Measures to improve the blast resistance of glazing

Introduction

This Guidance Note replaces NPSA EBP 01/14: April 2014 - Guidance Note: Measures to improve the blast resistance of glazing. It has been issued to provide updated guidance on the use of glazing to reduce potential hazards from flying glass in the event of an explosion, and to provide guidance on the specification of more specialised glazing systems such as glazed curtain wall systems which are frequently used in new commercial developments.

The measures contained in the Guidance Note should be considered as the minimum requirements for blast resistance. Other requirements such as Health and Safety and wind loading may be more stringent and result in thicker panes of glass or larger fixings being required. Where window systems require a level of blast resistance they should be specified by designers competent in the field of blast design.

NOTE 1: Suitable engineers should be members of the Register of Security Engineers and Specialists (RSES) (www.rses.org.uk) or be able to demonstrate that they have the training and experience to meet the appropriate RSES competences.

Measures covered

The measures covered by this guidance note can be broken down into two broad categories; mitigation measures and protection measures.

Mitigation measures – are used where the potential for flying glass must be reduced but it is not practical to use other protection measures because: it is not possible to replace the windows because the building is listed or leased; the building is not considered a likely target but the potential for flying glass caused by the blast from an explosion at some distance needs to be reduced. In such situations the installation of the following measures can be considered:

• Anti-Shatter Film (ASF) (sometimes referred to as Anti-Fragmentation Film) and Bomb Blast Net Curtains (BBNC)
• Laminated glass in normal window frames.
• Balanced designed glazed facades

Protection measures – are used where a specified level of protection is required and where it is possible to replace the existing windows, incorporate secondary window systems or influence new construction design. In these cases the building owner will need to agree the threat (charge mass and range or pressure and impulse) and the acceptable hazard level with the glazing designer.

In such situations the following measures are recommended:

• Blast resistant windows
• Secondary windows
• Blast resistant glazed facades

In addition to the measures identified above, combinations of laminated and toughened glass are frequently used in large window panes in areas classified as ‘crowded spaces’, such as shopping centres and transport related buildings, where a level of blast resistance may be required. In these situations, the driving factor for determining the appropriate pane thickness will often be compliance with the Building Regulations or wind loading and not the minimum thicknesses specified in this guidance note. In such situations the assistance of designers competent in the field of blast design should be sought (see Note 1).

Anti-Shatter Film (ASF) and Bomb Blast Net Curtains (BBNC)

• These should be used in normal window frames as a retrofit for existing annealed glass, only where the windows cannot be replaced.
• This option approximately halves the stand-off range at which a hazard would occur from flying annealed glass shards. ASF holds the broken glass together and BBNC will stop the glass and ASF being projected further into the building.
• ASF typically has a warranty of approximately 10 years, after which it may need replacing.
• More details can be found in NPSA EBP 08/13: December 2013 – Guidance Note: The Use of ASF and BBNC.

Laminated glass in normal window frames

• This option provides better blast-mitigation than ASF and BBNC, subject to suitable frame fixings into the adjacent structure.
• Recommended in lieu of ASF and BBNC for new builds or window replacements.
• As a general principle laminated glass should be specified for all new works as it provides a significant improvement in blast resistance for a minimal increase in cost. Compliance with Building Regulations for heat loss generally dictates the use of double glazed units which are able to accommodate the increase in thickness required for laminated glass.
• For single glazing not less than 6.8mm laminated glass should be used. In double glazed units (DGU) the inner pane should not be less than 6.8mm laminated glass and the outer pane should be a minimum of 6mm toughened or 6.8mm laminated glass. These dimensions relate to panes with maximum dimensions of approximately 1.5m x 1.2m. Where the pane dimensions are greater than this the guidance of an engineer with experience in designing glazing systems to withstand blast loads should be sought (see Note 1). This is applicable for external facing windows where there is no likelihood of impact by people. If used in internal locations where the glass may be impacted by people, then the glass should also be designed in accordance with BS6262: Glazing in buildings - Part 4:2005 Safety related to human impact which may require toughened laminated glass to be used for both panes.
• For laminated glass further slight improvements in safety may be achieved by selective strengthening of the glazing assembly e.g. by adding fixing screws to beading and/or the frame.

Balanced designed glazed facades

• In certain situations where a large glazed façade is required the designer may recommend a façade using mullion and transom sections to form a glazing support structure, or where clear glazing lines are required, glass bonded to an internal frame or utilising a fixed point glazing system.
• These systems are generally weak against blast loads. Tests have shown that the response of the systems against a blast load can be improved if the system is designed as a balanced design i.e. all components are designed to avoid early component failure.
• Further guidance on how to achieve a balanced design for glazed facades with a unit size of 1.4m x 3.6m can be found on the NPSA webpage. Where the unit and pane dimensions differ from this, guidance from an engineer with experience in designing glazing systems to withstand blast loads should be sought (see Note 1).

Blast resistant windows

• These should be designed in accordance with the agreed threat and acceptable hazard level and consider the glass and the window frame as a complete system.
• For example, for windows with maximum pane dimensions of approximately 1.5m by 1.2m minimum pane thicknesses for DGUs are: the inner pane should not be less than 7.5mm laminated glass and the outer pane should be a minimum of 6mm toughened or 6.8mm laminated glass. Where the pane dimensions are greater than this the guidance of an engineer with experience in designing glazing systems to withstand blast loads should be sought (see Note 1).
• The glass should be well held in suitable gaskets in 35mm rebates or be in at least 30mm rebates if held in well bedded clamped gaskets or, preferably, bonded with silicone or polysulphide sealant. The latter will reduce the hazard range to about a quarter of that for plain annealed glass.
• Other designed combinations of toughened and/or laminated glass can be specified with appropriately designed and tested frames and fixings.
• In all cases the surrounding structure and fixings must be capable of withstanding the increased loads expected.

Secondary glazing

• In situations where a greater level of blast resistance is required, and it is not practical to replace the window with a blast resistant window, secondary glazing units may be considered.
• These window systems are designed to prevent the external glazing from entering the building and are either fixed within the window reveal or, where there is a reduced wall thickness, to the inner face of the wall.
• Such window systems are designed to be openable to provide access to the inside of the external glazing for cleaning and maintenance.

Blast resistant glazed façades

• Where a greater level of blast resistance is required, it may be necessary to select a specialist glazed façade system which has been designed against the agreed threat and acceptable hazard level.
• These are specialist systems which are beyond the scope of this guidance note. If blast resistance is required, the façade and support system must be appropriately designed, checked by façade designers competent in the field of blast design (see Note 1) and should be tested in accordance with NPSA Test Standard for the Explosion Resistance of Windows and Curtain Walling.

Other considerations

For window systems designed to resist blast using laminated glass the interlayer specification and the glass bonds between the supporting structure and the edge of the glass and between the glass and the interlayer are crucial to ensure that the laminated glass is retained in position. Incorrect specification of the interlayer may result in the expected blast performance not being achieved. Further guidance is available on the Laminated Glass webpage.

Material defects such as edge delamination will reduce this bond and reduce the blast-resistance of the laminated glass. Further guidance is available in NPSA EBP 04/13: July 2013 – Guidance Note: Influence of Delamination of Laminated Glass on its Blast Performance.