By definition, it is hard to prevent an explosive device from being brought into a public space. Intelligence, monitoring, and CCTV with facial recognition all have an important role in prevention, but should a device detonate then physical protection measures are required. Most of the content below focusses on improving the facades of buildings that look onto or even contain the public space. Litterbins are in that space and barriers are there to keep some devices at a safer distance. Each of the sections below could be expanded hugely, but the aim here is to provide a primer and to arm the reader with a grounding and the ability to then ask the right questions of potential suppliers or to better understand their potential vulnerability.
Understanding the Problem
The film and television industry, in search of visual spectacle and entertainment, has not served the public or even the general security industry with a proper understanding of blast effects or the distinct threat mechanisms contained within. It is therefore worthwhile spending a bit of time to cover what is really going on.
For this, the focus will be on explosive devices rather than industrial or processing accidents or the gas explosion in a domestic kitchen.
A block of or vessel full of explosives is of finite volume. When given the appropriate shock by means of detonator or detonator and booster charge, the chemical process results in an instantaneous and very large increase in volume. The air that was around that explosive is forced outwards as an expanding sphere. Depending on the quantity and type of explosive used the chemical reactions can continue and sustain the release of energy over a longer period – still measured in milliseconds. This explosive event creates a high level but short duration burst of heat and the explosive mass might be held in a case that fragments or it could be packed with items designed to add to the add to the fragmentation.,
Shockwave and Impulse
The shockwave is visible in this video of the detonation of a large mass of black powder. The higher density air held inside the expanding sphere acts as a lens which, with the appropriate background, shows the shockwave. The products of the chemical reaction can be seen behind the shock-front which has detached and races outwards at up to 7000 metres per second.
The peak blast pressure is at the surface of the sphere. As with any sphere, an increase in its radius creates a significant increase in its surface area. The peak pressure is a function of this increasing surface area and so drops accordingly. This effect is dramatic and blast pressure decays quickly.
There are two graphs that can be employed to illustrate this effect.
The first is the classic pressure time graph. This shows the pressure profile at a given point near the explosion, with respect to time. The baseline of the graph is 1 bar which is the ambient, normal air pressure. The sudden increase in pressure shown is over and above this ambient pressure and so is termed ‘overpressure’. Repeated experience and experimentation shows that even on a timescale measure in milliseconds the pressure trace is almost vertical and instantaneous. Having reached a peak the pressure then drops away quickly.,
How quickly depends upon the nature of the explosive event. A small charge, close by such as a postal device might reach peak value ‘Y’ but there is not the mass of explosive to sustain this and the drop is very quick. At a given distance from a much larger device such as a truck bomb the peak overpressure might reach the same value of ‘Y’ but the decay will be comparatively slower and the total energy delivered will be larger. The total energy delivered is shown by the area under the pressure decay curve and is termed the ‘Impulse.’
With the explosion have driven the ambient air outwards, along with the gaseous products of the event, there comes a point when this is no longer reinforced and a partial vacuum is created in its wake. This is filled by a return of air to the seat of explosion. This is a much smaller scale event than the original explosion, but for large devices does have real world effects that will be discussed later.
Measurement of time on a millisecond timescale is not intrinsically easy for people to grasp. A more tangible means of understanding the rapid drop in pressure from a peak is to look at the maximum overpressure that could be achieved at different distances from a charge. The example left shows the overpressure generated by 20kg of TNT at distances from 1m out to 20 metres. It can be seen that most of the decay is within the first 2m metres and from about 6 metres the decay becomes gradual by comparison.
The shockwave and impulse have different effects on the environment around them. An intense shockwave has a strong cutting effect. Damage that shows jagged edges indicates that it was close to the seat of the explosion where the shockwave was intense. The impulse is less intense but of longer duration and in general acts to push and displace items in a more sustained manner. An illustration of the difference can be seen in the two buried IED vehicle attacks shown below.
The front end of the Landrover has been severed from the rest of the vehicle and the jagged damage is indicative of shockwave damage. The Stryker vehicle on the right was designed to better maintain its structural integrity in the face of a shockwave and has some shaping to redirect a portion of the impulse. It was however still lifted and thrown by the impulsive effects and without specialist seating and other measures crew survivability would still be under threat.
