Protection against attacks with the use of explosives

• Author: Vasilis Karlos - Martin Larcher
• Pages: 20-55

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3!Protection against attacks with the use of explosives

3.1!General

Blast is defined as a very fast chemical reaction involving a solid, dust or gas, during which a sudden release

of energy and hot gases takes place. During an explosion the produced hot gases expand and occupy all the

available space generating a wave type propagation that grows outwards from the surface of the explosive.

Along with the produced gases the surrounding air also expands leading to the creation of a blast wave that

impinges on structures located in its path. At the present guideline, the examined blast waves are the result of

detonations, since terrorist attacks are usually performed with solid materials characterized by fast reaction

rates. This means that the resulting pressures rise nearly instantaneously from atmospheric to their maximum

value and rapidly decay in a matter of milliseconds. Only part of the available explosive energy is transformed

into blast waves, as the rest of the detonation products mix and burn with the surrounding air. This afterburning

process does not affect the produced blast wave as it occurs at a later stage, but should be considered in the

case of confined and internal explosions.

Blast waves propagate outwards in all directions (spherically if the explosion takes place in mid-air or hemi-

spherically if the explosive is placed on the ground) at speeds greater than the speed of sound. The front of the

blast wave, i.e. the shock front, is characterized by high pressures (or better overpressures as they are calculated

in ambient conditions), and their amplitude reduces with increasing distance from the detonation centre, while

its duration increases. When the blast wave comes to contact with a rigid surface a reflection occurs that results

in increased applied pressures on the surface, known as reflected pressures.

The characteristics of a blast wave at a certain location depend on the distance from the detonation centre, the

type and weight of the explosives, and the interaction with the ground or other obstacles situated in its path.

Three types of non-contact, unconfined explosions are considered in this guideline, as shown in Fig. 9. Each type

depends on the relative position of the detonation centre to the point of interest, i.e. its height from the ground

and its horizontal distance between the projection of the explosive to the ground and the structure.

a.!Free-air bursts: The charge is detonated in the air and the created blast wave propagates

spherically outwards impinging onto the structure without interacting with obstacles or the

ground.

b.!Air bursts: The charge is detonated in the air and the created blast wave propagates

spherically outwards, but bef ore impinging onto the structure it has first interacted with the

ground.

c.!Surface bursts: The charge is detonated almost at ground level and the blast wave

immediately interacts locally with the ground and propagates hemi-spherically outwards

before impinging onto the structure.

Figure 9. Types of external unconfined explosions: (a) Free-air burst, (b) Air burst, and (c) Surface burst.

In blast design, localized damage is usually not excluded, as long as it does not jeopardize the safety and load

bearing capacity of the structural system and the effects on humans are kept to a minimum. The structure

should still be able to fulfil its original purpose with a minimum level of disruption in its use while simultaneously

ensuring people’s safety. Several strategies exist for reducing the risk of failure and mitigating the effects of

an explosion. One option is eliminating or minimizing t he probability of occurrence of the action and applying

principles of capacity design, such as sacri ficial components that are able to reduce the explosion effects

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(EN1991-1-7, §3.2.3). Another approach is upgrading the perimeter of a site and increasing the distance of the

detonation centre from the target structure decreasing the resulting blast loads. Increasing the strength and

ductility of key design members is also an option that results in increased energy absorbing capacity. Also, the

presence of alternative load paths in case of member failure, prevents the development of a progressive

collapse mechanism (see Section 3.8.3) that could result in a large death toll.

3.2!Identifying worst-case scenarios

The probability of a structure or a building to be the target of an explosive terrorist attack is very low, but should

not be ignored since the outcome of such an attack could be devastating. The identification of potential targets

is a complex and challenging task and requires the analysis of various data, as has already been emphasized

in the previous section. Terrorists usually choose their targets desiring to cause human casualties and create a

psychological and economic impact to the society. Well-protected targets are often avoided by terrorists, as the

chances of a successful attack are lower. An attack scenario may involve weapons, vehicle-borne or person-

borne improvised explosive devices. Clearly, the larger the transportation vehicle, the larger the amount of

explosives it could transfer and the greater the resulting blast. Fig. 10 shows an estimate of the quantity of

explosives that could be carried by various means of transportation.

