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Blast Resistant Glazing

• Introduction
• Base specifications
• Enhanced specifications

Introduction

• Providing a degree of blast resistance to buildings has been an issue for building developers and designers since the IRA bombings in London (Baltic Exchange 1992, Bishopsgate 1993, South Quays 1996) and Manchester (City Centre 1996). There is, however, virtually no published information providing detailed information for designers who have to rely on the advice of specialist consultants.
• Consultants will generally start by carrying out a threat assessment for the building, based on its location and use. Architects should be aware that threat assessment is not an absolute science and an appropriate design response will inevitably involve a degree of compromise between specification and cost which can only be resolved by detailed discussion with the client.
• Although providing a degree of blast resistance to the glazing is only one of the many aspects to be considered, this page summarises the current thinking as it relates to commercial buildings in major urban areas of the UK. It assumes that the blast will be external and that the purpose of the ’resistance’ is to maximise safety to the occupants and minimise disruption to the business.

Base specifications

• For most major commercial projects, the pane size, compliance with the Building Regulations, environmental requirements, and thermal stress calculations for the glass will generally produce a ’base’ specification of an insulating glass unit comprising:
  Outer pane: Heat-strengthened, toughened, or (occasionally) laminated glass.
  Inner pane: Safety glass - either toughened or laminated.

Standard framing + Toughened outer pane + Toughened inner pane

• Most curtain walling systems utilising pressure-plate gasket glazing techniques have a glass ’rebate’ of 15mm to 20mm. It is generally accepted that disengagement of the glazing from the frame, under a ’typical’ blast load likely to be experienced in a major urban area, should be seriously considered although there is no consensus of opinion as to whether this is inevitable.
• For: The supporters of the toughened/toughened approach take the view that the numerous small fragments of toughened glass offer less risk of serious injury than the larger pieces resulting from the breakage of laminated glass. They also point out that the nature of an explosive blast (a short-duration high-intensity positive pressure, followed by a longer-duration lower intensity negative pressure) means that most of the fragments resulting from the breakage of toughened glass will actually be drawn outside the building. This approach is equally suitable for pressure-plate-glazed systems or structural-sealant-glazed systems.
• Against: The supporters of other approaches take the view that toughened glass is more likely to break and spray fragments into the building than laminated glass and that, therefore, toughened glass is more dangerous.

Standard framing + Toughened outer pane + Laminated inner pane

• This approach is only suitable for pressure-plate-glazed systems. The use of structural-sealant-glazing with toughened/laminated panes should generally be considered as an ’enhanced’ specification.
• For: The supporters of this approach take the view that the laminated inner pane will provide a degree of (uncalculable) blast absorption and, in most blast situations, is unlikely to disengage from the frame. There is, apparently, a video in restricted circulation which shows, in slow-motion, the effect of a bomb blast on a curtain wall of this type. We understand that this shows the outer pane breaking, then the inner pane bowing and absorbing the blast before, finally, disengaging and falling onto the floor immediately adjacent to the wall. The crucial aspect of this video, for the supporters of this approach, is that the disengaged pane does not ’fly around’ the interior of the building.
• Against: The supporters of other approaches maintain that, as it is impossible to predict the severity of any particular blast, one cannot take the risk of the pane disengaging from the frame and causing extensive damage within the building. It is also possible that a laminated pane might need to be thicker, and therefore heavier, than a toughened pane of the same strength.

Enhanced specifications

Enhanced framing + Toughened/Laminated outer pane + Laminated inner pane

• This is the first step towards a fully-engineered approach to blast resistant glazing and will always require specialist advice from others. It is normal for the consultant to specify the static load that will replicate the effects of the dynamic load resulting from the possible blast. In general, architects should never take responsibility for specifying an appropriate static load as it is outside their normal level of expertise.
• The curtain wall should be designed to ensure that:
• the glass will not break.
• the glazing will not disengage from the framing.
• the fixings will prevent the framing disengaging from the structure.
• Some deformation of the framing and fixings is generally considered acceptable and there is an assumption that the wall may need replacement after the blast. Load absorption and the effects on the structure are generally ignored. There is an assumption that these performance requirements can be achieved by minor modifications to existing curtain wall systems and proved by calculation only. The enhancements generally include:
• increasing the PVB interlayer thickness from 0.38mm to 0.76mm or 1.52mm.
• increasing the glazing ’rebate’ to 30/35mm.
• increasing the size of the framing members or providing additional stiffening.
• the selection of appropriate fixings.

Strengthened framing + Toughened/Laminated outer pane + Laminated inner pane

• Providing any further strengthening of the curtain wall should only be considered as part of the overall structural design of the building and will always require the input of a specialist consultant working in conjunction with the structural engineer.

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