These distinct mechanisms provide different damage or injury mechanisms to structures or people across a range of circumstances. A small device held by a person could result in a shockwave dominated traumatic amputation whereas a larger device, further away in unlikely to do this but could throw that person bodily potentially into fixed obstacles.
Overpressure and Injury:
There has been much historical research into the effects of blast overpressure exposure and the thresholds and lethality implications on the human body. The graph to the right shows that the susceptibility of the ears, lungs and general lethality. Designed to pick up sound, which is just small pressure fluctuations in the air, the ears are not surprisingly the most sensitive to blast damage.
It can be seen from this that mitigating or managing blast pressure to below 3 or 4 psi will provide some confidence in avoiding direct blast pressure injuries to personnel at that location. Ongoing research will refine these values and provide more specific detail on mechanisms such as mTBI (see making brainwaves page here) but they are a well established and a viable guide.
Incident and Reflected Pressure:
Someone looking into protecting a structure and perhaps examining potential suppliers test reports will see the terms ‘Incident’ (or free field) pressure and ‘Reflected’ pressure. These are different and worthy of explanation.
Incident/Free Field pressure measurement is used more as a research and quality control measure. It measures the pressure wave without interacting or interfering with it to any significant degree. Reflected pressure is more a real world measurement as it records the pressure build up on a surface normal to the direction of the pressure wave. This is what a surface such as a door, windows or walls would experience and is invariably a higher value than the incident pressure generated by the same explosive event at the same distance. The image left shows how pressure gauges can be orientated to record the incident and reflected pressure gauges. For the reflected pressure a more accurate measurement can be achieved by setting the gauges into a larger, flat face structure, often called a reference block.
Heat and Fragmentation:
A explosive blast will create a flash and pulse of radiant heat. This is of very short duration and unless supported with a fuel source to make a blast incendiary there is often remarkably little damage that could be attributed to heat. Fragmentation is far more significant concern. Many conventional munitions such as artillery shells, mortars, hand grenades, claymore and bounding mines utilise the explosive event to drive out carefully engineered fragments for maximum lethality and injury. IEDs have adopted this strategy, either by modifying conventional weapons leftover as the remnants of war, or by the starting from scratch such as with pipe bombs or nail bombs. Suicide bombers often pack out the their explosive vests with nuts and bolts or ball bearings to increase the range and opportunity over which to kill those in the vicinity.
Fragments that are designed or deliberately placed around the explosive as part of the device are termed Primary Fragmentation. A device such as a car bomb can break up the ground surface on which it sits and throw up stones, glass or parts of damaged structures outwards at great speed. These can be just as injurious but are termed Secondary Fragmentation. We have seen that the blast overpressure decays very quickly with distance. The effective radius of fragmentation will almost always be far greater than blast effects and is the driver in setting safe evacuation distances at a suspect bomb location or when conducting explosive trials for research.
Left: The 1999 bombing of the Admiral Duncan pub in Soho, London was made much more effective by the large number of nails packed around the device in a sports bag.
Right: The safety distances for different sized devices. Comparing the outdoor evacuation distance for the 50lb briefcase device with the similar 20kg charge in the graph above illustrates the huge difference in blast and fragmentation ranges.
A Sunny Day in Munich
This image of plaza in Munich will serve the purpose of highlighting various elements and options for protection that will be discussed in more detail below. By turn the issues to be considered are glazing, doors, walls, litterbins and barriers. These all comprise physical, direct protection systems. Other options such as CCTV with facial recognition technology, systems to control pedestrian flow and behaviour and other intelligence based systems fall outside the remit of this article.
Every location is different and each presents its own challenges. Some buildings such as Government offices maybe an obvious target but if your building shares the same vicinity then you need to be aware and act accordingly to protect your personnel and assets, both from a basic responsibility point of view and for business continuity. Lack of preparedness and long term disruption is a win for the terrorist.
Glazing
Aside from impact from fragmentation, one of the biggest causes of injury with blast effects in a public, urban environment is flying glass. As would be expected much of this threat is to personnel inside buildings when the broken window panes are projected at them. At larger stand-off distances glazing maybe damaged just enough to break and while some will enter the building, a portion of it will fall down the outside into the street. We have also seen that for large devices there is a negative overpressure phase which can also pull glass back towards the seat of the blast and way from the building. This issue of large amounts of glass in the street is one of the drivers for the philosophy of ‘invacuation’, which is moving personnel to predetermined stronger areas within a building, away from the windows, rather than sending them out into the street.
