Standing up to Terrorism
|St Ethelburga in Bishopsgate, London: left, two days after it was torn apart by an IRA bomb (photo: Doctor John Schofield); and, right, as rebuilt (Photo: John Critchley, Building Images). The gilded globe of the weather vane can be seen at the top of the rubble.|
The destruction of the medieval church of St Ethulburga’s by an IRA bomb in 1993 serves as a graphic reminder that it is not only people that are vulnerable to terrorism. Many of our most important buildings are iconic structures, institutions and centres of national security, law, finance, culture and government, and therefore potential targets of terrorism. Many are tourist attractions. Furthermore, counter-terrorist measures are not confined to these buildings alone, since structures are so closely packed that the risk of attack on an individual building cannot be isolated from that of its neighbours. Precautionary alterations to many city-centre buildings are now being widely considered.
However, in the rush to protect people and property from any terrorist threat, it should not be forgotten that historic buildings are also vulnerable to the anti-terrorist measures themselves. While the protection of people must remain our primary aim, it is vital that all alterations to our historic places are carefully considered and sensitively handled, if character, charm and historic interest are not to be the first and, perhaps, the only victims of the threat.
The most difficult task is assessing the perceived level of risk of attack against the building, and its likely size or potency. Only then is it possible to determine performance criteria for the solution.
Solutions for protecting a building against the impact of an explosive device, the most prevalent form of terrorist attack, fall into three distinct areas: security measures and stand-off zones; structural reinforcement of the building; and the use of proprietary blast-mitigation devices such as blast screens, suspect device isolators or blast-absorbing walls that can be mobilised when a threat is identified. Or combinations of all three may be used.
SECURITY, SCREENING AND STAND-OFF
The first decision is whether to strengthen the building or prevent the blast from reaching it. The latter is best achieved by ensuring a large area of stand-off or set-back from areas accessible to the public. A prime example of this approach can be seen at Buckingham Palace.
Emphasis on stand-off is increasingly being seen in new building design. Some schemes, for example, are using a modern variation on the historic concept of the castle and keep, to create a protective tranche of ground between the building and public areas. New schemes are also being designed to keep car parking and car access as far from the building as possible to reduce the risk of car bombs or even ramming. This requires extra acreage or may necessitate a reduction in on-site parking.
With or without the benefit of stand-off, high tech security and policing, using CCTV surveillance and the reaction of the security forces, can be effective in keeping distance between an explosive device and its target. Security systems can work through a hierarchy of security zones radiating out from the building, designed to intercept different types of risk at an appropriate distance from the building. For example, entry by trucks over 2.8 tonnes might be prohibited at the outside boundary of the security ring. Closer to the building, you might reduce vehicle access to cars-only, and then exclude cars altogether within, say, 25m of the building. People may then be searched at entry points to the building. This ensures that trucks, cars and other potential carriers of larger explosive devices are screened out well away from the building, leaving only foot-borne and therefore much smaller devices to be screened out closer to the building.
The majority of buildings are located on public roads or other public areas where this sort of security system is logistically impossible. The solution may be to strengthen the building and/or deploy proprietary blast defence structures, such as concrete blocks, sand bags or water-filled screens to mitigate a blast.
There are no current standards defining levels of terrorist threat. The degree of reinforcement needed to make an existing building ‘bombproof’ depends on the type of attack that is most likely. This is clearly hard to predict, and criteria will be based in part on past experience. Solutions can be engineered for different scenarios based on the possible size of bomb and distance from the building. The security services and police, as well as the appointed blast protection engineer, can provide advice on the degree of attack that the building might suffer to help determine the level of protection needed.
Of course, budget considerations will inevitably influence the equation. The blast protection criteria set may require a design/ construction plus policing solution that exceeds the desired budget spend. Rationalising the perceived threat to fit the budget will obviously undermine the security scheme.
Justification of spend on blast protection in building design, new or retrofit, is made all the more difficult as it has little impact on insurance premiums in comparison to designing out other risk factors. Fire is by far the biggest risk factor affecting liability. Consequently, fire detailing in buildings has a significantly bigger impact on premiums than blast-resistance detail.
Windows are the most dangerous element of a building structure subjected to the impact, even some distance away, of a bomb blast. Around 85 per cent of all blast-related injuries are from flying glass, so windows are the first consideration in any blast protection solution.
