There are five general categories of munitions damage mechanisms: blast, fragmentation, cratering, shaped charge penetration, and incendiary effects. A given target is usually most vulnerable to one particular damage mechanism, though it may be vulnerable (to a lesser extent) to several damage mechanisms. The factors governing determination of the primary damage mechanism for a given target are: target construction, target location (relative to the point of warhead detonation), warhead damage effects pattern, and the desired type and level of damage.
Fragmentation is caused by the break-up of the weapon casing upon detonation. Fragments of a bomb case can achieve velocities from 3,000 to 11,000 fps depending on the type of bomb (for example GP bomb fragments have velocities of 5,000 to 9,000 fps). Fragmentation is effective against troops, vehicles, aircraft and other soft targets. The fragmentation effects generated from the detonation of a high-explosive bomb have greater effective range than blast, usually up to approximately 3,000 feet regardless of bomb size. The fragmentation effect can be maximized by using a bomb specifically designed for this effect, or by using a GP bomb with an airburst functioning fuze.
The cratering effect is normally achieved by using a GP bomb with a delayed fuzing system. This system allows bomb penetration before the explosion. Since the explosion occurs within the surface media, the energy of the blast causes the formation of a crater. This effect is most desirable in interdiction of lines of communication (LOC) and area denial operations
Armor penetration is accomplished by shaped charges or kinetic energy penetrators. This is an effective damage mechanism for tanks, assault guns, armored personnel carriers, and other armored targets. A major problem associated with both shaped charges and kinetic energy penetrators is the lack of visible damage. This may result in repeated attacks to produce battlefield evidence that a target is no longer a threat.
Fire is effective in interrupting operations of enemy personnel and in damaging supplies stored in the open. Incendiaries produce intense, localized heat designed to ignite adjacent combustible target materials. The true incendiary produces no fireball and relatively little flame. The basic damage mechanism of firebomb weapons comes from the fireball and burning residual fuel globules, impact momentum of the fuel and container, and damage from fires started by the weapon. The sharp cutoff of casualty-producing mechanisms outside the incendiary pattern allows delivery close to friendly troops, usually parallel to the forward line of battle, with minimum risk. Munitions have been developed with full fragmentation and penetrating capabilities coupled with reactive incendiary devices. These improved incendiaries are highly effective against fuel and other flammable targets. A drawback, however, in planning for the employment of incendiary weapons is that incendiary/fire effects are not evaluated in current weaponeering methodologies.
Blast is caused by tremendous dynamic overpressures generated by the detonation of a high explosive. Complete (high order) detonation of high-explosives can generate pressures up to 700 tons per square inch and temperatures in the range of 3,000 to 4,500º prior to bomb case fragmentation. It is essential that the bomb casing remain intact long enough after the detonation sequence begins to contain the hot gases and achieve a high order explosion. A consideration when striking hardened targets is that deformation of the weapon casing or fuze may cause the warhead to dud or experience a low order detonation. Approximately half of the total energy generated will be used in swelling the bomb casing to 1.5 times its normal size prior to fragmenting and then imparting velocity to those fragments. The remainder of this energy is expended in compression of the air surrounding the bomb and is responsible for the blast effect. This effect is most desirable for attacking walls, collapsing roofs, and destroying or damaging machinery. For surface targets blast is maximized by using a general purpose (GP) bomb with an instantaneous fuzing system that will produce a surface burst with little or no confinement of the overpressures generated by excessive burial. For buildings or bunkers the use of a delayed fuzing system allows the blast to occur within the structure maximizing the damage caused by the explosion.
In World War I and World War II, it was observed that injuries to the thoracic and abdominal viscera and to the central nervous system resulted from rapid changes in the environmental pressure, for instance, from an air blast. When persons are exposed to the effects of nearby shell or mortar explosions, death may occur with minimal evidence of external injury. Hemorrhages and lacerations may occur in the lungs, abdominal viscera, or brain. The effect of blast on personnel is confined to a relatively short distance (110 feet for a 2000 pound bomb).
An explosion is an extremely rapid release of energy in the form of light, heat, sound, and a shock wave. A shock wave consists of at supersonic velocities. As the shock wave expands, pressure dereases rapidly (with the cube of the distance) and, a surface that is in line-of-sight of the explosion, amplified by a factor of up to thirteen. Pressures also decay rapidly over time (i.e., exponentially) and have a very brief span of existence, measured typically in thousandths of a second, or milliseconds. Diffraction effects, caused by corners of a building, may act to confine the air-blast, prolonging its duration. Late in the explosive event,the shock wave becomes negative,creating suction. Behind the shock wave, where a vacuum has been created, air rushes in,creating a powerful wind or drag pressure on all surfaces of the building. This wind picks up and carries flying debris in the vicinity of the detonation. In an external explosion, a portion of the energy is also imparted to the ground, creating a crater and generating a ground shock wave analogous to a high-intensity, short-duration earthquake.
Conventional structures are designed to withstand roof snow loads of 30 pounds per square foot (1.44 kilopascals) and wind loads of 100 miles per hour (161 kilometers per hour). The loads equate to 0.2 pounds per square inch (psi). Comparing these loads with the design capacity, it is evident that conventional structures, which include aboveground storage facilities, contribute little to propagation protection from either blast or fragments. Propagation protection is provided by distance and/or barricading.