Anti-shatter Film and Anchoring of Frames:
Clear polymer film, with a high tensile strength, bonded to the rear face of a glazing pane can prevent shards of glass from being individually accelerated inwards. The introduction of such films created an opportunity to upgrade glazing through retrofit . This often involved the application of film up to or to within a few millimetres of the edge of the frame.
This was a step forward but it only partially solved the problem. Now the whole window pane would act as a single unit and be blown from its frame acting as a large, high energy projectile. The large surface area will create a lot of drag which will slow it relatively quickly but you wouldn’t want to be in its way.
Refitting a window so that its anti-shatter film could be extended into and bonded to the frame is a logical step in protection. Done properly this not only prevented flying glass but also helps to create a cohesive surface across the building to keep the blast pressure out. The images below illustrate these step changes most effectively.
The benefits of a high tensile strength polymer are exploited further in laminated glass. Here, multiple layers of polymer are alternated with glass within the window pane. The overall thickness and number of layers is adjustable at the manufacturing stage to meet different threat levels. This increase in potential protection performance is of little use if this pane is not bonded into a suitably robust frame with deep rebates which itself anchored into the surrounding wall. Then the wall itself needs to be able to support those anchors and be robust enough in its own right to resist the blast loading. There is no point having high spec windows if the walls collapse. For new builds there is control over the specification of all these elements. For upgrades it is not uncommon that the quality of build does not fully reflect the design that was intended.
The images below are from a survey conducted outside the UK on a building that was potentially going to be occupied by an official body. It can be seen that from the inside this particular window looks to be well fitted and to a high cosmetic standard. A closer look at the frame showed that it was held in place with some simple screws and given the size of the window, there wasn’t many of them. Further examination on the outside showed that there were in fact large gaps between frame and the wall and where many of the screws were used they weren’t actually attached to wall but to some wooden packers.
Performance Testing
The market place is populated with a myriad of products for anti-shatter film, laminated glass, frames and retrofit anchors. The customer needs to be able to compare the performance of these products against a realistic scenario that matches their perceived threat level.
A few, broadly similar, specifications exist that have defined threat levels, test arrangement and resulting performance rankings. The test outcome is not a straight pass or fail but a series of ratings governed by how far glass is projected into the building. Obviously glass going no further than just below the window is a better situation than entering by 4 or 5 metres. An issue with testing glazing systems is that to achieve both the peak pressure and impulse, large charge sizes are required; even on a scaled basis the entry level is 30kg. Larger charge sizes increases the cost of trials and also limits the number of facilities in which they can be conducted. ISO 16933 is a widely used specification and it contains a series of threats for vehicle and person borne devices. The vehicle threat table is shown with the peak overpressure and associated impulse highlighted for each level.
Doors
Doors are a potential weakness in a facade. They have the added complication over windows in that they need to open. We have seen that anti-shatter film on windows works by being elastic and ‘rolling with the punch’. For doors however, held in place by hinges and catches, stiffness is the key to success. A door with significant flex could pull out its own shoot-bolts and open or at best it will create gaps around its edge that let in blast overpressure.
A door that opens outwards towards the public space and the likely source of the blast will be pushed back onto its frame. Local planning regulations might not allow outward opening doors, but if they can be used then it is welcome from a blast perspective. An inward opening door relies entirely on its catches and hinges.
A single leaf door is a simpler design proposition than a double leaf door. Inherently stiffer and with no weak plane of connection between the two door halves. These factors have to be balanced against daily operational needs and why the door is there at all.
As with glazing, the attachment to the frame and that frame’s attachment to the surrounding structure is critical so that every element acts together to resist the loading. Much of the technology required to increase the blast performance of a door is the same as that required for increasing its inherent security rating. This includes using a panel that is resist physical attack, robust door hinges and catches and more of them. Testing the performance of doors follows a lot of the same protocols as that for glazing. The same threat levels are widely used. In the same way that the glazing is tested as part of a self contained cubicle, efforts need to be made to ensure that blast pressure is prevented from getting around behind the door where it could start to equalise the pressure and so reduce the net loading.
