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The Role of External Façade in Protecting Building Occupants against Terrorism and Impacts

David Hadden – Arup Security Consulting

Andy Lee – Arup Facade Engineering

Abstract In response to violent terrorist attacks on civilians around the world in recent years protective measures are being included in the designs of increasing numbers of commercial and public buildings. This paper focuses primarily on the role of glazed building façades in providing protection for a building’s occupants against deliberate attack, particularly from bomb blast. Protective design strategies are discussed as well as the performance under such extreme loading of various types of glass and glazing configurations.

Introduction There are many factors that building designers, whether they are architects or engineers, have to consider when planning a new building. The forces of nature acting on the building, such as gravity, wind and seismic loading, need to be evaluated and resisted. Imposed loads related to the function of the building must be considered. The façade of the building has to provide protection for its occupants against wind and rain and the extremes of temperature and humidity. There will also be aesthetic requirements and constraints on cost and programme that need to be balanced in the building design. But even this wide ranging list of topics does not cover all the challenges faced by 21st century architects and engineers. The perception, and indeed the evidence from around the world, that terrorists can strike anywhere mean that building occupants are demanding more than ever before in modern times that the building they are in will protect them from deliberate, life-threatening attack. Consequently the designers of all kinds of building, commercial and public buildings as well as those related to Government or other official bodies, are increasingly called on by their clients to incorporate protection against bomb blast or other severe forms of attack into their designs while still meeting all the other criteria referred to earlier. The building façade is often where the greatest degree of collaboration is required between designers, suppliers and constructors if all of these design criteria are to be satisfactorily resolved.

Protective Design Criteria & Strategies What can a designer do to reduce the risk to those in and around a building in the event of, for example, a nearby terrorist bomb explosion? One answer is to build heavy concrete bunkers or fortresses, but buildings are occupied by people who, to be comfortable and able to function, need light, air, and an awareness of the world outside. A reasonable balance must be struck between blast protection and all the other criteria for a successful building. The starting point is for the designer and client (the developer, owner, or tenant) to agree on the level of threat to be considered and the objectives of any blast protection measures. Assessing the terrorist bomb threat to a particular building is not an exercise in precise mathematics or statistical analysis and there is an almost infinite range of possible combinations of device size

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and distance to which a building might be exposed. The objective of the bomb threat assessment is for the project participants to agree on one or more combinations of charge weight and location to be protected against. The blast protection objectives must also be realistic. To expect any building, other than a hardened military facility, to withstand unscathed a large vehicle bomb immediately outside the front door is unrealistic. However, it might be accepted, for example, that on a large façade the most severely loaded windows closest to the seat of the explosion can be allowed to produce a certain level of hazard on grounds of economy, on the understanding that other windows, which are offset from the seat, remain more protective. In other cases, it may be required to fully protect the most exposed rooms at the closest range. It should also be decided whether to glaze the whole facade to the same details as required for the most heavily loaded point at ground level, or whether a graded approach to protection is to be adopted. Another key influence on the design is whether the façade or other building components are expected to be reusable after the “design explosion” or whether a level of permanent damage and distortion is acceptable, necessitating replacement. Thus threat assessment and defining protection objectives are part of a process to select a position on the scale of possible events from which to develop measures consistent with those objectives. But before commencing the design of the elements of the building structure or its facades to resist a particular blast load or the impact of a particular vehicle all other measures that might reduce the severity of the threat should be examined. For example it may be beneficial to change the orientation of the building to give better protection to critical areas. It may be possible to utilize the landscaping to prevent vehicle impact or to increase the stand off distance between the building and a possible vehicle bomb. Or relocating ventilation intakes may make them less vulnerable to the introduction of a chemical or biological agent. Every possible measure that will reduce the exposure of the building to attack should be considered under the following headings first set out by Elliott, Mays and Smith [1].

