Bomb shell

Such measures have been broadly successful, with overall incidents of fires coming down in number (and deaths from fires in buildings falling quite markedly).

That said, there's little or no room for complacency. With the current climate created by international terrorism, it would be a natural progression to assume that future Building Regulations be enhanced to require protection against certain terrorist acts.

Security and counter-terrorism measures in the design of new buildings are not a new concept in the UK. With the troubles in Northern Ireland spilling over onto the mainland, for decades now public buildings have incorporated specific measures to counter the various terrorist threats. Down the years these threats have grown in magnitude, from personal attacks in the form of parcel bombs through to more sophisticated devices in bags or packages and car or lorry bombs (used to great effect in both London and Manchester).

Then there were the mortar attacks on Heathrow Airport and 10 Downing Street.

With the Northern Ireland peace process having taken effect, the degree of severity posed by the Irish terrorist threat has declined somewhat, although the threat of international terrorism remains. The well-documented events of September 11 2001 and the UK's continuing participation – alongside the US – in overt action against known 'rouge' states (in particular some of those in the Middle East) ensure that this country will be a prime target for many years to come.

Nature of the threat
The nature of the terrorist threat has changed in recent times. As well as the more traditional forms of attack experienced by this country, there's now the new threat of the suicide bomber. Thankfully, we've not yet experienced that form of attack on home shores.

The atrocities of September 11 were a further manifestation of terrorism, but that type of event cannot be effectively prevented by the design of buildings (although overall structural robustness and escape route capacity may be enhanced).

Before considering what steps can be taken to deal with terrorism in building design, the nature of the threat needs to be understood. It's sobering to note that the Brighton bomb consisted of just a few kilograms of Demex, a commercially-available explosive. 60 kilograms of that same material could easily fit in to a common suitcase.

The most extensive building damage arising from a bomb blast is caused to windows. Typically, flying glass shards account for 90% of all injuries from a blast. Depending upon the size of the explosive, the area of structural damage is small compared to the area in which there's glazing damage. In the case of a typical car bomb, there would be significant damage from flying or falling glass over a 140-metre radius from the seat of the explosion, whereas significant structural damage would only extend over a 55-metre radius.

If the glazing damage zones were superimposed over the centre of a major city, more than 700 buildings could be affected. It should be noted that the blast effects on people can be more widespread. The survival threshold is 40 metres. Anyone within that area will suffer lung damage from the blast overpressure which is likely to prove fatal. If they're in clear line of sight of the explosion, the blast will almost certainly result in fatal trauma. At a distance of 55 metres, people will suffer from eardrum rupture and there'll be a temporary loss of hearing up to 300 metres from the seat of the explosion.

Traditional versus the modern
In terms of structural damage, traditional buildings are far less robust than modern forms of construction. The latter tend to have structural frames of either reinforced concrete or steel which, being flexible, are more able to sway and absorb the blast impulse. More importantly, the external envelope of the building will be able to absorb the blast effects by deformation. This will result in significant damage to one or more building elevations, much of it superficial, although the overall integrity of the building will be unimpaired.

Older buildings without framed elements, where the load-bearing structure is in the external envelope, will suffer probably less façade damage due to the increased mass of the external walls. However, if the walls fail the end result will be partial or complete collapse of the structure.

The IRA bombing of Brighton's Grand Hotel back in the 1980s during the Conservative Party Conference lives on as a perfect example of how a load-bearing structure was partially collapsed by a fairly small amount of explosive. If the hotel had been of a framed construction, the explosion would have probably caused a number of walls and floors to be blown out, but the structure wouldn't have been significantly affected – and structural collapse simply wouldn't have occurred.

All external-facing areas should be circulation spaces like atriums and corridors, while entrances ought to have two sets of doors for added protection. Extensive glazed facades on a building should be avoided if at all possible

In essence, there are six major effects caused by a bomb blast which must be considered in building design to negate the effects of terrorist attack. These are as follows:

• the direct blast wave from the explosion itself, which is a line of sight effect;

• side-on blast pressure – this affects the side, roof and rear elevations of the building that are not in direct line of sight of the bomb;

• reflected blast – blast waves 'bouncing' off adjacent hard, solid objects such as buildings opposite the target building;

• ground shock – the transmission of shock through the ground;

• the impact of fragments from the bomb itself, and the immediate surroundings of the bomb (such as a car or lorry);

• flying debris, including building cladding, earth and glass.

To minimise the impact of these effects on a building, there are three basic rules which security professionals may follow. Rule Number One is to keep bombs out of buildings. Rule Number Two is to keep bombs as far away as possible, and Rule Number Three states that you should comply strictly with Rules Number One and Two! For measures to be most effective, the security of the building(s) against terrorist attack demands to be considered at the earliest possible opportunity in the design process. These considerations should be undertaken by a properly qualified specialist working closely with the architect and structural engineer.

The first rule of good design
The first rule is fundamental. There needs to be a physical stand-off distance so that potential vehicle bombs cannot get anywhere near to the building. Delivery vehicle access needs to be controlled. If the operation of the building requires a large number of deliveries, have the delivery/processing area in a separate building away from the main complex. Currently, all passenger parking at UK airports, for example, is required to be at a minimum distance of 30 metres from any airport building.

The use of unobtrusive physical barriers (such as changes in level) and water features can often prove highly effective in providing stand-off, while at the same time maintaining an attractive environment for visitors and employees alike.

