4.2 Damage Mechanisms

Damage due to the air-blast shock wave may be divided into direct airblast effects and progressive collapse.

Direct air-blast effects are damage caused by the high-intensity pressures of the air blast close to the explosion. These may induce localized failure of exterior walls, windows, roof systems, floor systems, and columns.

Progressive collapse refers to the spread of an initial local failure from element to element, eventually resulting in a disproportionate extent of collapse relative to the zone of initial damage. Localized damage due to direct air-blast effects may or may not progress, depending on the design and construction of the building. To produce a progressive collapse, the weapon must be in close proximity to a critical load-bearing element. Progressive collapse can propagate vertically upward or downward (e.g., Ronan Point) from the source of the explosion, and it can propagate laterally from bay to bay as well.

The pressures that an explosion 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 pressure on the floor system. In terms of sequence of response, the air blast first impinges the exterior envelope of the building. The pressure wave pushes on the exterior walls and may cause wall failure and window breakage. As the shock wave continues to expand, it enters the structure, pushing both upward on the ceilings and downward on the floors (see Figure 4-1).

Floor failure is common in large-scale vehicle-delivered explosive attacks, because floor slabs typically have a large surface area for the pressure to act on and a comparably small thickness. Floor failure is particularly common for close-in and internal explosions. The loss of a floor system increases the unbraced height of the supporting columns, which may lead to structural instability.

For hand-carried weapons that are brought into the building and placed on the floor away from a primary vertical load-bearing element, the response will be more localized with damage and injuries extending a bay or two in each direction (see Figure 4-2). Although the weapon is smaller, the air-blast effects are amplified due to multiple reflections from interior surfaces. Typical damage types that may be expected include:

• localized failure of the floor system immediately below the weapon;
• damage and possible localized failure for the floor system above the weapon;
• damage and possible localized failure of nearby concrete and masonry walls;
• failure of nonstructural elements such as partition walls, false ceilings, ductwork, window treatments; and
• flying debris generated by furniture, computer equipment, and other contents.

More extensive damage, possibly leading to progressive collapse, may occur if the weapon is strategically placed directly against a primary load-bearing element such as a column.

In comparison to other hazards such as earthquake or wind, an explosive attack has several distinguishing features, listed below.

• The intensity of the localized pressures acting on building components can be several orders of magnitude greater than these other hazards. It is not uncommon for the peak pressure on the building from a vehicle weapon parked along the curb to be in excess of 100 psi. Major damage and failure of building components is expected even for relatively small weapons, in close proximity to the building.
• Explosive pressures decay extremely rapidly with distance from the source. Pressures acting on the building, particularly on the side facing the explosion, may vary significantly, causing a wide range of damage types. As a result, air blast tends to cause more localized damage than other hazards that have a more global effect.
• The duration of the event is very short, measured in thousandths of a second, (milliseconds). In terms of timing, the building is engulfed by the shockwave and direct air-blast damage occurs within tens to hundreds of milliseconds from the time of detonation due to the supersonic velocity of the shock wave and the nearly instantaneous response of the structural elements. By comparison, earthquake events last for seconds and wind loads may act on the building for minutes or longer.