Blast Effects on Buildings and People: A Primer for Users of the ISC Security Design Criteria
In addition to natural and technological hazards, in recent years owners, tenants, and design professionals have had to consider design criteria to address an additional hazard to public buildings—the deliberately placed bomb. Whether a new federal facility is being built or an existing one renovated, physical protection from bombs has become integral to the planning process. The challenge for the building project team is to design and construct federal facilities that provide protection from terrorist explosive threats while at the same time offering desirable workspaces in attractive buildings that are well integrated with the surrounding neighborhood. When these structures are situated on urban sites, it is often difficult to restrict access to preserve effective standoff1 distances. Architectural aesthetics may also conflict with blast mitigation. Given the limited resources available for physical protection, these competing objectives are often addressed through design solutions that seek to protect occupants while accepting significant damage to the assets and structure. There is a need for a better way of allocating scarce resources to the highest-priority threats.
Comprehensive protection against the full range of possible threats is prohibitively costly—and probably not even possible. However, the committee believes that a level of protection that reduces the risk of mass casualties resulting from terrorist attacks can be provided for all occupants
at a reasonable cost. Full implementation of the ISC Security Design Criteria will provide some protection against most threats while significantly reducing the risk of injuries.
PURPOSE AND USE
The many possible users of the ISC Security Design Criteria will have differing degrees of familiarity with the topics addressed; in general, the audience may require an introduction to the subjects of both blast effects on people and buildings and the technical basis for blast-resistant design. Among those who should benefit from a general description of the basic concepts of blast-resistant design are architects and engineering professionals, facility owners, agency and private project managers, and tenants and other users of the facilities. Therefore, this basic introduction to protective design is provided to supplement the technical discussions of other parts of the ISC Security Design Criteria.
Understanding both features for physical protection of buildings and operational security features can inform decisions on physical protection. This explanation may help dispel some misconceptions about protective design and elicit more cooperative participation by all stakeholders in implementing effective design strategies from the outset. Stakeholders can readily see the benefits of such basic ways to protect buildings from bomb damage as establishing a secure perimeter, preventing progressive collapse, isolating internal threats from occupied spaces, and mitigating the glass and debris hazard.
Improving the performance of a building in response to an explosive event requires the services of a professional experienced in both conventional and protective design. First, qualified security professionals, in consultation with the building owner, manager, and occupants, should assess how vulnerable a facility is to various types of threats and assign levels of risk. A threat assessment and risk analysis (TARA) can help guide the owner, principal tenants, and security staff in balancing the operational, technical, and physical aspects of security services to maximize protection within the budget available.
BLAST EFFECTS—BASIC INFORMATION
The Nature of Explosions
Explosive materials are designed to release a large amount of energy in a very short time. Part of the energy is released as heat and part as shock waves that travel through the air and the ground. The shock wave (air blast) radiates at supersonic speed in all directions from the explosive source,
diminishing in intensity as the distance from the source increases. These waves reflect off the ground, adjacent structures, and other surfaces, rein-forcing the intensity of the blast’s effects. These reflections are most pronounced in dense urban environments where neighboring structures can create a canyon effect or where pockets of blast energy can become trapped in re-entrant (concave) corners.
Blast wave reflections also occur within structures, greatly increasing the potential for injury to the occupants and damage to building systems and contents. When a shock wave encounters a structure, it subjects all surfaces to the force of the blast. If the shock wave enters through windows or door openings, the interior elements—both structural (e.g., floor slabs and columns) and nonstructural (partitions, lighting fixtures, etc.)—the contents, and the occupants will also be subjected to the effects of the air-blast. Moreover, there are dynamic pressures that occur somewhat later caused by the mass movement of the air itself. This is essentially a high-velocity wind that can propel blast-generated debris with great force.
In very large explosions, the shock wave propagated through the earth may induce ground motion similar to that resulting from a short-duration earthquake. As shown in Figure 3.1, immediately after an explosion the pressure increases to a peak value almost instantaneously. As the pressure then decays from its peak value and interacts with the structure, it can cause enormous damage. Impulse (a blast parameter of momentum) is the time
during which the pressure wave decays back to its pre-explosion ambient value; it is represented by the area under the pressure-time curve. For high explosives, the shock wave traverses the structure very quickly and the positive-phase duration, t0, is measured in milliseconds. Nuclear weapons have a much longer positive-phase duration, which partially explains their capacity to cause far greater structural damage.
