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4
Blast-Effects Mitigation Potential for Commercial Buildings

This chapter explores opportunities to apply blast-effects mitigation techniques to commercial buildings. Such opportunities may occur in several areas: in the architectural planning process, specifically, for example, in site selection or physical space planning; and in the design and placement of critical building systems, such as electrical and communications systems. Also discussed are special considerations for blast-effects mitigation in commercial buildings, such as below-grade vulnerabilities and the stack effect in high-rise buildings (two areas of vulnerability shown dramatically in the World Trade Center bombing). Where issues applicable to civilian buildings are addressed by military technical literature, these are pointed out as potential transfer opportunities.

Assessing Threats to Civilian Buildings

In the aftermath of attacks on civilian buildings in the United States, many building owners must now consider the following questions when deciding whether their property might potentially be a target of terrorism:

• Who or what is the threat?
• Is a bomb a possible choice of weapon?
• What are the most likely scenarios or tactics for introducing a bomb into or near the building?

If an owner decides that there is reason to believe the property could be a



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Page 49 4 Blast-Effects Mitigation Potential for Commercial Buildings This chapter explores opportunities to apply blast-effects mitigation techniques to commercial buildings. Such opportunities may occur in several areas: in the architectural planning process, specifically, for example, in site selection or physical space planning; and in the design and placement of critical building systems, such as electrical and communications systems. Also discussed are special considerations for blast-effects mitigation in commercial buildings, such as below-grade vulnerabilities and the stack effect in high-rise buildings (two areas of vulnerability shown dramatically in the World Trade Center bombing). Where issues applicable to civilian buildings are addressed by military technical literature, these are pointed out as potential transfer opportunities. Assessing Threats to Civilian Buildings In the aftermath of attacks on civilian buildings in the United States, many building owners must now consider the following questions when deciding whether their property might potentially be a target of terrorism: • Who or what is the threat? • Is a bomb a possible choice of weapon? • What are the most likely scenarios or tactics for introducing a bomb into or near the building? If an owner decides that there is reason to believe the property could be a

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Page 50 target for a terrorist bomb, then the consequences of taking action should be considered: • What resources, including technologies, are available to respond to the threat? • What are the costs of applying those technologies? • What will building tenants and occupants tolerate in the way of inconvenience or added expense for security measures? U.S. Department of the Army (1993) Technical Manual TM 5-853 (Security Engineering) provides a systematic methodology to analyze ''aggressor threats and tactics,'' including a system for rating potential risks and developing appropriate responses. The Defense Nuclear Agency (DNA) has developed an integrated systems approach, the balanced survivability assessment method, to evaluate the survivability of facilities against a wide spectrum of threats. Both of these techniques are directed primarily at military needs, though they could be used effectively for civilian applications. Answers resulting from these analyses would need to be weighed in the context of the functional and financial goals of the building owner. Architectural Planning Process Having decided that a proposed or existing building requires protection from attack, the owner must assess what can be done to mitigate the effects of an explosion should one occur. Blast-hardening refers to all measures to reduce or eliminate the effects of an explosion. This included techniques such as physical space planning, that deliberately use architectural location and organization of spaces and other nonstructural features to minimize the effects of an explosion on people and property. These options are discussed in U.S. Army Technical Manual TM 5-853 to a limited degree, though the planning techniques in this manual are strongest in the area of supporting access control to the facility. All such options can be applied in the normal design of a building, just as designers now routinely incorporate seismic, wind, and fire protection features and systems in buildings. The architectural planning process itself involves less technology than awareness and design skill, although technology may be needed to validate or revise empirical planning and design assumptions. In considering opportunities for technology transfer, it is useful to discuss here these planning techniques and consider how they might supplement or be supported by the technologies of hardening. Improving the performance of conventionally designed civilian buildings following an explosion begins with site selection and the architectural planning process. One of the first steps in protective planning for a new facility is to select

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Page 51 a building site that allows access to the facility to be easily controlled so as to make delivery or placing of an explosive as difficult as possible. Locations to avoid, for example, include congested urban areas where it is difficult and costly to secure distance between the building and uncontrolled public rights-of-way. Preferably, the site, its entrances, and the building itself should all be placed out of alignment with potential high-speed approaches by vehicles, such as would be the case of an entrance drive opposite a street. Distance between the building and streets or parking areas where potential vehicle bombs could be detonated is one of the most effective means of minimizing damage from explosions. The cost of land, however, must be considered in this measure. Control of surrounding streets and adjacent off-site parking is ideal, but rarely available to civilian commercial building owners. As to the building itself, earlier chapters discussed some technically sophisticated tools, especially computer codes, for designing structures to respond acceptably to explosions. The following discussion explores less technical design approaches that, when applied with common sense, can economically reduce the effects of an explosion on the people and contents of the building. Of course, hardening parts of a building may also be necessary. Many planning and design response options could be considered for a new building or retrofitting an existing structure. All the following options could be applied to new or retrofit situations. Identifying potentially hazardous functions, such as mail- and freight-receiving and handling facilities. If a letter or package in freight is considered a likely means of introducing a bomb into a building (such as buildings housing celebrities who might be targets and who receive large volumes of unsolicited mail), providing such facilities in either a hardened area or a remote location, perhaps off site, might be considered, even at the cost and inconvenience this might involve. (TM 5-853 addresses bombs introduced through mailed or shipped packages in considerable detail.) Controlling or eliminating hazardous material storage on site. Fuel storage, trash holding, paint shops, and pressure vessels can contribute to the fire and smoke generated in an explosion, and might be located remotely from areas vulnerable to the introduction of a bomb, from means of egress, and from life-safety systems such as fire pumps. Locating vulnerable functions away from uncontrolled public traffic areas to minimize blast-effects on occupants.High-profile potential targets (and facilities for the particularly vulnerable such as children, the elderly, or handicapped) might be placed in remote, inconspicuous locations rather than, say, at the front of the building over the entrance. Offices and other continuously occupied spaces including day care centers can be located, for example, on the sides of the build-

