Protection against attacks with the use of explosives

AuthorVasilis Karlos - Martin Larcher
3!Protection against attacks with the use of explosives
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
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
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
(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.
10 −.
Truck with
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

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