The role of facades in blast hazard mitigation

Building security is most effectively achieved through a holistic approach that blends together overall planning, active procedures and physical enhancements. In a previous issue of the Security Monitor¹, my colleague Professor Christopher Elliott described how terrorist attacks can cause buildings to collapse. A bomb that leaves a building standing can still result in deaths, injuries and extensive internal damage if the facade of the building - its windows and cladding - is breached. However, if this outer envelope is designed to remain intact, or at least to fail in a relatively safe manner, the lives of occupants can be protected and the disruption inside minimised.

This article outlines some principles for good design of building facades to mitigate the hazards arising from an external bomb explosion, and their relative effectiveness.

Glazing hazard

The most widespread cause of injuries and internal disruption from an external bomb blast is the fragmentation and inward projection of window glass. This truth of this has been observed in large explosions around the world, from London and Manchester to Oklahoma and Nairobi. Plain annealed glass, of the sort most of us have in our homes, is the most hazardous type as it breaks into dagger-like shards. These shards are thrown at high speed by an explosion deep into the building causing laceration injuries. Blast pressures entering through shattered windows can also cause potentially fatal lung damage or eardrum rupture, and may throw people against walls and other solid objects.

The most effective glazing type to provide protection against blast is laminated glass. Even if cracked by blast pressures, the outer glass layers remain bonded to the inner plastic interlayer rather than forming free-flying shards. By securely bonding the glass to suitably enhanced frames, it can bulge inwards (as illustrated below) but stay in place due to the remarkable properties of the polyvinyl butyrate interlayer.

If it remains untorn, the interlayer prevents blast pressures entering the building and at most a fine glass dust is detached from the inner surface. The major causes of injury are thereby removed. Eventually, if the interlayer is stretched beyond its limit, it will tear but even after tearing has commenced the blast wave will be restricted until the opening is substantial. The effectiveness due to this ductile behaviour of laminated glass, in combination with appropriate frames and fixings to the building structure, is well proven in tests and actual terrorist bomb explosions.

Laminated glass, even when held in normal frames with conventional neoprene gaskets, will still provide a useful reduction in blast hazard to occupants. Used in this way the laminated glass may crack and even detach from its framing. However, it will lose inward velocity more rapidly than individual particles or shards of non-laminated glass and reduce the extent of the occupied floor where the hazard level is high.

Where windows are double-glazed, laminated glass should always be used in the inner layer. It is, however, reasonable to use monolithic glass (preferably toughened) in the outer layer as the inner laminate will impede the flight of the broken glass and limit its throw into the building.

Toughened or fully tempered glass shatters at higher loads than annealed glass, and forms dice-shaped particles rather than elongated razor-sharp shards. Although generally less hazardous, these toughened 'dice' may travel at even higher velocities and can still cause injuries deep inside a building. It is of course possible to design windows in toughened glass (or even annealed glass if sufficiently thick) to resist a given bomb blast up to the point of cracking. However, should the blast exceed the 'design bomb' the glass will then shatter completely. The disadvantages of solid glazing compared to laminated glass include:

• Brittle failure with no reserve of protection once cracking occurs;
• Once shattered the debris is likely to detach from its frame;
• Shape and higher velocity of glazing debris makes it hazardous over wider floor area;
• Requires comparatively strong, stiff frames to utilise the full strength of the glass; and
• Can be unpredictably shattered by the impact of small bomb fragments.

Particularly in the case of a tall building, consideration is often given to the height to which glazing enhancement should be extended. If the glazing selection is based on a particular explosion, it may be found that above a certain level monolithic glass of a thickness, which satisfies the other design criteria, would not be cracked. In such circumstances the client may elect to install the monolithic glass if it offers cost savings. However, he must accept that, as noted above, such a solution offers no reserve of protection for those floors in the event of an explosion more severe than the design event.

