2.