• Deflect a terrorist attack by showing, through layout, security, and defences, that the chances of success for the terrorist is small; targets that are otherwise attractive to terrorists should be made anonymous;

• Disguise the valuable parts of a potential target so that the energy of attack is wasted on the wrong area and the attack, although completed, fails to make the impact the terrorist seeks; it is reduced to an acceptable annoyance;

• Disperse a potential target so that an attack could never cover a large enough area to cause significant destruction, and thereby, impact; this may be suitable for a rural, industrial installation, but is probably unachievable for any inner-city building;

• Stop an attack reaching a potential target by erecting a physical barrier to the method of attack; this covers a range of measures from vehicle bollards and barriers to pedestrian entry controls. Against a large vehicle bomb, in particular, this is the only defence that will be successful;

• Blunt the attack once it reaches its target by hardening the structure to absorb the energy of the attack and protect valuable assets.

The analogy of the layers of an onion is often used to visualize an ideal model for the security of a building. While this idea can be applied to the layering of physical and operational security working inwards from the building perimeter, the headings above can also be thought of as strategic layers of security each of which can contribute to the protection of the assets at the centre.

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The role of the façade in protecting a building’s occupants comes under the final heading above in which the designer seeks to limit the effects of any credible residual threats that still remain after taking into account the contribution of all other threat reduction measures that have been applied.

Design of Protective Facades

Glazing under blast load

Although the possibility of significant structural bomb blast damage should never be ignored without proper consideration, the performance of the façade of any building is usually critical to the safety of its occupants. The most widespread cause of injuries and internal disruption from an external bomb blast is the fragmentation and inward projection of window glass. The truth of this has been observed in large explosions around the world, from London to Jakarta to Oklahoma and Nairobi. Plain annealed glass, the sort most of us have in our homes, is the most hazardous type as it breaks easily 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 or may throw people against walls and other solid objects. Undoubtedly the most effective type of glass to provide protection against blast is laminated glass. Even if cracked by blast pressures, the outer glass layers generally remain bonded to the inner plastic interlayer rather than forming free-flying shards. Maximum protective performance can be achieved by securely bonding the glass to suitably enhanced frames. This is often achieved by using structural silicone sealant thus enabling the cracked glass to behave as a membrane and allowing it can bulge inwards (as illustrated in Figure 1) while remaining attached to its frame due to the remarkable properties of the polyvinyl butyral (pvb) interlayer.

Figure 1. Membrane action in edge bonded laminated glass under blast loading If it remains untorn and held at its edges, 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. By way of example a window of 7.5mm thick laminated glass and pane dimensions 1.25m x 1.55m can survive without tearing the effects of a 100kg TNT charge at distance of just over 30m or a 5kg TNT charge at just under 6m.

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If subjected to an explosion that stretches the interlayer beyond its limit it will tear, but even after tearing has commenced the blast wave infiltrating within will be restricted until a wide opening has formed. The effectiveness of laminated glass, due to this energy absorbing behavior, in combination with appropriate frames and fixings to the building structure is well proven, both in tests and actual terrorist bomb explosions. Laminated glass, even when held in normal frames with conventional neoprene gaskets, can 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, as illustrated in Figure 2, reduce the extent of the occupied floor over which the hazard level is high.

Figure 2. Low internal blast hazard due to use of laminated glass in conventional gaskets Where windows are double glazed, laminated glass should always be used in the inner layer. It is however generally considered acceptable to use monolithic glass in the outer layer as the inner laminate will impede the flight of the broken outer glass and limit its throw into the building. Greatest protection would undoubtedly be achieved by using laminated glass in both the inner and outer leaves but it is recognized that cost constraints may not allow such a solution to be adopted in all cases. Toughened glass (known also as fully-tempered glass) shatters at higher loads than annealed glass and mostly forms dice-shaped particles rather than elongated razor-sharp shards. It was formerly considered that injuries from the impact of fragments of shattered toughened glass were less severe than those from shards of annealed glass. However because of the greater effort required to shatter toughened glass its fragments may travel at even higher velocities and cause different types of injury that are just as hazardous to occupants as lacerations due to elongated shards. Mechanical interlock between individual fragments can also result in larger pieces of crazed toughened glass being thrown inwards at high velocity. It is of course possible to design windows in toughened glass (or even annealed glass if sufficiently thick) to resist a specified bomb blast without cracking. However, should the blast exceed the “design bomb,” the glass will then shatter completely. The disadvantages under blast loading of monolithic glass 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

• Can be unpredictably shattered by the impact of bomb fragments.

