PIRA bombing of Bishopsgate, 1993.
Modern buildings make much greater use of glass than older buildings, which means that most bomb casualties nowadays are caused by flying glass.
This page provides information, primarily aimed at architects and structural engineers, on the best glazing solutions available to protect against flying glass.
There are three main methods of protection against flying glass:
Use polyester film 175 microns thick (including adhesive layers of multi-ply) or film with equivalent properties. Consider using 300 micron film for panes over 6 square metres area, or over 8mm thick, or for ground floor windows of over 3 square metres. The specification can be lowered to at least 100 microns if bomb blast net curtains are also used.
The film must be fixed in clean and dust-free conditions. On new windows or areas being re-glazed, apply film to the glass to its extreme edges before fixing the frames. If, however, there is to be extensive building work after installation - in which case dust and debris may cause unacceptable scratching - it may be better to postpone fixing the film until work is complete. In this case the film should be applied as close as possible to the putty of glazing bars with an edge gap of less than 3mm. 5mm can be tolerated along particularly irregular putty edges.
Butt joints are acceptable if the film is insufficiently wide to cover the glass in one piece.
The film cannot be applied to the patterned side of frosted, figured or reeded glass.
With double glazed windows consisting of two separate frames, in which the inner frame can be opened independently of the other, treat both panes. If the inner pane cannot be opened independently, or a "sealed unit" is fitted, applying film to the inner panel is sufficient. Sometimes inner frames are only lightly fitted; if so, they should be fully secured.
Use bomb blast net curtains only in combination with anti-shatter film. If the film is of sufficient thickness they may not be needed, but they are vital with single glazed, small paned windows in Georgian frames.
Curtains should be of 90 or 100 denier polyester terylene curtain material and be made twice the width and 1.5 times the length of the window. The bottom hem must incorporate flexible weights at the rate of 400 grammes per metre. The excess length should be folded concertina-wise and placed in shallow troughs at window sill level.
Curtains should normally be installed 50 to 100 mm from the glass, but can be farther back if it is impossible to hang them closer.
Toughened (fully heat tempered) glass provides a degree of safety but not complete security and is therefore not recommended for external window or door use.
It can, however, resist high blast pressures without damage provided it is well supported in a strong and rigid frame. When it does break its fragments are less injurious than plain glass shards. When used on its own it should have anti-shatter film applied.
Laminated glass offers a higher level of protection than toughened glass.
The minimum overall thickness of laminated glass classed as blast resistant is 7.5mm, including a minimum polyvinylbutryal (pvb) interlayer thickness of 1.5mm. The laminated glass should be fixed in a frame designed to withstand the bending effects of a static load of 7kN per square metre over the complete area of the glazing and frame. The fixings of rebates to the frame and of frame to the structure should be designed to withstand a line load of 20kN per metre of perimeter.
These loads are broadly applicable, for nominal design without factors of safety, to most glazing systems incorporating 7.5mm laminated glass, but are based on requirements for panes of about a 2 square metre area. The loads should be factored up by 50% to match the increased blast resistance of smaller windows of about 1 square metre. The line load for fixings may be factored down by 25% to 15kN per metre of perimeter for windows of about 4 square metres overall area; but the 7kN per square metre should not be significantly reduced when designing the frames of large glazing systems.
Panes with an edge dimension of 1m or more should be provided with a frame having a glazing rebate of at least 35mm giving a bearing of 30mm. Greater protection may be provided by setting the pane in double-sided adhesive security glazing tape or ideally bonded in sealant.
If robust frames and deep rebates cannot be provided, a level of protection equivalent to anti-shatter film on plain glass with net curtains can be achieved using thinner laminated glass, e.g. 6.8mm thick.
In double glazing, the preferred standard is a 7.5mm laminated glass inner pane with a 6mm toughened glass outer pane in a robust frame with deep rebates.
The same design loads may be applied as mentioned above. Users may elect merely to limit the spread of flying glass fragments by using laminated glass in less robust, standard frames. In this case the laminated inner pane may be reduced to 6.8mm thick (with 0.76mm pvb), with a 4mm toughened outer layer where panes are less than about 2 square metres. Even with standard frames some attention should be given to the strength of fixings of the frame to the surrounding structure. It is recommended that fixings should be designed to resist not less than 25% of the values given above, i.e. not less than 5kN per metre. Fixings should be at a maximum of 350mm centres.
Information reproduced with kind permission from MI5.gov.uk