On April 19, 1995, the Alfred P. Murrah Federal Building, Oklahoma City, Oklahoma was bombed, causing extensive damage to the structure, the loss of 168 innocent lives, the victimization of the families of those who lost loved ones, hundreds of non-fatal injuries, and substantial property damage in the vicinity.
The media and the Executive branch reported that the sole source of the devastation was a single truck bomb consisting of 4,800 pounds of ammonium nitrate, transported to the location in a Ryder Truck and parked in front of the building. It is impossible that the destruction to the building could have resulted from such a bomb alone.
To cause the damage pattern that occurred to the Murrah building, there would have to have been demolition charges at several supporting column bases, at locations not accessible from the street, to supplement the truck bomb damage. Indeed, a careful examination of photographs showing the collapsed column bases reveals a failure mode produced by demolition charges and not by a blast from the truck bomb.
To understand what caused the damage to the Murrah Building, one needs to understand some basics about the use and nature of explosives.
First, blast through air is a very inefficient energy coupling mechanism against heavily reinforced concrete beams and columns.
Second, blast damage potential initially falls off more rapidly than an inverse function of the distance cubed. That is why in conventional weapons development, one seeks accuracy over yield for hard targets. That is also why in the World Trade Center bombing (where the only source of blast damage was a truck bomb) the column in the middle of the bombed-out cavity was relatively untouched, although reinforced concrete floors were completely stripped away for several floors above and below the point of the bomb's detonation (see Time Magazine, 3-8-93, page 35).
By contrast, heavily reinforced concrete structures can be destroyed effectively through detonation of explosives in contact with the reinforced concrete beams and columns. For example, the entire building remains in Oklahoma City were collapsed with 100-plus relatively small charges inserted into drilled holes in the columns. The total weight of all charges was on the order of 200 pounds.
The detonation wave pressure (1,000,000 to 1,500,000 pounds per square inch) from a high detonation velocity contact explosive sweeps into the column as a wave of compressive deformation. Since the pressure in the wave of deformation far exceeds the yield strength of the concrete (about 3,500 pounds per square inch) by a factor of approximately 300, the concrete is turned into granular sand and dust until the wave dissipates to below the yield strength of the concrete. This leaves a relatively smooth but granular surface, with protruding, bare reinforcement rods__a distinctive signature of damage by contact explosives. The effect of the contact explosive on the reinforcement rods themselves can only be seen under microscopic metallurgical examination. (The rods are inertially confined during the explosion and survive basically in tact because of their much higher yield strength and plasticity.)
When a reinforced concrete structure is damaged through air shock coupling and the pressure is below the compressive yield strength of the concrete, the failure mode is generally compressive structural fracture on one side and tensile fracture on the other__ both characterized by cracks and rough fracture surfaces. Such a surface texture is very different from the relatively smooth granular surface resulting from contact explosives.
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