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Particularly in the case of a tall building consideration is sometimes 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 that 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 the client must accept that, as described above, such a solution offers no reserve of protection for those floors in the event of an explosion more severe than the design event. Of course many building facades already include laminated glass for reasons other than blast protection. Before considering replacement of existing windows it is always worthwhile determining the make-up of the glass they contain and assessing the level of blast protection that it already provides relative to the threat being considered. When planning new windows in which a thin interlayer would satisfy normal glazing safety requirements consideration can be given to using an increased membrane thickness that would both satisfy non-blast related safety and enhance the explosive protection that the windows offer. There is as yet no universally accepted analytical procedure for determining the blast resistance of laminated glass in its post-crack state, although a significant amount of empirical data exists which can usually be made available to those with a genuine need. This data, however, is based around a limited range of window geometries and considerable expertise is required to apply it to other configurations. In Arup we have developed a methodology that enables us to determine the response of cracked laminated glass under blast load and the conditions under which the interlayer would just tear. This process involves a combination of non-linear membrane analysis and a time stepping analysis of the dynamic response of the glass to the intense but extremely short-duration loading produced by a bomb blast. 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 that 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 manner in which it is adhered 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 onwards and was extensively used by the UK government, 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 often leading to replacement within 5 to 10 years

• The likely reduction of transparency in service due to scratching and marking during cleaning or accidental contact.

More recently, some manufacturers and installers have developed the use of anchored ASF in which the edges of the film are secured, either by mechanical fixings or adhesive bonding, to the perimeter frames with the objective of generating membrane action similar to the behavior of bonded laminated glass, as described earlier. While in principle this technique might enhance the level of protection that is achievable, its effectiveness will be highly dependent on the ability of the perimeter anchorages to resist the membrane forces generated. One important influence will be the higher stiffness that polyester films generally possess compared to the polyvinyl butyral

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used as the interlayer in conventional laminated glass. This high stiffness means that polyester film will generate high edge forces, which will be onerous on the edge restraints, and will not lend itself so readily as pvb to the absorption of blast energy through elongation. Calculation methods for predicting the blast response of glass with anchored film are not yet as developed as those for laminated glass. While some blast test data for this use of ASF exist, extrapolating these results to predict the outcome of other window sizes and/or blast loading conditions should be undertaken with caution. Ballistic rated glazing is designed primarily to provide high impact resistance but does so 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 stiff and consequently transfers large forces to its supporting frames, which therefore must be made strong enough to avoid premature failure. The unyielding nature of polycarbonate under short duration blast load means that its failure mode in these conditions tends to be sudden detachment from its frames and vigorous inward throw of the whole pane. Despite the amount of research that has taken place into the behavior of glazing under blast load, because of the importance of appreciating the high speed dynamic behaviour involved, this remains a field in which levels of expertise vary greatly. Critical issues involving input from clients, designers, suppliers, and contractors need to be addressed early in a project to ensure a successful outcome [2].

