An explosion is a rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave travels outward, in all directions, from the source of the explosion and is the primary source of building damage. The duration of the shockwave is fast, measured in milli-seconds rather than seconds (think of a blink of the eye) and the forces imposed on anything it its path (be it a building or a person) are enormous – many times greater than hurricanes.
Many factors contribute to how a building will respond to an explosive event, the most critical are.
• The size of the bomb
• The distance from the bomb to the building
• The type of building construction.
The size of the bomb (also termed Net Explosive Weight) and the distance from the bomb to the building (standoff distance) determine the magnitude of the pressure (force over area) and the duration that the pressure acts on the building element. Pressures decrease as the standoff distance increases and durations tend to increase with greater standoff. Shorter durations and smaller pressures cause less damage than higher pressures and longer durations.
The type of construction is also a significant factor in how much damage a building will experience from an explosion. It is important to remember that the vast majority of buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to a bomb does not mean that the building was poorly designed. Blast loading is an extremely abnormal event and stresses buildings in ways never anticipated during the original design and construction. Buildings are generally designed to hold up gravity (downward) loads and lateral wind loads. In earthquake regions, they are also designed to withstand forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitudes associated with explosions. Very lightweight buildings and buildings built of unreinforced masonry (brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings built in high seismic zones tend to respond the best.