Fragmentation of monolithic glass can be inhibited by applying a polyester anti-shatter film (ASF) to its inside face. In the event of an explosion which cracks the glass, the film holds it together and dangerous shards are not released. The effectiveness of ASF depends on the properties of the film itself, the adhesive that fixes it to the glass and the care with which it is installed.

The use of polyester film to reduce the glazing blast hazard in this way was developed from the 1970s, primarily by the UK Government and generally in conjunction with bomb blast net curtains, as a retrofit measure for existing buildings when faced with a sustained terrorist bombing campaign. Although effective as a means of reducing widespread injury in such circumstances, ASF is rarely an appropriate choice for blast hazard reduction in a new building, due to:

• The superior protection provided by laminated glass, even in non-enhanced frames;
• The inevitable degradation in performance of ASF that occurs with time, leading to the need for peel-adhesion testing during its service life and replacement, often within five-10 years; and
• The likely reduction of transparency in service due to scratching and marking during cleaning or accidental contact.

Ballistic-rated glazing is designed primarily to provide high-impact resistance at the expense of the ductility and retention of debris that is desirable for blast hazard reduction. In this category polycarbonate is a very tough transparent material, which can be used on its own or laminated together with plies of conventional glass. Although capable of resisting considerable blast loading, polycarbonate is very stiff and consequently transfers considerable force to its supporting frames. These therefore must be made strong enough to avoid premature failure. Unlike the ductile behaviour of the interlayer in normal laminated glass, the polycarbonate under blast load tends to fail through sudden detachment from its frames and vigorous inward throw of the whole pane.

Effectiveness and cost of glazing enhancement

The above diagram illustrates the relative areas (centred on a football pitch to give a sense of scale) over which high levels of hazard are likely to arise from a large vehicle bomb acting on different glazed facades. By assuming that these areas give a measure of relative risk from an attack in an urban location, it can be seen that by progressing from thin annealed glass to laminated glass in normal frames, and then to laminated glass in enhanced frames, the relative risk to the building occupants is significantly reduced at each step.

As costs are often highly building-specific, it can be misleading to quote 'typical' figures. However, it is reasonable to expect the percentage uplift in building costs at each of these steps to be in single figures. These increases compare favourably with the associated reductions in risk that can be achieved.

It is logical to ensure that non-glazed areas of cladding provide comparable bomb blast protection to adjacent glazing. Without considering the behaviour of the materials in question this may not automatically be the case.

• Precast reinforced concrete cladding with robust fixings to the primary structure can be effective at resisting blast load through a combination of mass and strength.
• Masonry is brittle and more vulnerable to blast loading and its fixings to the building need to be considered carefully.
• Lighter metal cladding depends on its bending strength and ductility for its blast resilience but can be effective especially with appropriate fixings that enable in-plane membrane action (similar to laminated glass) to be developed.
• For all cladding types the fixings back to the primary structure should ideally be designed to remain undamaged under blast loads so that they are reusable after an explosion even if the cladding units themselves have to be replaced.

Interface between cladding and structure

Concerns are sometimes expressed that by enhancing the building facade more blast load will act on the main structure making it more vulnerable. As noted by Elliott², it is usually damage to a few critical members rather than an overwhelming load on the whole structure that causes buildings to collapse.

By designing the cladding, whether glazed or not, to span vertically between floors rather than fixing it to columns any blast forces from the facade will be largely resisted by the building's enormous mass thereby minimising the risk of collapse due to overloading the perimeter structure.

Summary

The facade of a building is the first line of defence for its occupants against the effects of an external bomb explosion. By selecting appropriate materials for both glazed and solid areas of cladding and adopting suitable methods of restraint and attachment to the building structure significant reductions in the likely levels of blast injuries to occupants and disruption to the internal environment can be achieved at reasonable cost.

David Hadden is Associate Director at Arup Security Consulting

NOTES

1 C.Elliott, 'Terrorism - why do buildings collapse?', RUSI Security Monitor, December 2002/January 2003 Vol.1 No.5

2 Ibid