This section defines the missile resisting requirement for the equipment/materials purchased under this specification.
Missile resistant doors shall take into consideration a missile generated by natural phenomena which is a small rigid missile of a size sufficient to just pass through any openings in protective barriers.
Missile resistant doors except personnel doors shall also be designed to withstand a force of large soft missile without exceeding the specified minimum yield strength of materials
The missile characteristics are as follows

a. 1800 kg automobile
b. 125 kg 8 inch armor piercing artillery shell
c. 1 inch solid steel sphere

The 8" artillery shell is represents the bounding missile to be generated by natural phenomena. The treatment of the artillery shell as an armor piercing blunt nose projectile is acceptable. The door and its frame shall be designed to not allow the missile to penetrate the door or cause the creation of a secondary missile. Hence the door shall be designed to prevent missile penetration and shall be designed to prevent catastrophic failure of the door and its frame due to the resulting impact load.
Horizontal velocity shall be 80.4 ft/sec(35% of the maximum horizontal windspeed of the design basis tornado)

Vertical velocities for all missiles are assumed as 70% of horizontal velocities except foir small missile, That is, 'C'.

Design Basis :
NURG-0800, 3.5.1.4. Missile Generated by Natural Phenomena, Spectrum I

Total impacting load of the missile ‘a’, automobile, is given by the following formula.
F(t) = Dc(t) + μ χ [m(t)] x V(t)
Where
F(t) = Total impact load
Dc(t) = The collapse load of the automobile
μ = A coefficient for change in momentum
m(t) = Mass of the automobile reaching the wall per unit time
V(t) = Striking velocity

After failure of the frontal area of the automobile, it can be assumed that remaining part of the automobile acts upon the wall as impactive load. Thus, the second term on the right-hand side of the above formula is

F=μHV(t)

Where F is the impactive load. For μ 0.625.

Design Basis : impactive Effects of Tornado Missiles and Aircraft A.K.Kar, Jpurnal of the Structural Division STII of ASCE Nov. 1979

The calculation formula for penetration of missile ‘b’ and ’c’ into steel is given below:
E/DF2 = S x (16,000 x T2 /F2 + 375 WT/F2) / 46,500

Where
E=critical Kinetic energy required for penetration, ft-lb
D=rod diameter, in.,
S=ultimate tensile strength of target plate, psi,
T= target plate thickness, in.,
W=length of side of square window in the target frame, in.,
F=scale factor =W/Ws: Ws=4 in
Design Basis :
U.S. Reactor containment Technology, ORNL-NSIC-5, Bol. 1, Chapter 6, Oak Ridge National Laboratory