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Page 52 ing away from streets or on interior courtyards. Street frontages can be reserved for circulation, nonvital equipment, storage, or other uses that could even be sacrificed to an explosion and absorb the initial shock. (TM 5-853 addresses this strategy briefly.) Providing dispersed, concealed, and controlled access to utility service entrances, fuel delivery, and storage facilities, and providing decentralized internal electrical and telecommunications distribution centers. Frequently, these important utilities enter buildings near one another, are easily identified, and are relatively easily accessed by unauthorized persons. An explosion in the area of the utility service entrances could destroy all services in one stroke if they are all close together. (TM 5-853 is concerned with the vulnerability of utility openings to forced entry or introduction of chemical and biological agents, but not with protection of the utilities themselves.) Furnishing redundant electrical and telecommunications supplies when necessary. Where it is impossible or not cost-effective to locate utilities out of harm's way entirely, redundancy of vital systems, such as switchgear, primary feeders, power generators, sprinkler mains, and fire pumps, may be advisable. Wherever possible, the alternative source should be located in a protected area and remote from the primary source. Determining the practicable application of physical security systems. A wide assortment of surveillance, access control, and access-denial products are available to use to reduce the risk of bomb deliveries by persons or vehicles. These products are identified in such manuals as TM 5-853 and in commercially available sources. Enhancing areas of refuge. Modern multistory buildings are allowed by many building codes to use "horizontal exits" in addition to the usual exit stairways. This concept permits large floor areas to be compartmented by fire and smoke barriers, so that persons fleeing fire and smoke in one area may move horizontally to an adjacent area. Occupants can either continue to exit by stair or remain in the area of refuge as long as necessary. Incapacitated persons need not negotiate fire stairs, at least immediately. As a result of an explosion, lower floors and means of egress from the building may be so severely damaged or filled with smoke that refuge areas may be needed for some period of time before rescue can be attempted successfully. The construction of the walls and partitions surrounding such areas might be strengthened to resist breach by flying debris. Additional accommodations to this situation can include emergency medical equipment, blankets, toilets, drinking water, radios, and flashlights in a designated area of each compartment.

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Page 53 Continually assessing existing conditions against the availability of new technologies that mitigate blasts-effects. Any building owner or occupant who believes the building may be a target for bombings should undertake systematic threat and vulnerability assessments periodically and decide on any new courses of action if new technologies are available. The Balanced Survivability Assessments technique developed for military applications by DNA would provide an excellent basis for this task after being adapted to civilian situations. Controlling or eliminating parking and loading under or within the building. Vehicle parking and loading operations within or under an occupied building pose a major hazard, since vehicle bombs can be very large and powerful. Vehicle bombs may also elude detection, especially in high-volume facilities where inspection of vehicles with any regularity or thoroughness may be operationally and financially unacceptable. Where parking or loading cannot be excluded, limiting the number of vehicles, such as those of tenants only, and providing machine-readable identifiers, vehicle-weight sensors, and spot checks are often acceptable compromises. Having a well-developed emergency operations plan to aid occupants after an explosion. Essential components of an emergency plan are the appointment of trained wardens, conducting practice drills, and regular review and update of emergency procedures. These measures are seldom appreciated by occupants in a peacetime environment and in situations other than high alert. They are often neglected or abandoned as soon as the sense of real urgency is lost. Disaster planning and emergency operations are two areas which can potentially return enormous dividends in terms of lives saved and suffering averted. A sample of the considerable literature in this field is included with the references to this chapter. Relocating functions and installing or upgrading fire- and life-safety features such as smoke control and evacuation. Fire- and life-safety code requirements are becoming more stringent, especially for new buildings in urban locations. Periodic review of a facility's fire- and life-safety features and upgrades to new building standards seem prudent in any case, but such measures are more urgent for a facility that could be a target of terrorism. Avoiding architectural features that magnify blast-effects. If there are architectural features that focus or increase blast-effects, they should be identified and avoided. (TM 5-853 indicates that re-entrant corners tend to cause blast pressures to build up.) Research is needed in this area to determine what other building configurations may have these properties.

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Page 54 Incorporation of controlled venting and other measures to mitigate the effects of internal explosions. There is a considerable body of knowledge on techniques to mitigate the effects of an internal explosion which has been developed as protection against accidental explosion in military, explosive, and chemical environments. (For example, U.S. Department of the Army [1990] Technical Manual TM 5-1300 (Structures to Resist the Effects of Accidental Explosions) discusses the use of blast valves for the controlled release of blast pressure from accidental explosions; Air Force Engineering and Services Center [1989] ESL-TR-87-57, [Protective Construction Design Manual] discusses the same topic for blasts resulting from weapon detonations.) Other methods of blast-effects mitigation available are blow-out panels for pressure relief and the use of "frangible" or sacrificial elements designed to fail to reduce the amplification of shock pressures. Temperature-reducing systems to retard the development of high overpressures are also available. Research on these techniques is needed to determine their applicability to civilian office structures where the survivability of the occupants is the paramount objective. Designing windows that minimize the effects of blasts or applying such technologies as high-strength glazing or fragment curtains. A number of studies have been done on security windows resistant to explosions (Chapter 7, vol. 3, of TM 5-853, and Chapter 5 of the American Society of Civil Engineers [1995] task group draft report on Structural Design for Physical Security provide important design guidance). Recently, research has been exploring more-robust window assemblies, including glazing for resistance to hurricane-blown debris. High-strength glazing materials, including glass block, tempered glass, and polycarbonates, and laminated and film-backed glass and fragment-entrapping meshes for fragment control are already in use for both security and storm resistance, but more research is needed to develop better assemblies and distribution of impact loads. Designing elevator and stair shafts to resist smoke penetration by pressurization or compartmentation. Elevator entrances and cabs are currently not resistant to smoke penetration. In a fire or explosion emergency, persons may be trapped in elevators and overcome by smoke. Research is needed to explore the possibility of sealing elevators against smoke penetration and to meet fire-resistance standards. At the present, the only option is isolation of shafts in smoke or fire compartments. Pressurization of stair shafts is a design problem, especially in tall buildings where shafts must be interrupted to decrease the height of pressurized chambers and to control pressure levels.

