Blast in OK City bombing. (was Terrorist Entropy vs...)

Kyle L. Webb (
Sat, 3 Aug 1996 21:26:29 -0600 (MDT)

Ian Goddard wrote:

> Ammonium nitrate exerts its strongest explosive force along a vertical,
> not a horizontal, axis. An ammonium nitrate / fuel bomb of the size used,
> or even much larger, could not exert enough pounds of pressure per square
> inch (from blast's epicenter to building) to cut the heavy steel re-bar
> reinforced columns, even deep inside the building, as was seen.

Ammonium nitrate and fuel oil is a disordered substance, thus having no
preferred direction in its detonation. Unless the geometry of the bomb is
such that it channels the blast wave, wouldn't the applied force be pretty
much isotropic?

(quoted by Goddard):
> " Blast through air is a terribly inefficient coupling
> mechanism against heavy reinforced concrete beams and
> columns; blast impulse and its potential for damage -
> drops dramatically when traveling through air, initially
> falling off more rapidly than an inverse function of the
> distance cubed. Even though the Oklahoma City truck bomb
> made an enormous impulse wave, it is wrong, Partin says,
> to be overly impressed and to attribute a force to that
> explosion which it clearly did not have. " (*)

The analysis is correct for free standing columns, since the only bending
moments are due to the difference in applied pressure along the beam.
However in a building, the connection to the floors provides bending fulcrums
which allow part of the concrete to be put in tension, causing the concrete to
break since it has little tensile strength. The connections to outside concrete
walls provide broad surfaces for the blast pressure to act over that couple
fairly well to the columns. The reinforcing iron is started bending as well,
and having lost the compression bearing structure of the concrete, then
buckles under the weight of the upper floors. The explosive doesn't have to
have the impulse to sever the beams, it undermines them and the weight of the
upper floors does the rest. During the falling of a building, some of the
re-bars will be placed under tremendous tension, and sheared along their width.
That damage looks superficially similiar to the tearing produced by a nearby
charge of high explosive along one side of the beam.

I can't rule out what Partin is saying, but I'm offering a known
damage mechanism that could reasonably produce a similiar effect
without needing to invoke conspiracies.

Kyle L. Webb Dept. of Physics + Astronomy University of New Mexico