The structural and other damage caused by an explosion is the building’s response to the enormous amount of energy produced. This energy can be either resisted through the use of massive elements that are strong or ductile enough to survive without failure or accepted in the form of partial damage to windows, facade, and structural members. The concept of “graceful failure” assumes that various elements will resist long enough to absorb a large amount of energy and then fail in a way that minimizes the risk of serious injury or death to the occupants. Modern protective design, which attempts to balance security and aesthetics, generally incorporates aspects of both initial resistance and graceful failure.
Blast Effects on Buildings
Buildings experience the effects of explosions in several stages:
The initial blast wave typically shatters windows and causes other damage to the building facade. It also exerts pressure on the roof and walls that are not directly facing the blast, sometimes damaging them as well.
In the second stage, the blast wave enters the building and exerts pressure on the structure. When directed upward, this pressure may be extremely damaging to slabs and columns because it acts counter to the design used to resist gravity loads. Air-blast pressures within a building can actually increase as the pressure waves reflect from surfaces and can cause injuries to the occupants directly by means of physical translation, ear, lung, and other organ damage, or debris from building elements and contents.
Finally, the building frame is loaded globally and responds as it would to a short-duration, high-intensity earthquake.
The blast pressures experienced by a structure are in a most general sense related to the amount of explosive used and the distance of the building from the explosion. The peak incident pressure, charge weight, and distance are mathematically related through an expression that varies as a function of the weight of the explosive and the cube of the distance. This relationship is critical to understanding the effects of explosions on structures. In particular, the pressure experienced by a building increases
with bomb size, but decreases very quickly with increasing distance between the building and the bomb. These factors form the two keystones of defensive design and, as is shown in subsequent discussion, limiting the size of a bomb through vehicle control and inspection and enforcing standoff distances from possible targets are two of the most important tools available to those charged with protecting people and buildings from bomb damage. Figure 3.2 illustrates how standoff distance can prevent major structural damage for typical commercial construction, structures retrofitted to increase blast resistance, and specially designed blast-resistant structures.
Usually, the standoff distance available and the assumed size of the explosive device will determine the blast-resistant features that must be provided. Large explosive devices detonated at relatively great standoff distances will produce a large but uniform pressure over the surface of the building; at lesser standoff distances, even a small explosive device can produce locally intense effects, such as shattering load-bearing columns. While the former scenario is likely to govern design of the facade to limit the formation of hazardous debris, the assumption of a smaller, close-in device is likely to control design of the first-floor load-bearing elements to prevent localized failure leading to progressive structural collapse.2 If a large explosive device is detonated close to the structure, global damage
and the size of the resulting ground crater may be increased to the point that the structure, foundation, or both will be overwhelmed and a catastrophic collapse may ensue.
One of the important first steps in the design of any structure is defining the loads. In security design, this must include defining the blast loading—a function of the expected charge weight and proximity. Although a vehicle-bomb employing conventional explosives still appears to be the most serious bombing threat, experience over the past decade has shown that hundreds of smaller bombing incidents occur annually (ATF, 1996). Although these smaller bombs lack the power to cause catastrophic structural damage, they can injure and kill people and cause localized damage, particularly to windows and nonstructural elements. To provide a proper basis for risk assessment and cost-benefit evaluation, the information used to define blast loads or expected charge weights must be as unambiguous as possible. This can best be accomplished by carefully analyzing charge weights from prior incidents, defining their probability distribution properly, and selecting design blast loads at specified probabilities.
Blast Effects on People
People exposed to explosions can be killed or injured by the intense heat and pressure generated at the site of the detonation, where temperatures can range up to 4,000ºC and pressures to several hundred times atmospheric. Such extremely high pressures can damage major organs, blood vessels, eyes, and ears. Shock-tube and explosive tests have indicated that human blast tolerance varies with both the magnitude of the shock pressure and the shock duration; the pressure tolerance for short blast loads is significantly higher than for long blast loads (U.S. Department of the Army, 1990).
Once the blast wave enters occupied spaces or in an indoor explosion, the pressure wave is reflected off walls, floors, and ceiling, forming in effect a series of pressure pulses. The response of the ears and lungs to repetitive pulses is similar to the response to a single long pulse. Because the damaging effects of blast pressure indoors often exceed those of an unconfined explosion of similar size, it is crucial to minimize the opportunity for blast pressures from outdoor explosions to enter occupied spaces and to protect such spaces from even small explosive devices (Cooper, 1996). How the ear and lung respond depends on the impulse and the orientation of a person’s body to the blast wave—the shorter the pulse duration, the higher the pressure that can be tolerated. The onset of lung hemorrhage begins in the range of 30 to 40 psi, with severe damage occurring above 80 psi and death in the range of 100 to 120 psi. The onset of eardrum rupture is about
5 psi; 50 percent of eardrums exposed to a rapidly rising pressure of 15 psi will rupture (U.S. Department of the Army, 1990).