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Page 55 Special Considerations for Hardening Existing Buildings Since there are many more existing buildings than planned new buildings, it is understandable that building owners and management would be more interested in ways to protect existing buildings. Structural Design for Physical Security, Chapter 8, "Retrofitting Existing Structures," (ASCE, 1995) discusses some approaches for possible improvements to existing structural systems. Sometimes little or nothing can be done to protect existing buildings from explosions, such as older wall load-bearing structures in congested urban settings where closing adjacent streets to vehicle traffic cannot be done. Defensive precautions, such as threat and vulnerability assessments, access control, and good intelligence and law enforcement, are always useful, but there are limitations to what building technologies can offer, and in some of these cases, the only option may be relocation. In addition to the architectural planning techniques discussed above, there are other means to harden typical existing civilian buildings against terrorist attacks. One consideration in commercial structures is that hardening features may be quite apparent when installed after the fact in an existing building, and most commercial building occupants do not want the appearance or the function of the building to be changed or to advertise their presence, if they are potential targets, by obvious security measures. Hardening a monumental structure such as the U.S. Capitol Building without changing its appearance or function would be technically challenging and extremely costly. Of course, many federal buildings have large security systems including on-site enforcement personnel. Yet the attack on the Murrah Federal Building in Oklahoma City demonstrates the vulnerability of most governmental buildings. Other prominent buildings, such as places of worship, communications centers, courthouses, and office buildings, are not as security conscious as airports, banks, and museums. In addition to the planning techniques discussed in the previous section, structural reinforcement of some building elements may be possible through the use of: • additional mass; • additional strength, through modification of boundary conditions (e.g., supports to walls or floors), reduction in spans, or reduction in loaded areas; • replacement of weak components; • redundancy of structure; • strengthening of exterior curtain wall (by attention to windows or doors); and • strengthening of interior partitions.

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Page 56 However, some of these techniques may present problems. For instance, consider the addition of mass. Loading from TNT is dynamic in nature, while adding mass to a building component has an effect where damage is caused by impulsive loading. Also, the addition of mass contributes to the weight of the building, which often places greater demands on the existing framing and foundations. It is usually difficult and expensive to add additional foundation capacity. Adding strength and stiffness to the structure is also difficult, especially when attempts are made to decrease spans. Such approaches usually require additional columns or walls and clearly are not suitable for inherently long-span structures such as courtrooms, trading floors, retail sales floors, convention centers, sports arenas, performing arts centers, museums, or worship areas. Reduction of loaded areas usually refers to earth beams placed against the exterior walls of the building. This solution has possibilities, depending on the site conditions and the exterior wall material, but it is certainly not a universal solution. Modifying boundary conditions also has its limits, especially when dealing with reinforced concrete structures. Wall load-bearing buildings are sometimes easily brought down by localized damage. Structural load-bearing members may need to be examined for their participation in the load-bearing system and to ascertain that they exhibit reasonable redundancy. Structural redundancy has potential, especially in precast structures or masonry-bearing wall structures. The designer may ensure that, when certain key elements (columns or walls) of a structure are damaged or destroyed, alternate load paths are provided so that the building will not undergo progressive or total collapse. Often this retrofit procedure only requires additional means of tying the structure together. For significant blast resistance, substantial and compact structural forms—such as box-type construction, with strong walls, roof, and floor—are often preferred. The building material most commonly used throughout the world, because of its relatively low cost and ease of fabrication, is concrete. Although concrete is a relatively brittle material, it is rendered ductile through steel reinforcing. For resistance to high pressures, care with the details of reinforced columns, connections, and walls is required; special reinforcing may include closely spaced ties throughout the element, with attention given to the reinforcing and tie-in to other walls and slabs at the edges where much of the resistance is developed. In some cases, the use of fiber-reinforced (metal or polymer) concrete can be effective. Quite obviously, anticipated loading conditions will influence the design, but reinforced concrete structures can be expected to be relatively thick (102 to 103 mm) to provide the mass and strength to resist blast pressures. Steel structures also need special attention where they are employed to resist intense blast loading or are expected to respond in an inelastic manner. Of particular concern are those connections that, if they fail, can lead to instability of the structure, and possibly collapse. For both steel and concrete structures, the de-

