up to Terrorism
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
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.
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
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
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.
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.
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.
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
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.
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
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.
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.
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.
FOR BLAST MITIGATION
deployable around building perimeters and in strategic positions,
the Cintec Water Wall uses water-filled engineered polymer to mitigate
the effects of a blast.
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.
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).