Preventing terrorist attacks on buildings

Buildings exist to provide a safe, serviced and moderated environment for a range of functions, including living space; manufacturing; utility generation and distribution; commercial business operations; transport hubs; and leisure and recreation facilities.

The design of buildings places emphasis on the comfort and safety of the occupants, cost-effective use of space, efficiency of business operations and the storage of records and business support systems such as complex information technology networks and computers. Increasingly they are designed to be flexible in use, resulting in large adaptable interior spaces and non-structural facades.

Most buildings have been designed to resist criminal activity and they often do that very well, but terrorism presents a scale of assault on the integrity of the building of a different character and magnitude. The purpose of this article is to explore the methods used by terrorists to attack buildings and some of the measures that could be employed to mitigate or defeat those threats.

A starting point is to consider the inherent weaknesses in buildings. The type of construction (as discussed in a previous article1), has a significant effect on the ability of a building to resist explosive attack. In outline, a building that has a solid, well-tied frame will be strong in this respect, while a building with masonry walls and without a frame will readily collapse. A building that has uncontrolled entry will allow a terrorist device to be delivered inside more easily. Terrorists will make use of pedestrian and vehicle access but also the access paths for the various utilities that serve the building. The location of the building in its surroundings will also play a part, for important security measures are sometimes impossible because of the constraints of a crowded site.

Terrorist attacks can appear as random and unpredictable, but study of the psychology of the terrorist and his or her aims gives some important clues. Terrorists desire a means of leverage that will gain them publicity to exert pressure on, or sometimes even replace, the existing government. It is often said that terrorists do not seek to cause casualties per se, but rather the impact triggered by casualties. Three broad methods are used to achieve those aims. First, by causing death or injury, terrorists aim to show that a government or organisation cannot protect its people. Second, by causing disruption, terrorists hope to demonstrate that a government cannot govern or an organisation cannot continue to function. Third, by causing or threatening unacceptable economic loss, terrorists hope to force compliance in the face of further loss. Unfortunately, attacks on buildings offer excellent opportunities for achieving all or any of these effects for the terrorist, which is what makes buildings such frequent terrorist targets.

Methods of Attack

Explosive attack: There are a number of ways in which a terrorist can attack a building. The most common, and often the most effective, is by explosive attack. The explosive charge can be delivered by vehicle, package or direct/indirect-fire weapons such as mortars and rocket launchers. Explosive effects decay rapidly as the distance from the explosion increases, so the terrorist will always seek to get as close as possible to the building or the valued parts of it.

For maximum effect the explosive should be placed inside the building, where the effects of an explosion can be magnified many times. Here the terrorist must make a trade-off between placing a much larger charge at a distance outside the structure, for example in a lorry bomb, or taking a smaller charge in a suitcase or bag into the building itself. Where a vehicle can be placed inside an unprotected building, such as via an underground car park, the terrorist will achieve the best result of all.

A mortar can also be particularly damaging, since a relatively large charge weight (100kg) can be delivered down through the roof of a building into its structural heart. The kinetic energy of the descending munition is considerable and assists this penetration. Furthermore, mortars are relatively easy to fabricate and carry light risk to the firer.

Incendiary attack: Many buildings have been successfully attacked by incendiary devices. These can take the form of small packages that will burn to create a bigger fire in time, or 'blast-incendiaries' (typically a combination of fuel and a small amount of explosives) that produce a fast moving directional fireball and cause a major conflagration in seconds.

Kinetic attack: Some spectacular attacks have involved crashing a large object, such as an aeroplane or a truck, into a building causing structural failure through a transfer of momentum. Frequently the terrorist will add some explosive or incendiary charge to exploit the entry achieved and/or the initial damage caused.

CBRN attack: Recently the idea of using chemical, biological, radiological or nuclear (CBRN) material to contaminate a building has been given widespread coverage. Toxic industrial chemicals might also be used. The attack would be prosecuted through contaminating the breathing, drinking or eating supplies to the building. The result would be attractive to a terrorist and such an attack will undoubtedly occur in time, but so far the terrors of the imagination have been much greater than the feasibility of carrying out this form of attack.

Disruptive attack: Terrorists may be able to achieve their aim merely by interrupting or preventing the proper use of a building. This can be done by sending a hoax warning of an impending attack or by creating a feeling of unease following a successful attack elsewhere, leading to an unwillingness to occupy a similar building.

