08.09 Security glazing

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Categories: Glass & Glazing

This Section covers glazing and fenestration products designed to resist intrusion.  Blast resistance is dealt with in Section 08.11.

When considering security against intrusion it is necessary to take a holistic approach.  Firstly it is possible to rely on an active deterrent such as alarm systems. Secondly, it is possible to deter casual intruders by providing lighting and so forth.  Advice on these approaches to security are available in 'Secure by design', and other police guidance documents.

Level of threat
Three levels of threat arise from the point of view of the window Specifier / Contractor:

  • Opportunistic burglary
  • Targeted burglary
  • Higher risks / values

Opportunistic burglary
The opportunist burglar will attempt to break into any property that they believe contains goods worth stealing.  They are interested in the goods to be stolen not the property attacked.  Such a burglar will attack a different property if the risk of being caught is t0o high.  This occurs if:

  • The tools required to gain entry are too large.
  • The attempt to break in takes too long
  • The attempt to break in creates too much noise.

It follows that windows and doors have to resist attack from a casual burglar using small tools for a short period of time.  A typical tool set is shown here.  It is generally reckoned that a window should be able to resist an intruder using small tools for a period of 3 minutes for it to present a defence.

BS7950 gives methods of test for windows with enhanced resistance to intrusion that apply to domestic casement and tilt-turn windows.

CWCT 'Guide to good practice for windows with enhanced resistance to intrusion'  may be applied to windows of other types.

BSI Product Assessment Scheme (PAS 024) gives methods of test for domestic doors.

Targeted intrusion
Buildings may be subjected to targeted intrusion for a number of reasons including:

  • Targeted occupant
  • Building unattended for long periods
  • High value contents

Likely targets include shops, some offices, some schools and so forth.  Windows in these buildings may offer a higher resistance to attack or may be protected in some other way.  For instance by shutters or bars.

Windows designed to resist intrusion at this level will have to resist possibly sustained attack by an intruder using much larger tools.  prENV 33.036 - 9 describes methods of attack, tools sets, and periods of attack for testing windows and doors required to perform at this higher level.

High threat levels
Some buildings face a high level of threat.  These include court rooms, military buildings, banks and similar secure buildings.  When designing and specifying for these buildings the building client will normally have their own specification and methods of test for security of doors and windows.

Specification for enhanced resistance to intrusion
For windows and doors subjected to a threat of opportunistic burglar it may be advisable to specify  for enhanced resistance to intrusion.  When specifying windows with enhanced resistance to intrusion it is important to consider the glass as well as the frame.

Frames can be specified to comply with BS7950 in the case of casement windows and doors for domestic use.  For other window types the CWCT 'Standard and guide to good practice for windows with enhanced resistance to intrusion can be used.  For doors compliance with BSI PAS 024 can be called for.

Glass is not covered by BS7950 or PAS 024 although it is sensible to use glass of an appropriate type.  The CWCT standard sets the following ranking of glazing materials for resistance to intrusion by an opportunistic burglar.

  • Toughened single glass
  • Annealed single glass
  • Sealed units comprising only leaves of toughened glass
  • Wired glass
  • Sealed units comprising at least one leaf of annealed glass
  • Laminated glass
  • Poly carbonate

Specification of security windows and doors
Where a higher level of security than that given by using BS7950 is wanted then the European Standards prENV 33.036 - 9 can be used.  These standards for window and door performance do make mention of glass performance and require the use of anti-bandit glass.

At the highest levels of performance it is unlikely that a glazed window will pass the test.  It will certainly be worthwhile exploring the use of shutters, bars and so forth as an alternative.

In the UK anti-bandit glass can be specified to comply with BS5544.  However, there are no British Standards for frames with opening lights that will hold anti-bandit glazing.  BS5544 could be used to specify glass for mounting in a fixed frame but it would still be necessary to assess the security of the glazing beads..

Design of security windows and doors
With the use of sealed glazing units and different framing materials intruders have changed their preferred method of entry from breaking glass.  The methods used to gain entry include:

  • Forcing the window latch

  • Simple cockspurs were easy to force open when used with hollow section frames of aluminium and PVC-U.  They were replaced by multi-point espagnollete locking systems but again these were easy to force if they comprised only cylindrical rollers that engaged with a keep.  Today secure windows use either shoot bolts that extend out of the head and foot of the opening casement to lock it in position or some other bolt that fully engages such as a claw.
  • Forcing the hinges

  • Traditional butt hinges offer little scope for the intruder to gain entry.  However, early designs of variable geometry, friction stay, hinge were easy to disengage allowing the intruder to tear the casement from its opening, starting at the hinge edge.  Secure hinges have horns that fully engage into openings in the body of the hinge (end caps).  These should fully encircle the moving arm of the hinge to prevent it being lifted over the restraint.
  • Unglazing the window

  • The use of dry glazing techniques has made it easier to de-glaze a window.  Most windows are externally beaded and some can be easily de-glazed by levering out the beads.  This can be prevented by the use of:
    • Beads that snap into short lengths
    • Glazing tapes that stick the bead into place.  Note these may be difficult to reglaze in the event of breakage.
    • Internal clips that prevent bead removal
    Windows may be made with internal beads that are not accessible to the intruder but these should be secure and resist removal by internal impact on the glass.

Hardware has to be competent and resist breakage.  It must also be designed to prevent disengagement.  The ease with which an intruder can disengage locks, catches and hinges depends on the performance of both the frame and the hardware in combination.

When an intruder attempts to lever open a window with an implement such as a screwdriver the opening casement experiences two forces:

  • An out of plane force that may cause it open,
  • An in-plane force that may cause the frames to part such that the hardware can disengage.

Equal and opposite forces act on the fixed framing member.

A skilled intruder will use two implements simultaneously and be able to cause two adjacent framing members to both part and slide past each other.  Under these conditions the hardware is more likely to disengage.

Windows may be tested in accordance with BS7950 or the CWCT document 'Windows with enhanced resistance to intrusion'.

There are three elements to the test programme:

  • Mechanical leverage test
  • Impact test
  • Manipulation test

In the mechanical leverage test, forces are applied to the window.  For a typical casement window comprising a fixed light alongside an opening light there will be a hinged edge and a locking edge on the opening casement, image. To simulate the effect that an intruder may have in parting a frame, loads of 1500N are applied perpendicular to the frame, image.  This represents the effect of a single implement and is the only in-plane force required by BS7950.  The CWCT method of test requires the application of a second 1500N in-plane force to force the frame in a direction perpendicular to the first force, image.  Finally a force of 3000N is applied normal to the plane of the window in an attempt to force it open, image.  This procedure is then repeated at all other security points including hinges, image.

The forces of 1500N and 3000N appear to be high but they are the forces that can be achieved manually with the use of a simple lever proportioned as a typical tool, image.  Windows are tested in a rig that applies the loads slowly and progressively through a series of pneumatic rams and suitable bearers or straps, image.

The impact test uses a sand filled leather bag to apply an impact load on glazing, infill panels, and inward opening casements, image.  The intention is not to test the glass for security but to check that nothing is dislodged.  For instance a glazing bead that would allow de-glazing of the opening.  For inward opening casements it is important that the hardware can withstand an impact load.

Intruders may try to gain entry by removing glazing beads and other components using small tools.  To ensure that windows are secure against this type of attack it is necessary to allow the tester to manipulate the window using a variety of small tools such as credit cards and small screwdrivers.