When designing protection measures to mitigate the threat of an explosive device, there is a lack of accurate data on which to base calculations and therefore assumptions have to be made. These assumptions may not be sufficiently close to reality when considering terror events and vehicle born improvised explosive devices (VBIEDs) in particular.  VBIEDs are the most common type of explosive device used by terrorists, and will be the focus of this analysis.

The dynamics of an explosive event is determined by a range of factors, including:

• Explosion velocity and gravitational weight;
• The shape of the charge;
• The location and strength of the initiation process;
• The explosion's package.

Usually a terrorist building a bomb is more interested in camouflaging the device to ensure that it will not be discovered than in the initiation mechanism, which is usually home-made and not particularly effective. As a result, optimum efficacy of the explosive effect (pressure and impulses) will not be achieved. It can be expected that a maximum of 70% to 80% is achieved in the best case, but it is estimated that typical efficacy is usually 30%- to 40% or lower.  A number of VBIEDs that will be analyzed in this paper exhibited efficacy of less than 3% to 5% of the optimum results.

In addition to variations in the explosive effect, there are at least four ways in which a large explosive event may affect a structure:

• Contact blast shattering phenomenon;
• Blast pressure (direct and reflected);
• Large debris;
• Heat wave.

In all four phenomena, text books and software modeling do not take into account the difference between the dynamics of a VBIED event and military grade explosives. For example, in a VBIED event, we usually ignore the contact blast effect as that is very low compared to a military grade explosive device. The pressure wave is relatively long and the temperature high as a result of a poor initiation and subsequent explosion sequence as well as poor quality of the explosive material. Fragmentation and debris may be characterized by a large variety of fragments from the car's engine and axle flying with high velocity into building walls and columns. In this way, a VBIED event differs greatly from any military style event typically analyzed in our text books.

In this paper, we focus on defining a VBIED’s charge quantity and expected explosion output, leaving the other phenomena for further study.

 

Defining the problem

In terror related security projects, levels of protection and structural enhancement are determined based on the damage expected if and when a large explosion, usually a VBIED, occurs in the vicinity of a building. It could be suggested that in this case the more protection, the better, but in the real world this often means that the proposed solution is unaffordable or unacceptable and that the site remains unprotected.

In order to overcome this problem, American authorities recommend protection levels based on the size of the vehicle that can approach the building and are able to enter into the "protected" area. The threat level is determined according to the internal vehicle volume where explosives can be concealed, which is then converted to an equivalent weight of military grade TNT (see Figure 1 as an example). The risks and consequences of such explosions are shown in Figure 2.

What these tables do not take into account, however, is that low explosive density, an inefficient detonation mechanism, poor design of the geometry of the explosive device, or inefficient explosive material can all result in a reduction in the efficacy of the explosive event to the level of far smaller TNT equivalent weights.  As will be shown below, data suggests that the efficacy may be as low as 33% of comparable quantities of TNT.

Not enough information is currently available in open sources (there may be more available from classified sources) on the performance of the VBIEDs that exploded in terror attacks around the world compared to properly designed TNT equivalent explosive devices.

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Uzi More, M.I.P. Security Ltd., Israel