CHARACTERISTICS OF THE DAMAGE CAUSED BY THE ATOMIC BOMBS

CHARACTERISTICS OF THE DAMAGE CAUSED BY
THE ATOMIC BOMBS
The damage to man-made structures caused by the bombs
was due to two distinct causes: first the blast, or pressure
wave, emanating from the center of the explosion, and,
second, the fires which were caused either by the heat of
the explosion itself or by the collapse of buildings containing
stoves, electrical fixtures, or any other equipment which
might produce what is known as a secondary fire, and subsequent
spread of these fires.
The blast produced by the atomic bomb has already been
stated to be approximately equivalent to that of 20,000
tons of T.N.T. Given this figure, one may calculate the expected
peak pressures in the air, at various distances from
the center of the explosion, which occurred following detonation
of the bomb. The peak pressures which were calculated
before the bombs were dropped agreed very closely
with those which were actually experienced in the cities
during the attack as computed by Allied experts in a number
of ingenious ways after the occupation of Japan.

The blast of pressure from the atomic bombs differed
from that of ordinary high explosive bombs in three main
ways:
A. Downward thrust. Because the explosions were well
up in the air, much of the damage resulted from a downward
pressure. This pressure of course most largely effected
flat roofs. Some telegraph and other poles immediately
below the explosion remained upright while those at
greater distances from the center of damage, being more
largely exposed to a horizontal thrust from the blast pressure
waves, were overturned or tilted. Trees underneath
the explosion remained upright but had their branches broken
downward.
B. Mass distortion of buildings. An ordinary bomb can
damage only a part of a large building, which may then
collapse further under the action of gravity. But the blast
wave from an atomic bomb is so large that it can engulf
whole buildings, no matter how great their size, pushing
them over as though a giant hand had given them a shove.
C. Long duration of the positive pressure pulse and consequent
small effect of the negative pressure, or suction,
phase. In any explosion, the positive pressure exerted by
the blast lasts for a definite period of time (usually a small
fraction of a second) and is then followed by a somewhat
longer period of negative pressure, or suction. The negative
pressure is always much weaker than the positive, but
in ordinary explosions the short duration of the positive
pulse results in many structures not having time to fail in
that phase, while they are able to fail under the more extended,
though weaker, negative pressure. But the duration
of the positive pulse is approximately proportional to the 1/
3 power of the size of the explosive charge. Thus, if the
relation held true throughout the range in question, a 10-ton
T.N.T. explosion would have a positive pulse only about 1/
14th as long as that of a 20,000-ton explosion. Consequently,
the atomic explosions had positive pulses so much longer
then those of ordinary explosives that nearly all failures probably
occurred during this phase, and very little damage could
be attributed to the suction which followed.
One other interesting feature was the combination of flash
ignition and comparative slow pressure wave. Some objects,
such as thin, dry wooden slats, were ignited by the
radiated flash heat, and then their fires were blown out
some time later (depending on their distance from X) by
the pressure blast which followed the flash radiation.