Parcel and letter bombs tend to be identified in the Mail Room. Again, if large volumes of mail are anticipated, locate the Mail Room in a separate building in order to keep potential package bombs out of the main facility. If that's not possible, locate the Mail Room on the exterior of the building with a large vent on the perimeter wall. In this way, any explosion will be directed outwards causing minimum internal damage to the adjacent areas.

Control of personnel access to the building is an operational security issue, although design does have a part to play in that the number of building entrances should be kept to a minimum and not be obscure.

How to minimise blast effects
Having maximised the stand-off to a bomb, the next consideration for the in-house security manager and the building design team is how to minimise the blast effects should the bomb detonate. The vital core areas of the building – such as the computer room, safe room, archive suite, meeting rooms and other areas where there is nearly always a high degree of people concentration – should be located at the centre of the building.

All external-facing areas should be circulation spaces such as atriums and corridors, while entrances ought to have two sets of doors for added protection. Extensive glazed facades – so beloved of many architects these days – should be avoided if at all possible, although it's duly acknowledged that such a design is very often a requirement for high value corporate buildings. If deployed, glazed facades must be specifically enhanced against bomb blast (of which more anon).

The inclusion of recessed façade elements and overhangs should be avoided for they tend to trap and focus blast pressure on the façade. Stone cladding on flexible framed buildings should also be avoided as this will crack, be expensive to replace and present the risk of stone panels falling from the structure (which could prolong the recovery period post-blast).

The very best plan form for a blast-enhanced building would be a cylindrical shape. This minimises the effective façade area exposed to the blast, the majority of the blast pressure being deflected around the building. This contrasts with a rectangular building footprint where the flat elevations will experience full blast overpressure with minimal dissipation.

It should be stressed that cylindrical building forms are not always practical, although the inclusion of smooth, curved facades on building structures can assist with the dissipation of blast overpressure. Basements that extend beyond the building line should also be avoided due to their vulnerability to bombs located above them.

In addition to the above points, a significant corporate-style building should also be designed so that it can be re-glazed from the inside. That is particularly important for high rise buildings. Also, a cladding system should be installed that allows for easy removal and replacement of individual elements. The building must have a robust core, and a framed structure (ie no load-bearing masonry) incorporating careful joint detailing, floor slabs designed for upward and downward loads and the redundancy of members.

With improvised mortars serving as a potential threat, the inclusion of a sacrificial roof – which can provide a stand-off to the structural roof slab, will absorb the detonation and so leave the building largely unscathed – may need consideration.

Protecting the workforce within
Having enhanced the building envelope and structure, the third major consideration centres on the protection of staff in the event of an explosion. Although the building enhancements will provide a significant amount of cover, all-too-often people are evacuated during bomb alerts to what are thought to be safe locations, whereas they may be being placed directly in harms way.

The bomb refuge area must be robust enough to be shielded against perforation from primary bomb fragments and debris. The integrity of the enclosing structure should be maintained, although some minor deformation of the structural members is permissible

The incidence of specifically-designated bomb refuge areas within buildings is increasing, and their provision is a valid element of building design. Again, there are a number of considerations for the practising security professional to bear in mind.

These areas must be protected from blast pressure such that the internal space pressure will not exceed the ear drum rupture threshold. The glazing hazard must be minimised. There should be no windows at all, or they must be designed to prevent a low flying glass.

The bomb refuge area must be robust enough to be shielded against perforation from primary bomb fragments and debris. The integrity of the enclosing structure should be maintained, although some minor deformation of the structural members is permissible. Refuge areas must also be large enough to accommodate all members of staff, while travel distances to them must be to a minimum from all occupied spaces in the building.

Crucially, refuge areas must have sufficient life support systems (ie ventilation, sanitation and communication). There needs to be an alternative means of escape available should post-blast building evacuation be required, and such areas must be located well away from any fuel storage and gas intake areas.

The need for robust glazing
Glazing robustness is a major design consideration for security and counter-terrorism professionals. Falling or flying glass, no matter what size the shards, can give rise to very serious injury.

The foremost hazardous type of glass is annealed or plate glass, the sort that's found in most homes.

There are various types of glass used in building construction today, some of them potentially more hazardous than others. They include the aforementioned annealed glass, toughened glass, laminated annealed glass, laminated toughened glass, Georgian wired glass and polycarbonate variants.

Bomb blast will invariably find the weak point in any building or structure. A balanced design approach is required, as merely fitting the most robust façade to a structure may well result in more severe loading of that structure than if a more flexible façade system were adopted. The means of fixing the glazing to the frame (and the way in which the frame is fixed to the building itself) is also important. There's absolutely no merit in installing robust glazing if its support fixings and structure subsequently fail.

Another effective means of enhancement – primarily in existing buildings with annealed glass windows – is the use of anti-shatter film and safety net curtains which act to 'catch' glass and debris from the blast, and deposit it harmlessly on the floor close to the window.

Some of the design measures outlined involve minimal additional capital expenditure, whereas others could entail a considerable increase in the build cost. It has been demonstrated that, for a 5% increase of capital build cost, a significantly enhanced level of security may be incorporated within the design. The majority of international and major UK clients accept this axiom and duly incorporate the necessary enhancements in public buildings.

Public and defence sector buildings frequently require significantly enhanced security and resilience against attack, although that means significant extra cost.

On a final and more ominous note, although past threats have mainly emanated from packages or vehicle bombs of varying magnitude, we are now having to face up to the prospect of suicide bombers. The future also holds an even more menacing danger in the form of biological, chemical and even nuclear threats. If such attacks do materialise, building design will have to be enhanced still further.