MadSci Network: Earth Sciences

Re: How do you scale an explosion by it's size?

Date: Mon Apr 22 01:35:09 2002
Posted By: Joseph Weeks, President
Area of science: Earth Sciences
ID: 1013984645.Es

Determining the size of an explosion is not an exact science because there 
are a wide range of variables.  For example, the size of crater produced 
by an explosion depends on a variety of factors, specifically how strong 
is the material subjected to the blast, what is the distance from the 
explosive to the ground, and what are the characteristics of the 
explosive.  So, looking at the size of a crater isn't necessarily the best 
way to determine the size of an explosion.

When an explosive detonates, it generates a pressure wave which expands 
more or less spherically from the point of the explosion.  The pressure 
wave decreases to the third power with distance from the source of the 
blast, i.e., double the distance from a blast and the overpressure, or 
pressure higher than atmospheric pressure decreases by a factor of 8.  The 
following link: presents 
some simple formulas for calculating blast overpressures.  Of course, 
buildings, hills, and other features can interact with the blast wave, 
causing overpressures to increase or decrease.

Overpressures developed by nuclear explosives have been studied 
extensively.  The following link:
als/amedp6/PART_I/chapter3.htm provides a description of the effects of a 
nuclear blast.  Table 3-II in 
that reference shows the overpressures necessary for certain types of 
damage to occur.  For example, an overpressure of 3 to 7 kPa results in 
glass being shattered.  So, if investigators are attempting to determine 
the size of a specific blast, they can measure the distance how far away 
from the blast that windows are broken and then back calculate to 
determine the approximate size of the blast.

There are computer models available to predict the size of a blast and its 
effect on structures which enable structural engineers to design oil 
platforms and chemical processing equipment to be safer to operate.  These 
models are also used to determine safe distances for storing explosives so 
that in the event of an accident, an explosion doesn't propagate from one 
cache of explosives to the next.

Thanks for giving me the chance to talk about explosives.  It was a blast.

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