Yet many organisations need bomb-proof older buildings. Governments like their embassies to be located in city centres in grand classical buildings rather than in anonymous bunkers on the edge of town. Government ministries enjoy pomp and circumstance too – Whitehall is an obvious example. Also, many private companies are housed in old buildings and are not prepared to go to all the hassle of moving for an event that probably will never happen. So if an organisation wants to stay where it is, but in a more secure environment, what should it do?
One person who can help is Stephen Ward. He spent 24 years in the army as a civil engineer specialising in construction, and part of his work involved advising on making buildings more resistant to terrorist attack, particularly in Northern Ireland. He even has an MSc in weapons effects on structures. In late 1999, he left the army to run Blastec, the new blast protection arm of building reinforcement specialist Cintec.
Cintec specialises in reinforcing existing masonry structures. For example, it will reinforce weak structures that have deteriorated or become damaged, or protect buildings against structural collapse in earthquake zones. The system is very simple in principle (see "The Cintec system", right) and consists of drilling along the length or height of masonry walls and inserting steel reinforcing bars. The anchors are also ideal for stopping old walls caving in under the blast pressure from a bomb.
Ward is no salesman for the system and takes a refreshingly detached view of how to protect older buildings against blast. He says there are essentially two approaches to protecting a building's occupants against terrorist bombs: "The first question to ask is, do you mitigate the effects of blast or harden your building?" Mitigating the effects of bombs is best done by making sure there is a large area of "stand-off"; Buckingham Palace is a good example of this, with a huge empty space between the palace and public areas where a bomb might be placed. This is fine for the Queen, but most buildings are in spitting distance of a road or other public area.
If a building owner wants to make a building more bomb-proof, the first task is decide on the likely level of attack. "The problem is nobody knows the size of the bomb and where it is going to be planted so you argue for the worst-case scenario from the engineering point of view," says Ward. In practice this could turn out to be prohibitively expensive, so Cintec prepares a number of possible scenarios based on bombs varying in size and how close they are likely to be placed to the building. The security services and police also offer advice on likely scenarios. From this, the owner can decide on the appropriate level of protection for the perceived risk.
"Windows are the first thing you look at," says Ward. "Flying glass kills people." One approach is to stick film over the glass to hold it together in one piece, and to install bomb-proof net curtains behind the glass as a secondary measure to catch the glass in case it is blasted out in one piece. However, the curtains prevent occupants from seeing out of the window, and the film only lasts 10 years. "After two cycles, it would probably have been no more expensive to use bomb-proof windows in the first place," concludes Ward.
If you don’t allow the windows to break, you need more wall reinforcement as the force of the blast isn’t dissipated
Stephen Ward, Cintec
The window frame has to be securely fixed to the structure so that the whole lot doesn't get blown out as a unit. Conventional anchors are normally used for this job. At this point Ward puts on his salesman's hat and extols the virtues of the Cintec system for anchoring windows. "Cintec anchors can be engineered to be stronger than conventional window bolts so their number can be significantly reduced. And because they are much longer they also reinforce the surrounding masonry at the same time."
In any event, building owners may be forced to reinforce walls when installing blast-proof windows. "If you don't allow the windows to break you need more reinforcement in the walls as the force of the blast isn't dissipated," cautions Ward. He claims that if Cintec anchors are being installed to hold the windows into the structure, strengthening all the walls may not cost a lot more as much of the work is being done anyway during window installation.
As one of Ward's roles is as Blastec's research and development man, he has come up with two new ideas to prevent a blast reaching the building in the first place. The first idea is to use "water walls" – giant plastic bags filled with water to absorb the blast energy. Water also reduces the effectiveness of the explosive and its heat. The bags are small enough to fit into the boot of a car and are filled and emptied in situ, removing the need for heavy lifting equipment. The idea is not as crazy as it sounds; the Labour party used the bags at its last conference to protect delegates against potential bomb attacks.
Empty water bags could be hidden in pavements and activated when needed. They would rise up as a solid, water-filled wall up to 3 m high. This would be particularly useful for organisations that don't want to clutter their surroundings with huge lumps of concrete. A water wall could offer a discreet solution; if a security guard spotted a suspect vehicle they could deploy the water walls at the push of a button. These could provide full protection up to first-floor level and a lesser level of protection for higher floors.
The latest idea dreamed up by the eternally inventive Ward is "permeable blast walls". These are like gabion cages filled with rocks and the idea is to make the blast wave travel around all the rocks to dissipate its energy. The same principle is employed in a car exhaust by making the exhaust gases travel much further in the silencer to reduce the roar of the engine to a gentle purr. A water-filled core in the wall would perform the same job as the plastic water walls.
