In all the above examples, the shock wave was reflected, and the "responding" material effect on the blast wave was insignificantly different from that of a non-responding material. In all cases the shock wave reflected as if the structure was non-responding. This justifies and simplifies calculations of shock interactions with structures with the assumption that the structures are rigid and non-responding. The debris velocity of the structures reached a maximum of less than half of the material velocity behind the shock. The debris remains well behind the shock front because the shock velocity is greater than the material velocity at the shock front by the speed of sound. As the velocity of the air behind the shock slows and reverses, the debris falls to the ground or reverses direction and may fall closer to the detonation point than its original location. With the possible exception of a few buildings that may be in or very near the fireball, urban structures absorb little energy from the blast wave. The debris is almost always at least 1000 times denser than the air in the blast wave. The air responds 1000 times faster than the solid material. The kinetic energy of the debris is therefore much less than 1 % of the air blast energy. The debris from these structures generally falls within about four times the building height. Many years ago Hal Brode advised me to "not do hydro in your head." The above are several examples of why this is good advice for everyone. Most of the misconceptions about air blast waves and their interactions with structures are caused by a lack of understanding of the differences between overpressure and dynamic pressure. Further we must keep in mind that most solid materials, whether wood or steel, are at least 1000 times more dense than air and will therefore move one thousandth as fast as the air blast that loads them.
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