A comparatively simple measure is to apply anti-shatter film (ASF) to minimise disintegration of the glass and retain fragments. This may be complemented by the installation of bomb-blast net curtains (BBNC) across the window, to catch the glass should it be blasted out in one piece. However, BBNC requires regular cleaning and impedes visibility, and ASF is guaranteed by manufacturers for only ten years. Both systems are also visually intrusive on interior schemes, and could significantly impede the presentation of historic windows with stained glass or special leading. Bomb-proof secondary glazing is generally less intrusive, and studies have shown that two cycles of ASF and BBNC installation and removal may be less cost-effective than the initial installation of blast-resistant glazing.
Widely used in the ‘70s and ‘80s in the wake of attacks by the IRA, so-called ‘film and curtain’ solutions are no longer being recommended by the Police Scientific Development Branch.
With a design life of 30 years or more, bomb-proof windows could prove more cost-effective in the long term and may have less impact on the visual scheme. They may be used to totally replace existing windows or be fitted behind or on the exterior as secondary glazing. Where they replace existing historic windows, the originals should be protected and placed in storage until it is safe for them to be returned to their original location.
Many historic buildings benefit from thick wall sections and therefore deep window recesses, allowing bomb-proof window sections to be applied internally behind the existing glazing without changing the exterior look of the building. In terms of visual impact, modern blast-proof glass is optically much improved, allowing good visibility of decorative glass and casements behind. Exceptional levels of blast-resistance (and thermal performance) are now being harnessed in relatively slim and less obtrusive window sections. Even ballistic glass is being engineered in thinner sections to reduce visual disruption. The principal problem, as with all secondary glazing, is where there are fine plaster or timber mouldings on the inside, or shutters. Each case needs to be considered carefully, in conjunction with the conservation specialist, to determine the least damaging solution.
|Collins Barracks in Dublin. A number of the main walls and archways required stabilising and the ashlar granite face of the walls was tied back to the core and inner skin with Cintec anchors.|
The effectiveness of replacement bomb-proof windows depends on the rigidity of the window frame. This requires careful selection of fixing bolts or anchors to ensure that the window is not blown out of the wall under blast impact.
Consideration must also be given to the visual and structural impact of the frame anchoring system. In a heritage project, the ideal is a fixing system which is concealed, to minimise visual impact, and uses a small number of anchors to minimise mechanical disturbance of the wall.
In historic buildings there are two main concerns: how to attain a sound fixing in a structure of variable condition; and the affect of any intervention on it. Traditional masonry walls invariably include voids and loose material, and this variable nature must be taken into account in the design of the fixing system. Loads must be distributed, and some element of consolidation is inevitably required. This may require extensive intervention. Grouts, consolidants and other materials used should be selected to be as compatible with the original material as possible. For traditional masonry walls, this suggests the use of hydraulic mortars and cements as these tend to be more compatible with masonry than polymers such as epoxy resins. Steel einforcement must be stainless, as the development of rust causes expansion, leading to cracks in the surrounding masonry.
Manufacturers of blast-resistant window units often recommend the use of a multi-bar anchoring system where poor masonry is suspected. Multi-bar anchors are made up of several ‘strands’ of steel in the same section width as a single bar design. This provides increased surface area for dispersing forces for more robust performance. A variation on the multi-bar is currently being developed which will incorporate energy-absorbing anchors. Again, they disperse forces by displaying progressive failure rather than breaking, prolonging their ability to retain the window frame.
Standard anchor and resin systems are not considered suitable in these situations unless pullout and sheer loads can be demonstrated to be adequate in all areas.
While window systems are the most vulnerable part of a building, it may also be considered necessary to strengthen walls against blast impact. The masonry of very old buildings can weaken over time from the effects of weathering, erosion and movement. Strengthening walls helps to prevent blast waves entering the building and damaging ducting, suspended ceilings, partitions, lighting, and critical IT and telecommunications systems.
The object of reinforcing existing masonry walls is to provide increased strength together with improved ductility and/or restraint systems where possible. This can be achieved in a variety of ways, depending on the level of protection required, with different cost and work scheduling implications.
A steel column and plate system is particularly robust. Steel columns are connected into the building floor and ceiling level, making it ideal for applications involving load-bearing walls. Internal surface preparation is minimal but installation is demanding as each connecting weld must be sound. The engineering is complex and construction detail problematic, making it a relatively expensive technique.
Steel stud partitions involve the fixing of vertical steel studs between floors to hold reinforced gypsum board or laminated glass. The partition is placed at least 300mm inside the existing wall to act as a catcher screen. Although easy to install and fairly low cost, this method is suitable only for non-load bearing walls and relatively light blast loads.