Mention should also be made of two methods of attack that are possible but unlikely to be all that successful. First, the use of a rocket launcher would only cause a small penetration in the outside fabric of the building if used, yet carries considerable risk of capture for the firer. Second, a handgun or rifle to assassinate somebody in a building is an option, but the more likely location for this to happen would be while travelling to and from a building, rather than while the target is inside.

Terrorists have been increasingly willing to die in order to carry out an attack successfully, committing suicide in the act of delivery. This trend reflects the growing lack of opportunities for success by less extreme means, as buildings have been adapted in recent years to increase their inherent protection.


A design philosophy to prevent a successful terrorist attack on buildings has six elements. These are to:

·Deflect an attack by showing, through layout, security and defences, that the chance of success for the terrorist is small. Targets that are attractive to terrorists should be made anonymous;

·Disguise a valuable part of a potential target, so that the energy of attack is wasted on the wrong area and the attack, although completed, fails to make the impact the terrorist seeks. It is thereby reduced to unacceptable annoyance;

·Disperse a potential target, so that an attack could never involve a large enough area to cause significant destruction, and thereby significant impact. This may be suitable for rural installations, but is not readily achievable for inner-city buildings except by the provision of a remote disaster recovery centre;

·Separate the most likely site for a terrorist to cause an explosion from the most valued items in the building. Position redundant or absorbent material between the two wherever possible. Remove or modify material that could become lethal secondary missiles, such as glass, office furniture, and so on;

·Stop an attack reaching a potential target by erecting a physical barrier to the methods of attack. This covers a range of measures from vehicle bollards, barriers to pedestrian entry controls and the shielding of key structural elements. Against a vehicle bomb, in particular, this is the only defence that will be successful in minimising the risk of primary damage to the structure and façades; and

·Blunt an attack once it occurs, by hardening building fabric to absorb the energy of the attack and protect valuable assets. This will invariably require a step change upwards in costs and may compromise the overall purpose of the building.

Successful Building Design

A successful design approach would be to deploy the first five elements above to mitigate the risk of a successful terrorist attack. The next step would involve considering whether the considerable increase in expenditure necessary to make the sixth element successful (hardening the building fabric) is really required and, if so, to what degree. The extent of hardening needs to be judged carefully if the purpose of the building is not to be compromised; indeed, if a building were designed to be safe against all common threats it would become a hardened bunker and probably a most unattractive place in which to work.

Some relief from these hard choices can be gained if an assessment is made of the occupancy of the building according to value. People, critical business functions and emergency services (including security control rooms and evacuation paths) are considerably more valuable than secondary space - storage, for example.

The more valued items in the building should be distanced from the most damaging and likely threat direction, and selectively hardened as necessary. For example, if no alternative exists to a building facing an uncontrolled public highway, along which any vehicle has unrestricted access, it would make sense to position the least valuable elements on the side of the building that faced this greater threat. These could be some non-critical services or some architectural feature such as a voided space.

Other structural elements, such as a concrete diaphragm wall included to resist structural sway in the frame, can help by being placed between the valuable elements that needed protecting and the area of greatest threat.

For similar reasons, protection can be achieved if access to the interior of a building is controlled into separate zones, with the location of the valued items in the building placed inside several outer rings through which public access is progressively reduced. This achieves separation and filtering of access within the building.

It may be worthwhile to make the public area of a building, where a hand-delivered bomb could most easily be placed or CBRN material most easily introduced, structurally independent from the main building and with separate heating and ventilation services. This will not save those unlucky enough to be working in that area, but it may mitigate the scope of the damage caused by an attack.

Efficient fire prevention systems will deal with small incendiaries, although extensive water damage may be caused if the sprinkler system is not zoned effectively. By contrast, a blast-incendiary is far more difficult to counteract. Here, it will help if the building is divided up into many separate physical zones with fire doors, if there are no accessible external ledges and sills where blast incendiaries could be left and if lower windows are made of laminated glass.

Buildings are, and will likely remain, highly attractive targets for terrorists because they provide a ready means of achieving their aims. Complete security could be achieved, but only at considerable extra expense and probably by compromising the purpose of the building to some degree. However, through innovative and clever design, it is possible to frustrate a terrorist attack and lessen the impact on the building and its contents.

Christopher Elliott is a consultant with Arup Security Consulting and is a visiting professor at Cranfield University

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