Ward has high hopes for the permeable blast wall. He says small-scale studies show it is 90% effective. "In the USA they are screaming out for this for airport car parks," he says." The Federal Aviation Authority regulations demand the introduction of increased stand-off, which means closing the lucrative car parking facilities near airport terminals." The permeable blast walls would surround the car park to contain any blast from a large vehicle bomb. "The rock-filled gabions are environmentally attractive," claims Ward. "It is possible to grow plants in them to offset their intrusiveness."
Unfortunately for British workers, most of Cintec's blast mitigation work is abroad. All the company's blast protection work in the UK is geared to preventing accidental blast damage. Most of Cintec's deliberate-blast protection work is in the USA and for American government properties in Europe. Ward says this is because UK organisations are very reluctant to spend money on blast protection. "The building owner will do the absolute minimum unless legislation forces them too do something," he says.
The Cintec systemNewport-based Cintec started off making cavity wall ties for Cardiff City council before developing its anchor system for structural masonry reinforcement. The system is particularly suitable for historic structures as it is invisible after installation. For example, Cintec anchors were used to stabilise and reinforce damaged masonry at Windsor Castle following the fire in 1992. The system was also used to strengthen the fractured walls of the ancient Mosque of al-Ghuri in Cairo after an earthquake in the same year. The system is regularly used to strengthen weak bridge arches and parapet walls, and is used by local authorities to stitch together crumbling council blocks and provide additional reinforcement in case of gas explosions. The first step is to examine the structure and carry out computer analysis to determine how many anchors are needed to reinforce the structure, and where they should be located. Holes measuring double the diameter of the steel anchor are drilled into the masonry. The anchor, enclosed in a fabric sock, is inserted and a cementitious grout is pumped into the sock, inflating it and locking the anchor to the surrounding masonry. The bars vary in diameter according to the application, and a recent development is the Multibar, which uses a series of small bars rather than one large one. This is particularly suitable for blast protection as it uses the same diameter hole yet provides a greater degree of strength because the surface area of steel against grout is greater. The bars are screwed together on site to form very long lengths; the company has installed anchors up to 50 m long and says there is virtually no limit. Cintec also says the system can be installed while buildings are occupied, and is comparatively quick to install. For example, a big project on the books is reinforcing the walls of two wings of a military hospital in Seoul – it will take 30 days to install the system in each wing of the hospital.
Join the resistance: Protecting your building against blast
- Install steel columns and plates behind the existing wall and connect them to the building frame at floor and ceiling level. The system provides very effective blast protection and is particularly suitable for structures where load-bearing walls support the floors above. The disadvantage of steel column and plate is that it is expensive and complex to install.
- Put in steel stud partitions similar to standard internal partitioning systems at least 300 mm behind non-load-bearing external walls. Fit reinforced gypsum boards or laminated glass between the vertical steel studs. The system does not provide full protection against blast but acts as a catcher screen for debris in the event of a light blast. The advantage, however, is that it is easy to install.
- Apply elastomeric sprays up to 15 mm thick to the back of an existing masonry wall. The coating means the masonry can flex considerably in the event of a blast and prevents the masonry spalling. However, it cannot be used on load-bearing walls so another load-bearing system is necessary for full protection. Although the coating is cheap it needs to be applied to a very clean, thoroughly prepared wall.
- Super-strong textiles such as Kevlar, used in bullet-proof clothing, can either be bonded to the back of a masonry wall, or mechanically attached to the floor and ceiling to catch spalled and broken masonry entering a building. As with elastomeric sprays, additional protection is needed for load-bearing walls, and it has to be applied to a thoroughly prepared wall or the fabric has to be very securely attached to the wall and floor.
- Building an internal concrete skin behind the existing exterior wall provides excellent blast protection but is a bit like building a second structure all over again. Foundations need checking to ensure they can take the extra weight and the structure of the building may need beefing up to take the extra dynamic loads in the event of a severe blast. There is a space penalty, as there has to be an air gap between the existing external wall and the new concrete skin, which will have to resist the blast and impact of the failed external wall. Needless to say, this is a very expensive solution.
- Another propriety solution is Durisol blocks, which are made in the USA. These are hollow concrete blocks made using mineralised wood shavings as the aggregate. The Durisol blocks work in the same way as the internal concrete skin – the system is cheaper to put up, but it cannot resist the same level of blast.