Elastomeric sprays consist of a polyurea or urea-based coating up to 15mm thick that is applied directly to the internal face of existing masonry. It forms a tensile membrane that enhances the flexural capacity of masonry and greatly reduces spalling. The coating is relatively inexpensive, but preparation is involved as the masonry must be thoroughly cleaned before installation. They can only be used on load bearing walls in conjunction with another load bearing system.
Derived from technology for stabilising weak soils, geotextiles effectively provide a fabric restraint system for preventing spalled and broken masonry from entering the building shell. The fabric is attached either mechanically to the floors above and below, or glued to the internal face of the wall. Considerable attention is needed in achieving effective fixing in both installation methods. Special arrangements must be made for load bearing walls and walls with windows. Carbon fibre sheeting is another relatively new technique that can be similarly applied.
The superficially applied solutions above are visually intrusive and cannot be installed while the building is occupied. The use of an internalised masonry reinforcement system, such as anchors, offers a less intrusive and exceptionally robust solution for the heritage application.
Anchoring systems generally involve minimal site preparation and can be installed to a fast-track programme while the building is still in use, saving the considerable expense of relocating staff to alternative facilities. In some systems, pattress plates will be visible on both sides of the wall at each anchor location, which can be very unsightly. It is better to use concealed anchors, leaving the exterior and interior of the wall visually undisturbed. Inconspicuous strengthening like this will be less emotive for occupants of a building and will give no visual indicators that might help potential attackers.
In one such system the anchors consist of a steel section encapsulated in a mesh fabric sleeve or ‘sock’. A cementitious grout is pumped under low pressure through the anchor body into the sock. This constrains the flow of grout, moulding the anchor to the internal contours of the wall, providing a strong mechanical bond. By containing the grout, the sock ensures none is lost and there is no undesirable migration into other parts of the structure. It is particularly effective in walls of unknown condition, moulding into voids and gaps.
The use of an integral anchoring system is especially advantageous for traditional masonry structures as unlike some modern engineered structures they do not have an external skeleton, so disproportionate collapse is a risk. In the event of a bomb blast, the walls of a structure rebound, causing roof and floor joists to lose their support. The internal skeleton of support provided by a masonry reinforcement system significantly improves the stability of the whole building shell under blast conditions, including non-malicious events such as a gas explosion, and seismic forces.
A heavier duty solution may be required where the design blast load is so large that the retrofit techniques outlined above would be ineffective. In these cases, the only solution will be to install an internal concrete skin. It is assumed that the existing wall will fail under blast load and deflect inward, thus relying on the inner concrete skin to resist the wave impact and remains of the failed masonry wall. A variation on this is the use of Durisol blocks, a hollow concrete block made of mineralised wood shaving aggregate, instead of sand and stone.
Although highly effective for more demanding blast protection regimes, these techniques introduce considerable additional loads and may require strengthening of the structural frame to prevent it collapsing. Foundations may also need underpinning to cope with the extra loads. There is also a noticeable loss of internal space to accommodate the width of the concrete skin plus air gap behind the existing wall.
DEVICES FOR BLAST MITIGATION
|Rapidly deployable around building perimeters and in strategic positions, the Cintec Water Wall uses water-filled engineered polymer to mitigate the effects of a blast.|
Structural solutions may be cost prohibitive, technically incompatible with certain historic buildings or unnecessary where risks of bomb attack are perceived to be low. The use of ‘stand-alone’ blast mitigating systems externally, to screen the building from a blast, may be the best option.
Traditionally, concrete blocks and sand bags have been used to create blast screens in front of buildings. These are permanent installations which are unsightly and take up space. One interesting alternative is to use inflatable water-filled structures which have been recently developed by Cintec and may be stored out of sight. The system is based on a self-inflating structure standing 3m high, constructed of a series of internally reinforced water-filled panels. These are inter-locked to neighbouring panels to create an unbroken wall to the length required for screening a blast.
Having a relatively high mass, water is extremely effective in mitigating the effects of explosive devices. The pressures behind the blast wave are substantially reduced, heat is absorbed and fragmentation is either eliminated or significantly reduced. Water is readily available in urban areas, so it is easy to inflate the system when and where necessary. The system has been tested against a car bomb containing 250kg TNT.
Such is the interest in this new technology that a range of products suitable for different sizes and types of explosive or attack has now been developed. The range includes a manually deployed barrier designed to provide temporary protection against moving vehicles of up to seven tonnes GVW, and a portable ‘suspect device isolator’ which can be stored and mobilised rapidly by first responders for mitigating the effects of blast and fragmentation from devices up to 20kg (44lbs).