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Page 57 signer should consider redundancy in supporting members, to help ensure the survivability of the structure even if some columns or other critical members are severed. Although current design procedures provide guidelines on how to enhance the breaching resistance, it is often impractical to protect against breaching and direct shear effects by conventional means. Alternative reinforcement details should be employed when heavy shear (diagonal, punching, or direct) is expected. Special materials and combinations of materials (e.g., high-strength concrete, perhaps combined with fiber reinforcement, and layering with energy-absorbing material) might possibly be employed in such situations. Strengthening exterior curtain walls may be quite effective against a bomb placed outside a building. Assuming the structure can resist the loading from the bomb, substantial damage can be avoided by not letting the blast wave into the building. Typically, doors and windows are the weak points. Windows can now be protected to some degree by several means: polyester fragment retention films, polyethylene terephthalate (PET) backing or interlayer, heat-strengthened and tempered glass, polycarbonate-sheet and urethane/glass composite glazing, and polyvinyl butyrate (PVB) interlayer or combined PVB-PET laminated glazing. Security windows and glazing are discussed in the U.S. Army Technical Manual TM 5-853 and Structural Design for Physical Security, Chapter 5, (ASCE, 1995). Various available shutter designs can decrease blast loading on the windows, although they are only useful with advance notice of a threat. Nylon and Kevlar® mesh curtains can also be used inside the windows to contain blast fragments, or special blast windows can be purchased. Special blast doors are available, though they can be too massive for high-frequency applications. Security doors are generally marketed for ballistic and forced-entry resistance, and while these doors are suitable for regular high-frequency use, their performance under blast conditions is unknown. Although these solutions have some impact on appearance, they can often be acceptable. (Security doors are also discussed in TM 5-853.) More research is needed on these assemblies, research that might also benefit design of windows and doors for protection from projectiles during high winds such as hurricanes. Selected metal stud and drywall interior partitions can be replaced with steel plate on hot-rolled steel framing. Steel-plate shear walls have been used in buildings before, to resist horizontal loads when thick, reinforced, concrete shear walls occupy too much space. Drywall interior partitions can also be replaced with reinforced masonry walls with a spray-on concrete/steel mesh or Kevlar® or ballistic nylon. When these interior partitions are tied into the existing structure, they increase the stiffness of the building, with minimal impact on the appearance or function of the interior space. Because of the high cost of retrofitting existing buildings for blast-effects mitigation, developing improved methods of monitoring and controlling the flow

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Page 58 of people into and out of facilities may prove to be the most cost-effective way to frustrate terrorist bombing attempts for the long term. Vulnerabilities of Civilian Structures The trend in new civilian building design for the last 50 years has been toward the use of lighter but stronger materials. This has led to more economical buildings, with the structure accounting for less of the floor area and lower first costs. During this same period of time, engineers have developed a greater understanding of building performance when a structure is subjected to dynamic horizontal and vertical forces associated with wind and earthquake. In a situation involving earthquake loading, the design forces decrease as the weight of the building decreases. Seismic design calls for the building to possess adequate strength (force and ductility-resistance characteristics) so as to resist the repetitive seismic motions in a manner that protects human lives and leaves the building usable, or, at the most, with damage that is easily repairable. When designing for wind and earthquake loads, therefore, it is advantageous, especially for the upper levels, to use lightweight nonstructural building materials such as metal stud and drywall partitions instead of masonry. The dynamic loading on buildings caused by explosions differs in important respects from dynamic loads imposed by earthquake and wind. These latter loads are of relatively low intensity, long duration (seconds to minutes), and essentially oscillatory (periodic in nature). Explosive loads, by comparison, are extremely large initially, act for very short durations of time (milliseconds), and are non-oscillatory (aperiodic). For explosive loads localized in the lower levels, characteristic of terrorist bombing incidents, the lower levels of a structure should be massive to effectively resist the large, short-duration loading. This goal is generally in keeping with seismic requirements where significant strength is called for in the lower levels. Sound engineering judgment, of course, should be used in the design of buildings to withstand short-duration explosive incidents. During the design process, particular attention should be paid to the following factors, because under certain conditions, or in combination with other factors, they may positively or negatively impact building performance following an explosion: • the use of lightweight materials—especially in nonstructural applications; • the use of very long spans; • the use of live-load reductions permitted by codes; • the vulnerability, especially of precast systems, to progressive collapse; • the strength of the exterior cladding and its effect on the structural system; • the effect of the loss of an individual column; and • the behavior of ductile framing systems.

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Page 59 A discussion of the design and behavior of structural components typically used in modern civilian buildings subjected to a transient blast-wave form is contained in Chapter 4 of Structural Design for Physical Security (ASCE, 1995). Although not specifically addressed to blast-effects, design for lateral loading for wind and earthquake is the subject of two important military publications (U.S. Department of the Army, 1986, 1991; and two civilian publications (ASCE, 1993; FEMA, 1994) as well as the three Model Building Codes. Vulnerabilities of Nonstructural Building Systems Recent instances of buildings that have suffered the effects of an explosion, such as the World Trade Center, have demonstrated that if the structure of the building survives and does not progressively collapse, the greatest problems are experienced during evacuation and rescue, when fire and smoke control and other critical building systems may not provide the necessary support. Most buildings are designed to resist events of seismic origin, fire, flood, wind, snow, and similar natural and human-caused events. Long experience with these perils has permitted rational codification of how a building and its systems must perform to achieve several critical goals to allow: • the maximum number of occupants to escape, • the minimum number of injuries and fatalities to be sustained, • the protection of property, and • emergency personnel to control or prevent further destruction of the building while these objectives are accomplished. Few civilian buildings in the United States are designed to withstand the effects of an explosion within or adjacent to the structure, although the design features for more common emergencies will also help to achieve the same objectives in the event of an explosion. After any disaster—fire, flood, or bomb explosion—a building's systems are major factors in the recovery period when evacuation, and, ultimately, reoccupancy and return to normal operation are the principal objectives. A modern building can be thought of as a set of interdependent systems and subsystems, rather than as a group of autonomous elements such as wall, floor, roof, and so forth. The structural system is probably the most vital, since failure of even part of this system may directly and immediately threaten all other systems. Extreme examples include the nearly total destruction of the Alfred P. Murrah Federal Building in Oklahoma City and the U.S. Marine barracks in Beirut. The structural system and its protection has therefore been extensively studied, and technologies for designing this system to resist failure following an explosion are discussed in Chapter 3 in this report. Nonstructural building systems have been given less attention in blast-ef-