Medical reports of past bombings and recent suicide bombing attacks cite shock and organ trauma as leading causes of death (Brismar and Bergenwald, 1982; Rivkind, 2002; Weightman and Gladish, 2001). For people within structures subjected to blast effects, penetration by glass fragments and impact by other blast-induced debris have been consistent causes of death and serious injury (Mallonee et al., 1996). This underscores the need for window treatments that minimize the production of fragments and for catcher systems to retain debris. People are also subject to blunt trauma from furniture, accessories, and nonstructural building components like overhead lighting and ductwork that become detached from their moorings. Smoke and inhalation of dust also cause blast-induced injury. That is why it is important to make every effort to keep blast energy outside the building and to secure furniture and nonstructural components. Earthquake hazard mitigation offers practical guidance here (FEMA, 1994). Numerous injuries are often sustained during evacuation, particularly if there is a large amount of shattered glass littering evacuation routes or if there is considerable smoke and dust.
Analysis of the 1995 Oklahoma City bombing (Mallonee et al., 1996) demonstrated that building collapse was the primary cause of death. In the 1996 bombing of the Khobar Towers residential complex in Saudi Arabia, the structure did not collapse; although the number of fatalities was far less than in Oklahoma City, their causes were similar to those in Oklahoma City that did not result directly from the building collapse (Oklahoma State Department of Health, 2000). Studies of the attacks on the U.S. embassy in Nairobi also found that the harm can be substantial even if the structure does not collapse. The board that reviewed the embassy bombings in Nairobi and Dar es Salaam in 1998 found:
The damage to the embassy was massive, especially internally. Although there was little structural damage to the five-story reinforced concrete building, the explosion reduced much of the interior to rubble—destroying windows, window frames, internal office partitions, and other fixtures on the rear side of the building. The secondary fragmentation from flying glass, internal concrete block walls, furniture, and fixtures caused most of the embassy casualties. (U.S. Department of State, 1999)
The Terrorist Threat
Although there is no theoretical limit to the size of an explosive device or the locations where it might be placed, there are practical implications of size and weight for explosive devices that could constitute terrorist weapons. The density of common high-energy explosives, such as ammonium
TABLE 3.1 Explosive Capacity of Typical Bomb Delivery Methods
Explosive Capacity (pounds/kilograms)
SOURCE: TSWG (1999).
nitrate and fuel oil (ANFO), TNT, and C4, is on the order of 100 pounds per cubic foot. The explosive capacities of various delivery devices ranging from a small suitcase bomb to a large truck can therefore be calculated: 50 pounds may be packaged in a large briefcase that fits unobtrusively on a luggage carrier. Smaller amounts may be packaged in backpacks and shopping bags, all of which arouse little suspicion from experienced security guards. Explosive devices weighing anywhere from several ounces to several pounds may be delivered through the U.S. Postal Service, other delivery services, or private couriers. Each of the several different sizes of the blast events typically considered in a terrorist threat analysis corresponds to the type of container or vehicle in which the explosive is delivered (see Table 3.1).
PROTECTIVE DESIGN STRATEGIES
Physical protection for buildings involves four basic actions:
Establish a secure perimeter.
Prevent progressive collapse.
Separate internal threats from occupied spaces.
Mitigate debris resulting from damage to the facade and building interior.
Other considerations, such as anchoring nonstructural components inside the building, are also design objectives that require special attention, as is protection of emergency services. The size of the explosive threat will determine the effectiveness of each of these protective features and the resources necessary to protect building occupants. Defining the threat to be designed against is fundamental to the protective design process and therefore requires careful consideration.
Though definition of the design threat depends on history, available intelligence, and assumed risk, from a practical standpoint it is limited by the means of weapon delivery. Because conventional explosives weigh approximately 100 pounds per cubic foot, a hand-carried device, if efficiently packaged, could occupy as little as half a cubic foot of space in a large briefcase or small suitcase. Such a device may be introduced deep into the structure, where it can do considerable damage to structural members, such as load-bearing columns, or cause considerable injury to occupants. As a result, screening stations at public entrances, mailrooms, and loading docks are critical for preventing these threats from entering occupied space.