Figure 10. Upper charge mass limit per mean of transportation.

* Person borne improvised explosive device (PBIED)

**Vehicle borne improvised explosive device (VBIED)

When an attac k with the use of explosives is of concern, the charge type and size are usually decided by the

building stakeholders and the responsible engineers. The need for a detailed or simplified analysis for an attack

scenario depends on a variety of factors, such as the importance of the asset, its occupancy level and the

perimeter protection, as has been demonstrated in the previous chapter. This analysis is the base of a well-

designed building security plan that can deter intruders from performing an attack and increases the chances

of building survival if such an attack materializes.

,−.

/0+−.

10 −.

/00+−.

200+−.

3000+−.

4,00+−.

3,000+−.

Letter-

Parcel*

Briefcase

Backpack*

Motorbike**

Passenger

car**

Pick-up

track**

Van**

Truck**

Truck with

trailer**

Threat

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The consequences of an external explosion to a specific target greatly depend on the stand-off distance, defined

as the distance from a possible charge location to the building’s façade. As shown in Fig. 11, the explosion

energy decreases rapidly with stand-off distance. Hence, increasing the stand-off di stance by deploying

perimeter protection measures is usually the most economical method for decreasing a site’s vulnerability. For

example, the use of fences, bollards, walls, planters, trees or other type of barriers prevent the presence of

unauthorised vehicles that may carry explosives close to the structure. Clearly, the increase of the distance

between the detonation centre and the structure of interest is not always feasible, as in the dense built

environment of metropolitan areas, buildings usually occupy nearly all of the available lot, resulting in cases

where the perimeter of the field coincides with the structure’s façade. The more distant is a building from a city

centre, the larger the chances of a bigger lot which translates to bigger space between the building face and

adjacent streets and communal spaces.

The stand-off distance value that is utilized in the design process is usually equal to the closest point a vehicle

or person can reach from the structure of interest. This is dictated by the abovementioned security measures

and could be a point at the fence perimeter (presuming that the fence cannot be breached), a parking spot near

the building, a location in a square or atrium in front of the building etc. Consequently, the first step towards

defining the potential detonation centre to be inserted in the blast design study, is the evaluation of the

surrounding area characteristics. The site topography, meaning the location of the building in comparison to its

surroundings, is important for evaluating the opportunities the aggressors might have to strike.

Fig. 11 shows the normal peak reflected pressure and normal reflect ed impulse at a point with respect to the

distance from the detonation centre for a hemispherical blast wave from the explosion of 10kg, 100kg and

1000kg of TNT. The diagrams are produced following the equations proposed by (Kingery, Bulmash, 1984). The

plots show that a small increase in the stand-off distance results in large reduction in pressure and impulse

values for all charge weights. Furthermore, the decrease rate of peak pressure values is larger than that of the

positive impulse for the whole distance range.

Figure 11. Peak reflected pressure and reflected impulse versus stand-off distance.

The protection measures that are deployed at the perimeter of a building are also important when defining the

charge weight for the design process. For explosive devices of substantial size that have to be transported by

a vehicle (Vehicle Improvised Explosive Devices-VBIEDs) the stand-off distance is usually calculated from the

closest point to the structure accessible by a vehicle, such as the fence, the access control point etc. The closest

point to a structure may not coincide with the worst-case scenario for a specific component, since the blast

wave energy at a lo cation is also dependent on the an gle of in cidence, as will be s hown later. Therefore, the

critical detonation point should be defined taking into consideration both its stand-off distance and its relative

location to the structure. Clearly, the security measures are expected to fulfil their purpose so that the hostile

vehicle cannot breach the site premises, by either penetration, deception or other techniques. Similarly, the