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Page 60 fects mitigation technology. However, the survival of some of these systems is essential to the rescue and evacuation of building occupants and to the recovery of the building to normal use. Therefore, the roles of these nonstructural systems in civilian buildings are addressed here. Three general types of nonstructural systems are defined for the present discussion. 1. The first type of nonstructural system consists of those systems that may be critical for survival and evacuation during and immediately after an explosion: • The electrical system supports lighting to aid in evacuation, fans that may be designed for smoke exhaust, occupied elevators at the time of an explosion, fire pumps, and numerous control systems that may ensure the function of security and communications systems. • Communications systems allow building occupants to be notified of the nature of the emergency and instruct them in evacuation and provide contact with outside rescue and emergency forces. • Plumbing systems deliver water for extinguishing fires, provide for hygiene and drinking during entrapment situations, and are used in administering first aid. • Ventilation systems provide pressurization and exhaust to control and contain smoke. • Circulation systems, including corridors, stairs, doors, and ramps, provide the means for occupants to escape and emergency personnel to enter the building. All of these systems must be operational and support each other in their various tasks to enhance the likelihood of survival of individuals not killed by the explosion itself. 2. The second group of nonstructural building systems may not in themselves play active roles in survival or rescue activities following an explosion, but they may attenuate, propagate, or contribute to the effects of an explosion: • Exterior wall systems consisting of interdependent subsystems: - wall construction—precast panels, metal panels, masonry, framed, and so forth—that is basically supported by the structural system; - sash supported by the wall system; - glazing supported by the sash, although polycarbonate glazing could hold the sash together; - sun-control system supported by the sash or soffit which could contain glass fragmentation, if in the form of a film or mesh, or contribute fragments, if in the form of blinds.

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Page 61 • Interior partition system - metal runners and studs supported by the structural system; - drywall panels supported by the studs and perhaps holding them together following an explosion; - finish materials supported by the drywall, and, in the case of some fabrics, possibly holding the drywall together under the effects of an explosion. • Interior ceiling system: - suspension grid supported from the structural system; - ceiling panels supported on the grid system; - lighting fixtures supported on the grid system: • Interior mechanical and electrical distribution systems: - heating, ventilating, and air-conditioning terminal units and ductwork supported by the structural system and perhaps easily dislodged by an explosion; - electrical conduit usually hung under the structural system and possibly severed by structural failure or directly damaged by an explosion if it occurs nearby (exposed or ruptured power lines may pose a threat to occupants and emergency personnel, or contribute to fire); - piping for water and gas under pressure usually hung under the structure, and, like conduit, possibly destroyed by the structure if it fails or is breached by the explosion itself (loss of fire-fighting water not only loses the fire-extinguishing function, but can pose a threat of flooding to occupants, other systems, and rescue personnel). These systems may perform in any number of ways in an explosion, either absorbing some of the shock, shielding people or property from fragments, or becoming lethal fragments themselves. In the case of an exterior explosion, loss of the exterior wall allows greater damage to the interior of the building. Strengthening the exterior wall and window systems has been studied, and design information is available in the TM 5-853 manual and elsewhere. However, little research appears to have been done on how these and other assemblies might be protected from the effects of an explosion in order to reduce the possibility of damage to critical systems. On the other hand, breach of any part of the building's envelope or of the enclosure of a space in which an explosion occurs will relieve pressures, possibly to the benefit of the building and its occupants. The effects of venting are discussed in the U.S. Department of the Army Technical Manual (1990) TM 5-1300 (sec. 2-14). The BLASTX computer code, discussed in Chapter 3 of this report, provides computational methods to predict the effects of explosions in vented conditions. 3. The third type of system assures the continued functioning or rapid repair of certain building systems for the continued beneficial use of the structure after the building sustains serious damage. The definition of this type of system de-

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Page 62 pends to some extent on the user's objectives and expectations, the type and location of the building, the season during which the event occurs, and what recovery-time interval is sought. Examples of this type of system include: • Mechanical temperature conditioning, as may be essential for computers supporting other systems, for certain laboratories and medical functions, and for food or other low-temperature storage needs. • Elevators. • Water and sewer service. • Security and perimeter control, such as may be needed to protect secure information, or sensitive or vital facilities, or to control inmates or prevent looting. Additional choices will have to be made according to the circumstances, financial considerations, and priorities of the building user or owner. Below-Grade Vulnerabilities in Civilian Buildings Many modern buildings and complexes have parking, loading, and service areas within the building, and because vehicles are capable of surreptitious transport of large explosive charges, the location of critical systems in such areas as basements, where the above-mentioned functions are often placed, is a common vulnerability. This arrangement often reflects the desire to reserve above-ground building areas for occupancies that benefit from access to daylight, and in some cases, it also reflects the desire to maximize the above-grade envelope permitted by zoning. (See also ''Architectural Planning Process'' discussed above.) Primary electrical switchgear is commonly located in a below-grade room (which is at a garage level in many buildings) simply because large commercial electric service is usually delivered underground. Although this equipment is housed in a separate, controlled access room, the enclosing construction is designed to meet fire codes only and is poorly equipped to withstand an explosion. If the room is located adjacent to or in the midst of the vehicle parking area, some of the main feeders from this equipment are also potentially exposed to an explosion from a vehicle bomb. If the building has a secondary power source from an alternative substation or even a power grid, the switchgear for that service is also typically placed in a similarly vulnerable location, if not in the same vicinity. If the building or complex has emergency power generation on site, the generator(s) might be located remotely from the main switchgear, but the generator controller and automatic-transfer switching equipment is typically mounted in the same room as the switchgear. An explosion that breaches the switchgear room also renders the emergency system inoperable, leaving the building with neither primary nor backup electricity. The generators themselves are usually situated for ease of heat and exhaust

