Introduction
The majority of injuries arising from vehicle borne improvised explosive devices (VBIEDs) are caused by flying or falling glass and there are several options for reducing this risk.
When assessing and addressing the vulnerability of windows NPSA divides the problem, and consequently our guidance, into two broad categories:
- punched window systems - 'stand-alone' windows incorporated into the façade of a building, typically each has its own frame and is mounted into the façade
- glass curtain walling systems - a 'wall' of glass supported by a purpose designed structure, often made from aluminium and typically spanning between floors, giving the external appearance that the building is made entirely from glass
The security properties of each of these types can be very different, however they are constructed using the same components and their resistance is linked to how they perform as a system. The components could include:
- glass
- seals and/or anchorage to the support structure/frame
- supporting structure/frame
- anchors connecting the support structure to the building frame/walls
- hardware associated with openable windows, for example hinges and locking systems
The level of protection should relate to the whole system and not just the glass.
When identifying glazing there are various types of glass to select, each with different properties:
- laminated glass - the preferred option for most security applications because of its unique properties. Care should be taken to ensure that the correct type of laminate is specified. It is important that the rest of the glazing system, e.g. support structure and fixings, is specified correctly.
- annealed/float glass - traditional window glass which forms sharp glass shards when broken. It is not recommended for use in any security solution.
- toughened glass, also known as tempered glass- this glass is approximately five times stronger than annealed glass. It will break into small chunks instead of glass shards.
- heat strengthened glass - similar to toughened glass but is twice the strength of annealed glass.
- laminated glass sandwiches - an interlayer between layers of glass designed to hold together if glass shatters.
- polycarbonate - stronger and lighter than glass and hard to break.
Anti-shatter film and bomb blast net curtains may be used in conjunction with any of the types of glazing.
Blast mitigation or protection?
Glazing measures are currently available which can either mitigate the effects from a blast threat by reducing the number of fragments entering the room or provide a higher level of resistance by protecting the occupants from a specified blast threat and the fragments by remaining in the frame.
The following are considered to be mitigation measures:
- the use of anti-shatter film (ASF) and bomb blast net curtains (BBNC)
- laminated glass in normal (non-blast enhanced) window frames where the glass is not bonded to the frame
The following are considered to be protective measures:
- laminated glass in normal window frames where the glass is bonded to the frame
- blast resistant glazing in blast enhanced frames or fixed point glazing
- secondary glazing using laminated glass
It is important to understand the acceptable level of risk in order to determine the appropriate glazing measures to adopt. Where blast enhancement measures are required, laminated glass in normal window frames is generally recommended as a minimum.
Additional security mitigations
Additional measures, such as bars and grilles, may also be incorporated to provide enhanced security. Thought should be given to whether these are placed inside or outside the glazing.
Obscuration measures can be used to mitigate both the threat of ballistic attacks and unauthorised observation. In the case of ballistic attacks, such measures will prevent aimed shots but may not stop un-aimed fire. Therefore, the manufacturing specification of glazing needs to be considered to determine whether it will withstand bullets from the selected threats (i.e. pistols, rifles or shotguns).
Additional expert advice
The measures detailed on each of the following pages, and in the associated guidance notes, highlight the key areas that security managers and project managers should take into account when considering how they can mitigate the impact of a blast load and other attack types to their building.
Additional advice should be sought from an engineer from the Register of Security Engineers and Specialists (RSES) if a higher level of risk has been identified in relation to the building you are concerned with. In this case a more detailed protection strategy will need to be developed to buy down the higher levels of risk.