Vehicles can carry significantly larger explosive charge weights. As a result, perimeters must be secured and parking or loading docks underneath or within occupied buildings must be protected by comprehensive screening procedures, performed continuously. The potential for threats to bypass screening procedures will always exist. Therefore, the choice of design-level explosive threat will be guided by standoff distance, site conditions, blast-resistant features of the building, and ultimately the amount of risk the building owner or principal tenant is prepared to accept.
Although it is theoretically possible to predict the effects of a certain charge weight of a known explosive at a specified standoff distance, the actual charge weight of explosive used by a terrorist, the efficiency of the chemical reaction, and the location cannot be reliably predicted. Thus the approach embodied in the ISC Security Design Criteria is to use predefined levels of protection based on an estimation of risk to the facility, taking into account its symbolic importance, mission criticality, and the consequences of loss.
Although this approach is a start for risk analysis, once a facility is perceived to be at risk of attack, initial assumptions about charge weight can be based on the capacity of possible delivery vehicles. In light of the large capacity of most trucks and vans and the practical limitations of structurally hardening general-purpose buildings to resist the effects of very large bombs, the most effective way to protect a structure is to keep the bomb as far away as possible—maximize the standoff distance.
To guarantee the integrity of the perimeter defense, antiram bollards, large planters, or other devices can be placed around the site at the desired standoff distance. Site conditions will govern the maximum vehicle speeds attainable. Both the barrier and its foundation must be designed to resist the maximum energy that can be developed by the delivery vehicle. If conditions limit the capacity of the barrier or its foundation, other means must be used to reduce vehicle speed or size.
Parking abutting the building should be secured or eliminated; uncon-
trolled street parking should not be permitted near the building. Though converting one lane of traffic into an extended sidewalk or plaza can increase standoff distance, the practical benefit of increasing the standoff depends on the charge weight. If the charge weight is small, even small increases in standoff distance will significantly reduce blast forces, but if the threat is large, the blast forces may overwhelm the structure despite the addition of 9 or 10 feet of standoff distance, and this measure may not significantly improve the survivability of the occupants or the structure.
The building exterior is the first real defense against the effects of a bomb; how the facade responds will significantly affect the behavior of the structure and the safety of its occupants. Hardening the facade is typically the single most costly and controversial component of blast protection because it is likely to change the appearance of the structure dramatically. The number and sizes of windows usually will be reduced and their attachments to the structure will become more rugged. Considering the large surface areas enclosing some buildings, the cost for increasing glazing system protection even modestly will be high. As a result, the optimal design solution is often one that seeks to improve how the facade behaves after some specified degree of damage.
Window assemblies can be designed to respond to many anticipated blast loads. For new construction, it may be best to specify laminated glass; for existing glazing, a fragment-retention film might be applied. Although these approaches do little to improve the strength of the glass (failure will occur), they can hold the shards of glass together to better protect occupants from hazardous debris.
Window solutions must be approached cautiously, however. There is a danger that strengthening glass could cause the entire window light to be dislodged in a single piece, injuring or killing anyone struck by it. Catchbar and other systems can provide additional safety if they are aesthetically and functionally viable, but ultimately, the risk of injury posed by many small shards must be weighed against that posed by a single large sheet.
The effectiveness of fragment-retention films depends on how they are applied, the thickness of the film, and how it is anchored to the frame. Common film systems range from simple edge-to-edge (daylight) applications to wet-glazed adhesion to a mechanical attachment to the window frame. Mechanical attachments are most effective when they are anchored to the underlying structure. Regardless of the method used, however, fragment-retention films raise architectural and life-cycle cost issues. Laminated glass is another option. It exhibits excellent post-damage behavior, is
available for most applications, and provides a high degree of safety to occupants.
The design of the window frames is as important as the type of glazing material used. If the window system is to fail safely, the glass must be held in the frame long enough for the stresses to cause it to fracture. Otherwise, the intact glass panel could separate from the frame at high speed and cause serious injury. Therefore, the frame system should be designed to allow the full mechanical capacity of the chosen glazing material to develop. Because the nominal strength of glazing is specified in terms of a prescribed number of failures per thousand, to ensure that the frames can resist the full mechanical capacity of the glass, a higher strength for the glazing material must be considered. Factors of two or three times the nominal capacity of the glass to resist breakage may be the basis for frame design. The bite (the distance the glass extends into the frame) must be adequate to assure that the failed glass is retained within the frame—possibly using structural silicone sealant. Finally, depending on the facade, the mullions may be designed to span from floor to floor or to tie into wall panels. They must also be able to withstand forces that will cause the window to fail.