Walls and Structures
The need for walls to act as the means for holding windows and doors has been discussed above. Here the focus will be on the walls as a barrier to blast in their own right.
For those looking for a detailed and authoritative work on building structures ‘Blast effects on buildings – Second edition’ is recommended. Ideal for anyone who likes heavyweight maths and a good graph.
This section is intended for a building owner to have a basic understanding on what they have, where it sits on a crude blast resistance scale and some options for upgrading its performance. It will help a building or facility owner at least have a grasp of what questions to be asking.
The essence of keeping blast and fragmentation out of your building is to have the whole facade act together. It doesn’t matter how sturdy your walls are if the windows and doors blow in. The same principle applies to the wall. Blockwork walls are made of small units with a low strength connection to each other. Blast pressure will always find a weakness and with a blockwork wall that weakness will be the start point for a rapid progressive collapse. Reinforced concrete walls are by default in a better position to meet the threat as a single unit – this obviously depends on the wall thickness, the internal reinforcement and the blast loading but their continuous nature works in their favour. Some upgrade options for additional protection are shown below.
Looking from left to right on the image above:-
The first three images cover systems that are applied externally. The first is a polyurea coating which deposits an elastomeric coating over the whole surface creating a continuous, high strength membrane that ties all the blockwork together. The second image is blast mitigation tiles (such as our own XPT system) mounted on sprung perforated profiles that decouple the tiles from the wall, trapping the blast to maximise the energy expended through mechanical work in destroying the tiles. The 0/90 degree arrangement of the profiles also braces the blockwork wall to better share the loading. The third image is fibre reinforced concrete cladding panels. These provide a high quality surface finish to a building and the high proportion of stainless steel fibres ties everything together in a much thinner structure than could be achieved with traditional rebar. The fibre reinforcement also helps prevent any rear face spall or loose pieces on concrete being thrown off the internal face as it is loaded in tension.
The fourth image is looking at the rarely pursued option of placing the reinforcement within the wall. A technology developed for preventing the collapse of old and decaying buildings, it can be just as well used to upgrade the blast resistance of otherwise structurally sound buildings.
The last image shows the benefit of using a high strength continuous fibre based matting on the inside of a wall. Very much like an anti-shatter film, it needs to anchored around it periphery to be fully effective. These membranes can be plastered over to create a normal, cosmetic standard of finish.
Litterbins
It was in Warrington, in the United Kingdom in 1993 that the issue of bombs in litterbins came to the fore. Devices placed in litterbins by the IRA were responsible for the death of two boys. This lead to a raft of proposed solutions and commercial offerings which was the catalyst for the Police Scientific and Development Branch (as it then was) to come up with the first UK test specification. This was used with success for many years but was eventually updated to a more comprehensive version that catered for a wider range of designs and technologies. The results of the tests contained within could also be used to derive a star rating which is a useful tool for the potential customer.
One way to get over the issue of devices in litterbins is to remove them altogether. This was the approach taken by the London Underground. It worked and passengers just took their litter home or looked for bins elsewhere. Where this is not an acceptable solution and a threat to the public exists then blast mitigation litterbins are employed. They tend to work by resisting lateral blast pressure and fragmentation and then allow it to be redirected upwards as the safest place for it. This lateral strength is often achieved with fibre reinforced composites such as glass fibre or aramids such as Kevlar. It can also carefully be done with steel in a sandwich construction.
It is important to note when considering where to put such litterbins that the blast pressure is released upwards as a wide cone rather than a neat cylinder. Using such equipment in an open space with a glass ceiling requires careful thought. Designs have been developed that absorb the redirected blast by making it do work and changing the state of fluids.
Barriers
Barriers, bollards, gates, fences and other measures are all about keeping the potential device away and so increase the stand-off distance. It has been shown above that even modest increases in stand-off from a device can make a big improvement to safety. For public spaces it is very hard to prevent the person borne device but at least these will be inherently limited in size. Preventing a vehicle from accessing a public space is now not uncommon and with the rise of ‘Vehicle as a Weapon’ attacks has even become expected and desirable.
Barriers are discussed in much more detail on our collected articles pages which can be seen here.
