Windows or punched window systems, are those which are 'stand-alone' windows incorporated into the facade of a building. Typically, each individual window comes with its own frame, which is mounted into the facade.
The measures detailed below and in the linked guidance note are suitable if you are considering general mitigation measures as part of your overall security strategy. These may be achieved through minor variations to your overall glazing specification.
Additional advice should be sought from a blast engineer from the Register of Security Engineers and Specialists (RSES) if a higher level of risk from improvised explosive devices (IEDs) has been identified in relation to the building you are concerned with. In this case a more detailed blast protection strategy will need to be developed to bring down the higher levels of risk.
The related pages below provide detailed guidance on the various aspects of glazing.
Key features of punched windows
The elements impacting the security resistance of a punched window are:
- the type, design and material of the window frame
- the fixing of the window frame to the façade
- the overall size of the window - this can impact on its blast resistance; bigger panes generally perform less well than smaller windows
- the rebate of the glass - the distance the glass protrudes into the frame
- the type and thickness of the glass used and whether or not it is a single pane, double or triple glazed - laminated glass is preferred for security applications, specifically on the inner sheet of glass
- how the glass is secured to the frame - for punched windows the glass is typically secured using 'beading' which, for a variety of reasons, should be on the side of the building interior
- window locking systems and hinges
- a variation of ribbon glazing where a series of windows are fixed in a continuous row
The majority of injuries arising from vehicle borne improvised explosive devices (VBIEDs) are caused by flying or falling glass within a building.
Where there is a need to mitigate this risk and reduce the number of fragments entering the room, it is recommended that the following are considered:
- use laminated glass in a normal (non-blast enhanced) window frame where the glass is not bonded to the frame i.e. glazing gaskets. (If a double glazed unit is used, the laminated glass must be on the inside i.e. the protected side)
- the laminated glass should be specified with minimum polyvinyl butyral (PVB) interlayer of 0.78mm
- the frame should be securely fitted to the structure, in accordance with the manufacturer's recommendations
Detailed guidance notes explaining the level of the protection provided in relation to the measures listed above are on the supporting pages.
If it is not possible to replace existing windows with laminated glass in a normal window frame, then the retrofit option should be considered (this option considers how either anti-shatter film (ASF), bomb blast net curtains (BBNC) or secondary glazing can be used).
Advice should be sought from a specialist RSES engineer if a higher level of mitigation or protection is required.