A curtain wall is a nonbearing exterior enclosure that can hold glass, metal, stone, precast concrete, or other panels and is supported by a building’s structural frame. Lightweight and composed of relatively slender members, curtain walls require careful design and considerable testing to ensure that the assembly has adequate wind and water resistance and meets the light and temperature differentials required of the exterior envelope. The ability of a curtain wall system to withstand the effects of explosive loading depends on how the various elements of the system perform. Although the glazing may be the most brittle component of a curtain wall system, the risk to the occupants depends on how the mechanical capacities of its elements interact. In addition to hardening3 the members that compose a curtain wall system, attachments to the floor slabs, or spandrel beams, require special attention. These connections must be adjustable to compensate for fabrication tolerances, accommodate differential interstory drifts and thermal deformations, and yet be capable of transferring gravity, wind, and blast loads.
An alternative approach is to allow the window systems to absorb a
considerable amount of blast energy through deformation while preventing debris from entering the occupied space. Explosive tests have determined that the inherent flexibility of curtain wall systems allowed their glazing to survive higher blast pressures than rigidly supported windows. The cable-protected window system takes this concept one step further; as the glass is damaged, it bears against a cable or muntin catch system, which in turn deforms the window frames. This system makes full use of the flexibility and capacity of all the window materials and dissipates large amounts of blast energy without hurling debris into the protected space. Extensive explosive testing has demonstrated the effectiveness of this approach, as have sophisticated computer simulations.
In addition to facing the hazard of glass and other facade debris being propelled into a building, its occupants may also be vulnerable to injury from much heavier debris resulting from structural damage. When an initiating localized failure causes adjoining members to be overloaded and fail, this progressive collapse causes damage that is disproportionate to the originating localized failure. A protective design will not use structural systems that either facilitate or are vulnerable to progressive collapse. In particular, new facilities may be designed to accept the loss of an exterior column for one or even two floors above grade without precipitating collapse.
These design requirements are intended to be threat-independent, providing redundant load paths should any damage occur due to abnormal loading. Threat-independence is intended to protect against an explosion of indeterminate size that might damage a single column. Although the alternate path approach is not associated with any specific threat that might cause damage, it is limited to abnormal loading conditions that would fail only one load-bearing member. Upgrading existing structures to prevent localized damage from causing a progressive collapse may not be easy using the alternate path method because loss of support at a column line would increase the spans of all beams directly above the zone of damage and require different patterns of reinforcement and different types of connection details than those typically used in conventional structural design.
Alternatively, columns may be sized, reinforced, or protected to prevent critical damage from a nearby bomb. Vulnerable concrete columns may be jacketed with steel plate or wrapped with composite materials. Steel columns may be encased in concrete to protect their cross sections and add mass. These approaches to preventing progressive collapse are generally more feasible in retrofits than attempting to supplement the capacity of connecting beams and girders; however, their effectiveness is predicated on
operational and technical security procedures that limit the magnitude and proximity of an explosive threat. These include effective perimeter protection, adequate screening of vehicles entering a parking facility or loading dock underneath or within the building, limiting parking adjacent to the building, and inspecting parcels carried into the building.
Transfer girders and the columns supporting them are particularly vulnerable to blast loading. Because transfer girders typically concentrate the load-bearing system into fewer structural elements, they do not provide the redundancy desired for blast resistance. Typically, the transfer girder spans a large opening, such as a loading dock, or makes it possible to shift the location of column lines at a particular floor. Damage to the girder may leave totally unsupported several lines of columns that terminate at the girder from above. The loss of a support column from below will also create a much larger span to carry critical load-bearing structure.
Transfer girders are critical sections; their loss may result in progressive collapse. If a transfer girder must be used, and if this girder may be vulnerable to an explosive loading, it is desirable that it be continuous over several supports. Furthermore, there should be substantial structure framing into the transfer girder to create a two-way redundancy and an alternate load path if there is a failure.
Other Building Spaces and Systems
The walls surrounding loading docks, mailrooms, and lobbies into which explosives may be introduced before inspection must be hardened to confine an explosive shock wave and must permit the resulting gas pressures to vent into the atmosphere. The isolation of occupied spaces from these vulnerable locations requires adequate reinforcement as well as connections that can resist the collected blast pressures. These structural features must be well integrated with the rest of the structural frame to ensure that any failure does not destabilize other portions of the gravity load-bearing system.