Glazing under impact load Wind borne debris thrown at high speed by hurricanes or typhoons is a familiar hazard in certain parts of the world. To reduce the vulnerability of windows to impact by such debris new interlayer compositions have been developed by the leading manufacturers that are particularly effective at resisting intense localized loading. Explosions can also be a source of impact damage to windows caused by fragments of the container or vehicle in which a bomb has been transported. Metal objects such as ball bearings or nuts and bolts are sometimes packed around a terrorist bomb with the intention of maximizing fragment injuries to personnel. Although the impact of large, dense pieces of blast debris - the drive shaft of the vehicle in which a bomb is delivered for example – would be difficult to withstand such occurrences are haphazard and sporadic. A particular window is more likely to be struck by one of the many smaller, lighter metal fragments that are produced when a bomb explosion shreds the body of the delivery vehicle. However the evidence from trials and actual bomb explosions indicates that against laminated glass with an interlayer of 1.52mm or more fragments of this type add little further hazard to that arising from direct air blast. Building debris created by an explosion can be another source of impact on glazing. Figure 3 shows testing of a glazed rooflight under impact by large pieces of stone dropped from a height of 10m. These tests showed that the thick laminated pane required in these rooflights to resist the specified air blast would be punctured but not penetrated by the subsequent impact of a piece of stone cladding dislodged from above by the explosion.

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Figure 3. Stone cladding drop test on laminated glass rooflight

Non-Glazed Cladding It is logical to ensure that non-glazed areas of a façade provide comparable bomb blast protection to adjacent glazing. Without considering the behavior of the materials in question, this may not automatically be the case. Some important points to bear in mind follow.

• 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.

• Unreinforced masonry is brittle and potentially vulnerable to blast loading. Its capacity

to resist out-of-plane blast load will be strongly influenced by the degree of in-plane restraint provided at the edge of each panel. Its fixings to the building structure need to be considered carefully if its full capacity is to be mobilized.

• 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, 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.

Protection versus Cost of Glazing Enhancement. Figure 4 illustrates in broad terms the relative areas (centered on a soccer stadium to give a sense of scale) over which high levels of hazard might arise from a large vehicle bomb acting on different types of glazed facade. 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. Since 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 favorably with the associated reductions in risk that can be achieved.

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Figure 4. Risk reduction achievable through glazing enhancement

Interface between Cladding and Structure. Concerns are sometimes expressed that by enhancing the building façade more blast load will act on the main structure making it more vulnerable. However 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 structural columns, any blast forces from the façade will be distributed throughout the structure by the floor slabs acting as diaphragms with high in-plane strength and stiffness. In this way the very short duration blast forces will be largely resisted by the building’s enormous overall inertia, thereby minimizing the risk of collapse due to direct blast damage to the vertical load carrying structure.

Figure 5. Preferred attachment of cladding to structure

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Conclusion The façade of a building has a vital role to play in the protection of its occupants against the effects of an external bomb explosion or other severe types of loading. By selecting appropriate materials for both glazed and non-glazed areas of cladding and adopting suitable methods of restraint and attachment to the building structure, significant reductions in the potential levels of both injuries to occupants and disruption to the internal environment can be achieved at reasonable cost.

References [1] C.L. Elliott, G.C. Mays, P.D. Smith. “The protection of buildings against terrorism and

disorder”. Proceedings of the Institution of Civil Engineers, Structures and Buildings, Vol.94, Issue 3, August 1992.

[2] D.C. Smith, D.Hadden. “Blast Hazard Mitigation Through the Use of Performance-

Specified Laminated Glazing Systems”. Glass Processing Days Conference Proceedings, June 2003.

Contact details David Hadden

Associate Director ArupSecurity Consulting

13 Fitzroy Street London W1T 4BQ, United Kingdom Tel: +44 (0) 20 7755 3319 Fax: +44 (0) 20 7755 2211 Mob: +44 (0) 7768 881 909

email: david.hadden@arup.com web: www.arup.com/securityconsulting

Andy Lee

Associate Director ArupFacadeEngineering

Level 5 Festival Walk, 80 Tat Chee Avenue, Kowloon Tong, Kowloon Hong Kong Tel: +852 2268 3223 Fax: +852 2268 3949

email: andy.lee@arup.com web: www.arup.com/eastasia

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Consultant of the Year 2002 & 2004