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Page 63 rejection on an outside wall in an areaway or even in a separate structure. In some buildings, however, the generators are also located in basement areas, making them subject to the same vulnerabilities noted above. Generator and switchgear rooms below grade are also vulnerable to water damage or flooding if an explosion ruptures water piping in the vicinity and there is insufficient drainage or pumping capacity. Building control centers for refrigeration and other building operating systems may also be located below grade in or near the central mechanical equipment room, if it is also below grade. This arrangement is vulnerable to the same risks in the event of an explosion. If this control center also includes security, communications, fire alarm, and other systems, its loss could be catastrophic. Buildings with early (nonenhanced) fire-detection and annunciation systems may have the main fire-panel controller and power supply in the switchgear room also. Buildings with enhanced systems have their fire-control panel located just inside the lobby doors for rapid access by fire officials, but in this location, the panel may be vulnerable to an explosion in the lobby area. As is typical of other commercial utilities, voice and data communications services usually enter a building underground, into a room that may also contain the main distribution frames. In some cases, this room also houses the electronic switching equipment that routes calls within the building and is the interface with the local telephone exchange. If the switching equipment is owned by a private telephone service provider, it may serve several adjacent buildings as well. A below-grade location adjacent to areas accessible to vehicles is vulnerable in the same way as the primary electric power service room. The main water and sewer services are also located below-grade level, and are usually co-located with the building's fire pump, siamese connection, standpipe, and sprinkler manifolds. This equipment may be enclosed in a room, but also may be placed behind a chain-link barrier that is intended only to discourage tampering and vandalism. Again, if this equipment is within or adjacent to vehicle parking areas, it may be lost in an explosion, and in addition to the loss of the services of water supply for fire and hygiene, the probability of flooding and interruption of other systems is high. Mechanical heating, cooling, and ventilation equipment may be located below-grade, at, or adjacent to, parking levels, especially if the building is subject to height or setback restrictions or other zoning constraints. This equipment and its attendant piping and ductwork are mostly in enclosed rooms, though the rooms are usually not resistant to blast-effects. However, air ducts are connected to the fan housings to bring outside air into the system and to distribute it into the building above, and chilled and hot water lines pass into and out of the room. Any of these ducts or pipes that traverse an area exposed to an explosive source is vulnerable to loss. Most modern buildings are totally dependent on mechanical systems, and the loss of these systems can jeopardize recovery of vital functions that permits rapid reoccupancy and resumption of the building's business.

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Page 64 Shafts for utilities are typically adjacent to stair and elevator shafts and subject to many of the same vulnerabilities. Utilities can also be severed in these shafts by explosion-generated debris. The most serious damage to building systems, then, can probably be accomplished from a vehicle-transported bomb that is detonated in a building's basement areas, simply because a number of critical building systems are typically found in this location, as is much of their control and distribution equipment. A garage detonation also has significant potential for fire and smoke because of the large amount of combustible materials present in and on the vehicles that may be parked there. Any basement vehicle parking areas, therefore, should be given particular attention when considering blast-hardening features or access control and detection, perhaps even at the expense of other measures when available funds are limited. Protecting Nonstructural Systems Blast-resistant, reinforced concrete walls can be constructed to decrease the effects of an explosion in or around buildings, including protection of many of the critical systems discussed above. Since little research has been done on the protection of systems or behavior of common building assemblies following an explosion, the following discussion is limited to a few of the planning techniques available to design professionals to remove at least parts of these systems from harm's way. As a general principle, critical services should be decentralized and, wherever possible, separated from garages, loading docks, and vehicle routes. Where emergency or alternative systems exist, they should be kept as remote as possible from primary systems. Although there are some first-cost economies in placing the electric switchgear, emergency generator controls, and fire panels in the same room, this practice places the building at greater risk of losing three critical systems in a single event. Physically decentralizing critical systems and their components may help one or more of these systems survive or may reduce the extent of damage to them, thus reducing the time needed to repair and restore them. Rooms containing critical systems may be placed behind or nested within other rooms deeper in the building to achieve greater distances from areas of vehicle access. Of course the same logic can be applied to noncritical systems as well. A design precaution may be to keep critical services away from exterior walls that face possible locations for a bomb at the building's exterior. A more drastic solution would be to place as many services as possible in floor raceways or in access floors, where they would be less exposed to explosions. Some hardening at little or no cost can be achieved during building design by locating lower floor chases behind protective building elements where they are less likely to be in the line of an approaching blast wave. It may also be useful to orient the

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Page 65 elevator lobby's lengthwise axis perpendicular to the anticipated location of an explosion, such as a parking place along the building's street frontage. Garage-level auxiliary rooms, such as those housing the primary electric service, are usually constructed of concrete masonry units that have poor resistance to side-loading, and it is doubtful that these assemblies could withstand a nearby explosion even if retrofitted with interior or internal bracing. Blast-response technology may permit the design of a device, baffle, or room shape that can deflect the shock wave away and toward a less critical building component or a component that is known to successfully resist blast-effects, such as underground perimeter walls. Some protection can be afforded to elevators and stair shafts by not having them be continuous from upper stories to basement levels. Separate elevators and stairs can be provided for the below-grade floors. This design is often desirable for security reasons as well. However, existing buildings have few if any options other than to construct stand-off barriers to keep vehicles as far as possible from elevators and stair shafts. Stack Effect in High-Rise Buildings The vast majority of the injuries caused by the World Trade Center bombing were not the result of direct impingement of fire or explosion, but from smoke and dust inhalation. Smoke and dust were driven upward through both towers by stack action, filling the stair shafts and many of the floors. The stack effect in tall buildings is similar to the behavior of a chimney: the colder, heavier air outside of the chimney pushes the warmer, lighter air inside of the chimney to the top; this action stops when the flue is closed. Each of the towers of the World Trade Center is like a chimney, with the warm air inside of the building pushed up by the cold air outside, generating an upward flow through the towers, particularly during the cold winter months. To minimize this air flow, an air lock was constructed around each of the two towers. Passing through the revolving doors is, in effect, passing through the air lock. With the detonation of the bomb, much of the masonry wall along the south wall of the north tower at the basement level, where the explosion occurred, was blown into the building. Other masonry work surrounding the shuttle elevator shafts was blown into those shafts. (Similar but lesser damage was done to the south tower.) In this way, the air locks were breached, particularly for the north tower. With the loss of the air locks, the smoke- and dust-laden air from the bombed area was drawn into and upward through the towers by stack action. The problem was aggravated in areas where tenants broke windows. It is conjectured that had the air locks survived the explosion, the smoke-laden air would have been confined to below-grade areas of the complex and would not have been forced into the towers, and perhaps several hundreds of millions of dollars of nonstructural damage would have been averted and per-