In the context of other hazards (e.g., earthquakes, winds, or floods), an explosive attack has the following distinguishing features.
The intensity of the pressures acting on a targeted building can be several orders of magnitude greater than these other hazards.It is not uncommon for the peak pressure to be in excess of 100 pounds per square inch (psi)on a building in an urban setting for a vehicle weapon parked along the curb. At these pressure levels, major damages and failure are expected.
Explosive pressures decay extremely rapidly with distance from the source. Therefore, the damages on the side of the building facing the explosion may be significantly more severe than on the opposite side. As a consequence, direct air-blast damages tend to cause more localized damage.
The duration of the event is very short, measured in thousandths of a second, or milliseconds. This differs from earthquakes and wind gusts, which are measured in seconds, or sustained wind or flood situations, which may be measured in hours. Because of this, the mass of the structure has a strong mitigating effect on the response because it takes time to mobilize the mass of the structure. By the time the mass is mobilized, the loading is gone, thus mitigating the response. This is the opposite of earthquakes, whose imparted forces are roughly in the same timeframe as the response of the building mass, causing a resonance effect that can worsen the damage.
The extent and severity of damage and injuries in an explosive event cannot be predicted with perfect certainty.Past events show that the unique specifics of the failure sequence for a building significantly affect the level of damage. Despite these uncertainties, it is possible to give some general indications of the overall level of damage and injuries to be expected in an explosive event, based on the size of the explosion, distance from the event, and assumptions about the construction of the building.
Damage due to the air-blast shock wave may be divided into direct air-blast effects and progressive collapse. Direct air-blast effects are damage caused by the high-intensity pressures of the air-blast close in to the explosion and may induce the localized failure of exterior walls, windows, floor systems, columns, and girders.
The air-blast shock wave is the primary damage mechanism in an explosion. The pressures it exerts on building surfaces may be several orders of magnitude greater than the loads for which the building is designed. The shock wave also acts in directions that the building may not have been designed for, such as upward on the floor system. In terms of sequence of response,the air-blast first impinges on the weakest point in the vicinity of the device closest to the explosion, typically the exterior envelope of the building. The explosion pushes on the exterior walls at the lower stories and may cause wall failure and window breakage. As the shock wave continues to expand, it enters the structure, pushing both upward and downward on the floors.
Floor failure is common in large-scale explosive attacks, because floor slabs typically have a large surface area for the pressure to act on and a comparably small thickness. In terms of the timing of events,the building is engulfed by the shockwave and direct air-blast damage occurs within tens to hundreds of milliseconds from the time of detonation.If progressive collapse is initiated, it typically occurs within seconds.
Glass is often the weakest part of a building,breaking at low pressures compared to other components such as the floors, walls, or columns.Past incidents have shown that glass breakage may extend for miles in large external explosions. High-velocity glass fragments have been shown to be a major contributor to injuries in such incidents. For incidents within downtown city areas,falling glass poses a major hazard to passersby on the sidewalks below and prolongs post-incident rescue and cleanup efforts by leaving tons of glass debris on the street.
Blast can cause significant casualties. During the bombing of the Murrah Federal Building, 168 people were killed. Severity and type of injury patterns incurred in explosive events may be related to the level of structural damage. The high pressure of the air-blast that enters through broken windows can cause eardrum damage and lung collapse.As the air-blast damages the building components in its path, missiles are generated that cause impact injuries. Airborne glass fragments typically cause penetration or laceration-type injuries.Larger fragments may cause non-penetrating, or blunt trauma,injuries. Finally, the air-blast pressures can cause occupants to be bodily thrown against objects or to fall. Lacerations due to high-velocity flying glass fragments have been responsible for a significant portion of the injuries received in explosion incidents. In the bombing of the Murrah Federal Building in Oklahoma City, for instance, 40 percent of the survivors in the building cited glass as contributing to their injuries. Within nearby buildings, laceration estimates ranged from 25 percent to 30 percent.
The amount of explosive and the resulting blast dictate the level of protection required to prevent a building from collapsing or minimize injuries and deaths. DoD correlates levels of protection with potential damage and expected injuries. The GSA and the Interagency Security Committee (ISC) also use the level of protection concept. However, wherein DoD has five levels, they have established four levels of protection.
- Severely damaged. Frame collapse/massive destruction. Little left standing. The total destruction of most buildings would result from 10-12 PSI incident pressure, while a blast of 6-9 PSI incident pressure would produce severe damage to reinforced concrete structures and 4-7 PSI incident pressure would produce serious damage to steel framed buildings.
- Heavily damaged. Major portions of the structure will collapse (over 50%). A significant percentage of secondary structural members will collapse (over 50%). Major deformation of primary and secondary structural members, but progressive collapse is unlikely. Collapse of non-structural elements.
- Damaged - unrepairable. 2.3 PSI incident pressure. Some sections of the structure may collapse or lose structural capacity (10 to 20% of structure). Major deformation of non-structural elements and secondary structural members and minor deformation of primary structural members, but progressive collapse is unlikely.
- Damaged - repairable. 1.8 PSI incident pressure. Minor to major deformations of both structural members and non-structural elements. Some secondary debris will be likely, but the structure remains intact with collapse unlikely. Minor deformations of non-structural elements and secondary structural members and no permanent deformation in primary structural members.
- Superficially damaged. 1.1 PSI incident pressure. No permanent deformation of primary and secondary structural members or non-structural elements.
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