Because a significant area of the exterior walls enclosing mechanical spaces is louvered for ventilation, hardened plenums are required to interdict a direct line between the louvered opening and the machinery beyond. The response of electrical and mechanical equipment to blast pressures has been studied using lethality and survivability algorithms developed for the Department of Defense. In the design process, all components composing the critical electrical and mechanical systems must be analyzed in detail to determine whether infill blast pressures or in-structure shock motions would disrupt service.
Nonstructural building components, such as piping, ducts, lighting units, and conduits, must be sufficiently anchored to prevent failure of
services and so that they do not become falling debris. To mitigate the effects of shock due primarily to the entry of blast pressures through damaged windows, these nonstructural systems should be located below raised floors where possible, or tied to the ceiling slabs using restraints appropriate for Seismic Zone IV (FEMA, 1994).
APPROACHES TO BLAST-RESISTANT STRUCTURAL DESIGN
Dynamic Versus Equivalent Static Analysis Methods
The pressure waves that result from detonation of an explosive device have a very high intensity peak value that diminishes to zero in milliseconds. Both the intensity of the blast pressures and their duration greatly influence their effect on structures. The relation between the brevity of the loading and the natural period of response of individual structural elements, as well as the ability of these structural elements to deform inelastically without collapse, will determine resistance. Massive structural components provide inertial resistance, which tends to reduce the magnitude of resistance required. While the strength of these members is critical to their response, their ability to deform inelastically in a ductile fashion will limit the forces that must be resisted. These effective loads may be fractions of the peak blast pressure intensity; blast analysis for a particular structural component will determine the required resistance. The design and detailing of structural elements make it possible for the structure to deform in a ductile manner to prevent a catastrophic brittle failure and allow for timely evacuation of the facility. Although significantly reduced from the peak intensity of blast pressures, these loads may still be many times greater than the design loads required for gravity, winds, or even seismic disturbances.
Seismic Versus Blast-resistant Design
Although design for seismic resistance and design for blast resistance share some common principles, the two types of design must not be mistakenly viewed as redundant. Unlike a seismic disturbance, in which the induced forces are proportional to the distributed mass, blast loading does not tax an entire frame uniformly. The localized loads will deform exterior bays of the structure much more than interior bays; as the blast loading progresses, the shear forces in each story of the building may not necessarily be distributed through the diaphragms in proportion to the framing stiffness. Furthermore, the characteristic patterns of loading and deformation in a blast event depend to a great extent on the standoff distances, which may be significantly different from those resulting from a seismic excitation
of the structure. Finally, if the initiating damage is widespread, the redistribution of forces to the already weakened structure may amplify the extent of the collapsed region; the resulting structural failure will be a result of global damage, not progressive collapse.
It is generally understood that increasing the ductile behavior of details in response to strong ground motions will increase the ductile behavior in response to blast loading. Yet though seismic design codes contain very useful detailing information that may be directly applied in blast resistance design, it is generally acknowledged that the zones of plastic hinge formation and the extent of ductility demands for seismic response are not necessarily useful. Attempts to link seismic and blast design requirements by simply comparing the lateral or shear forces on a structure produced by these events (an equivalent seismic base shear) perpetuate the erroneous impression that seismic design is an umbrella for blast resistance.
Assuming the initiating failure is localized to a single column, it is desirable and entirely possible to design a new structure to limit the extent of collapse to the bays on either side of the failed column for a height of one floor. The alternate path method, which assumes an idealized zone of initiating failure, allows the engineer to quantify the amount of continuously tied reinforcement that should be detailed into concrete members or steel sections. However, the zone of initiating failure assumption is an academic idealization. If the zone of damage is more extensive, the calculation of force redistribution will be in error; the definition of disproportionate collapse must be related to the entire region damaged by the explosion. Except for the near-contact satchel-charge scenario, the actual extent of damage may be significantly greater than the removal of a single column, and so the definition of disproportionate collapse must be revised to reflect this reality.
Furthermore, progressive collapse must not be confused with global collapse of structural systems. Progressive collapse cannot be prevented if the blast is so intense as to damage significant portions of the structure simultaneously. This would be termed a general collapse, and the alternate path approach will provide only limited resistance to global damage mechanisms. Localized hardening of vulnerable structural elements and improving robustness through ductile detailing of structural systems will improve resistance to more extensive blast damage.
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