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Page 66 sonal injuries would have been greatly reduced. Much less smoke was found in the south tower where the air locks remained largely intact. Controlling stack effect in tall buildings is mechanically complex and worthy of further study. Economic Considerations It is reasonable to expect that blast-hardening a building will cost an owner additional money, whether the building is in the design stages or already exists. A blast-hardened building may cost more to operate and maintain as well, especially for increased security staff. For a building still in the concept or design stage, additional first costs may include services of consulting experts, perhaps some testing or modeling, and the increased expense of nonstandard design, nonstandard construction practices, and, possibly, the purchasing and use of unusual materials. Existing buildings would entail these same costs plus the cost of retrofit demolition and perhaps the loss of some usable or rentable area occasioned by the relocation of subsystems or the creation of buffer spaces. For this discussion, properties may be divided into two broad classes: those constructed by an owner for the owner's use and those expected to perform as a financial investment. The economic factors in these two classes differ according to the owner's objectives. Buildings constructed by and for an owner include owner-occupied office buildings, such as corporate headquarters, warehouse and distribution facilities, process and production plants, and civilian government buildings. The facility typically does not need to compete financially directly with other similar facilities. The owner does not expect a return on the investment through leasing or renting these facilities to a second party, although some building owners may reduce their own floor space needs and rent or lease out other portions to lower their costs. When such a facility is being planned, the concern for protecting it against attack is based on the owner's assessment of risk. This assessment may depend on the owner's potential attractiveness to an attacker, perhaps for offering a controversial product (tobacco) or service (abortion), or for suffering an unpopular public image (a petroleum company or a multinational corporation that operates in many foreign countries, where it must try to satisfy diverse and often conflicting social and cultural expectations). While any owner must weigh the costs of protection against the perceived threat, this type of owner may be more willing to install security, because in an owner-occupied building convenience or preference of the occupants are less of a concern than for a commercial building requiring access by the general public. Since ease of public access is not a major determining factor, in this case the owner is also more likely to emphasize physical security measures to intercept an attacker through access control. Rather than taking measures to mitigate damage

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Page 67 from a successful attack, this type of owner may be more interested in avoiding disruption of whatever processes are taking place at the facility. This type of owner may also not be as concerned with space utilization as a commercial owner would be, and may be able to make more use of design strategies such as buffer space against outside walls and increased stand-off distances. Since the degree of occupant satisfaction is a prerogative of the owner, fenestration and other fragile building components can be reduced or eliminated, which could, in fact, result in lower direct costs of construction, operation, and maintenance than would have been the case for a building of conventional design and construction. On the other hand, buildings constructed as financial investments are expected to provide a return on their costs by generating revenues as a direct result of their use by others. Commercial office buildings, retail and mixed-use centers, and recreational facilities are typical of this class of structures. Moreover, these investments are made in a very price-competitive environment where even small-cost excursions can and do affect investment decisions. Appendix A compares the financial performance of a speculative commercial office building and the financial performance of the same building after it has incurred the additional costs of protection against attack. The discussion and accompanying cost models conclude that the construction premium for blast-hardening does not materially impact the financial performance of a commercial building. The committee believes that reasonable blast resistance can be accomplished for about a 5 percent premium in construction cost which equates to an increase of the lease premium of about 3.5 percent. However, there are still financial and other barriers to incorporating blast-hardening features in such buildings. Commercial developers and their investors are wary of anything that alters the traditional financial profile of a building or that injects an unknown element in its liquidity potential, refinancing value, or market position, or other factors that could in any way impact the property's performance as an investment. Since every element in the development of an investment-grade property must pass a rigorous cost/benefit analysis to ensure that the added element does not dilute the anticipated return on investment, the decision to add a nonrevenue element, such as blast-resistant construction, to a building would be much more likely if there were some cost-recovery mechanism available. Some of these mechanisms could be tax credits, reduced insurance costs, or more favorable lender terms. However, none of these mechanisms are available today, nor are they being contemplated. The most attractive and likely incentive for including blast protection in an investment property would be finding a long-term tenant, such as a government agency, that is sensitive to security issues and is willing to pay a higher rent for a blast-hardened building. Such a long-term lease would allow the developer to recover his costs. It also delays the time when it becomes necessary to find a replacement tenant with

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Page 68 the same security concerns, or to compete for another tenant against properties with lower cost bases and operating expenses. Finding a tenant who is willing to pay higher rent may mean advertising or promoting the property as blast-resistant, and most developers would be reluctant to do this because the advertising could be interpreted as a challenge and might actually invite an attack on what otherwise is an unremarkable target. Development and publication of security measures or blast-effects mitigating technologies may also carry the risk of liability potential, so the dissemination of technology may have to be done in a way that does not put the source at risk. Casualty insurers are most affected by damage to a property caused by an explosion, because of both repair costs and insured lost revenues. The potential for terrorist activity at a specific building can affect its insurability when an insurer believes that a tenant of the building is a high-profile target. A positive side of the economics of effective blast-hardening is the reduction in personal injuries or deaths with inevitable claims and the reduction in repair costs and lost revenues following an explosion. To the extent that the structure and its systems are able to resist blast-effects, occupants are better protected, repairs are more easily made, and the time the building is nonfunctional is reduced, along with the revenue losses to the occupant businesses. Additional benefits are possible to the extent that blast-hardening features improve a building's resistance to accidental explosions, which can result from the improper storage of chemicals, or fuel or gas leaks, for example. Representatives of the building industry, such as the Building Owners and Managers Association, generally corroborate the observation that there is a low level of continuing public concern about terrorist attacks on commercial buildings in the United States. In the case of the World Trade Center, public attention and concern were very high in the months that followed, only to subside as time passed. Time will tell if interest in the Oklahoma City bombing will be brief, or if a sustained level of concern will develop into widespread demand for blast-resistant measures in commercial buildings. Agents for Technology Transfer Successful transfer of relevant military technology to the civilian sector must overcome several significant barriers: inadequacy of professional education, inaccessibility of information, and lack of financial incentives. Agents for affecting this technology transfer must be selected with these barriers in mind. As already noted, specialized knowledge and resources are required to effectively realize the blast-effects mitigation potential for commercial buildings. Today, a select number of architect-engineer firms specializing in hardened military design and construction are also engaged in civilian building design and thus possess the requisite capabilities. Many of the design-oriented computer programs described in Chapter 3 were developed by research-oriented firms who frequently assist archi-

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Page 69 tect-engineer teams in the application of their design tools. However, if these technologies are to gain widespread acceptance, a major effort must be made to broaden the educational and experiential base within the profession. The agents for technology transfer are many and normally take months or years to be developed and disseminated. Past experience suggests that documents or reports that address certain aspects of the problem will become available; an example is the ASCE (1995) document currently being developed. Over time, if demand is sufficient, a series of such guideline documents could be expected. The sponsors for such documents could be technical societies, governmental laboratories, or contracted documents could be supported and funded by any number of sources. These guideline documents, even though not referenced in building codes, can normally be used in building design or retrofit. The guideline provisions normally far exceed code requirements, however, the professional would have to be sure that adopted procedures are not in violation of any building code procedures or other local, state, or federal ordinances. In time, such material will naturally find its way into texts and special courses in universities and colleges. This evolutionary process has occurred in many subdisciplines. In the civil engineering field, such design topics as wind, earthquakes, and other natural phenomena, offshore platforms, major pipelines, and criteria for nuclear power plants have evolved in this general manner. The schools of architecture, construction, and engineering in our country's universities have a unique ability to transfer technology through education, and they must be part of the process. So too must the professional societies, such as the American Institute of Architects, the American Society of Civil Engineers, the National Society of Professional Engineers, the American Society of Heating, Refrigerating, and Air Conditioning Engineers, the Association of General Contractors, the Association of Building Contractors, and the American Society for Industrial Security. Federal and state governments are also good candidates as agents of technology transfer, since they have a long-term concern to protect their own commercial office buildings. They are not subjected to all of the code issues that pertain to the private sector, and hence could incorporate blast-effects mitigation technology in their office buildings at a faster rate than the private sector. Also, the initial additional cost is not as great a concern when balanced against long-term objectives. Because the same designers typically work on both government and civilian office buildings, technology transfer would be much quicker. This same transfer technique is being used to introduce the metric (SI) system of units into civilian design and construction. Government agencies are requiring the SI units for their building projects and hence building designers who want to do the work are quickly making the necessary conversion.

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Page 70 References Air Force Engineering and Services Center. 1989. Protective Construction Design Manual. ESL-TR-87-57. Engineering and Services Laboratory, Tyndall Air Force Base, Florida. ASCE (American Society of Civil Engineers). 1993. Minimum Design Loads for Buildings and Other Structures. ASCE Standard 7-93. Washington, D.C.: American Society of Civil Engineers. ASCE (American Society of Civil Engineers). 1995. Structural Design for Physical Security—State of the Practice Report. Washington, D.C.: American Society of Civil Engineers, in press. Auf der Heide, E. 1989. Disaster Response: Principles of Preparation and Coordination. St. Louis: The C.V. Mosby Co. Berke, P.R., and T. Beatley. 1993. Planning for Earthquakes: Risk, Politics, and Policy. Baltimore, Maryland: John Hopkins University Press. Berke, P.R., T. Beatley, and S. Wilhite. 1989. Influence on local adoption of planning measures for earthquake hazard mitigation. International Journal of Mass Emergencies and Disasters 7:33–56. Drabek, T.E. 1986. Human Systems Responses to Disaster: An Inventory of Sociological Findings. New York: Springer-Verlag. FEMA (Federal Emergency Management Agency). 1994. NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings. FEMA 222A and 223A. Washington, D.C.: FEMA. May. P. 1985. Recovering from Catastrophes: Federal Disaster Relief Policy and Politics. Westport, Connecticut: Greenwood Press. U.S. Department of the Army. 1986. Seismic Design Guidelines for Essential Buildings. TM 5-809-10-1. Washington, D.C.: Headquarters, U.S. Department of the Army. U.S. Department of the Army. 1990. Structures to Resist the Effects of Accidental Explosions. Army TM 5-1300. Navy NAVFAC P-397, AFR 88-22. Washington, D.C.: Departments of the Army, Navy, and Air Force. U.S. Department of the Army. 1991. Seismic Design for Buildings. TM 5-809-10. Washington, D.C.: Headquarters, U.S. Department of the Army. U.S. Department of the Army. 1993. Security Engineering. TM-5-853. Washington. D.C.: Headquarters, U.S. Department of the Army. Vigo, G., C. Prater. P. Pramanik. J.E. Beeson, and M. Hall. 1993. Search and Rescue and Emergency Medical Services: A Multidisciplinary Annotated Bibliography, 2nd ed. College Station, Texas: Hazard Reduction and Recovery Center. Texas A&M University.