An explosion is a rapid release of stored energy characterized by a bright flash and an audible blast. Part of the energy is released as thermal radiation (flash); and part is coupled into the air as airblast and into the soil (ground) as a ground shock; both as radially expanding shock waves.

To be explosive:

1. The material must contain a substance or mixture of substances that remains unchanged under ordinary conditions but undergoes a fast chemical change upon stimulation.
2. This reaction must yield gases whose volume–under normal pressure, but at the high temperature resulting from an explosion–is much greater than that of the original substance.
3. The change must be exothermic in order to heat the products of the reaction and thus increase their pressure.

Common types of explosions include construction blasting to break up rock or to demolish buildings and their foundations, and accidental explosions resulting from natural gas leaks or other chemical/explosive materials.

Fire Protection Handbook. National Fire Protection Association, 20 Edition

Glasstone, S. and Dolan P. J. (Editors), The Effects of Nuclear Weapons. U.S. Department of Defense and the U.S. Department of Energy, Third Edition, 1977 Reprinted by the Federal Emergency Management Agency

Standard on Explosion Protection by Deflagration Venting. NFPA 68

The rapid expansion of hot gases resulting from the detonation of an explosive charge gives rise to a compression wave called a shock wave, which propagates through the air. The front of the shock wave can be considered infinitely steep, for all practical purposes. That is, the time required for compression of the undisturbed air just ahead of the wave to full pressure just behind the wave is essentially zero.

If the explosive source is spherical, the resulting shock wave will be spherical. Since its surface is continually increasing, the energy per unit area continually decreases. Consequently, as the shock wave travels outward from the charge, the pressure in the front of the wave, called the peak pressure, steadily decreases. At great distances from the charge, the peak pressure is infinitesimal, and the wave can be treated as a sound wave.

Behind the shock wave front, the pressure in the wave decreases from its initial peak value. At some distance from the charge, the pressure behind the shock front falls to a value below that of the atmosphere and then rises again to a steady value equal to that of the atmosphere. The part of the shock wave in which the pressure is greater than that of the atmosphere is called the positive phase, and, immediately following it, the part in which the pressure is less than that of the atmosphere is called the negative or suction phase.

A deflagration is an exothermic reaction (a moving flame front), which propagates from the burning gases to the unreacted material by conduction, convection and radiation. In this process the combustion zone progresses through the material (flammable mixture) at a rate that is less than the speed of sound in the unreacted material. In contrast, a detonation is an exothermic reaction characterized by the presence of a shock wave in the material that establishes and maintains the reaction. A distinctive characteristic of detonation is that the reaction zone propagates at a speed greater than the speed of sound.

Under proper conditions, flammable and combustible gases, mists or dusts suspended in air or another oxidant can burn when ignited. This could cause a deflagration-induced explosion to occur when the following conditions are met:

1. The presence of fuel mixed in proper proportions with the atmosphere (oxidant). Most gaseous fuels have lower- and upper-flammability limits for their concentrations in the air, and the concentration must be within these limits for a deflagration to occur.
2. The presence of air (oxygen) or other oxidant. Higher oxygen concentrations accelerate the rate of combustion, and low concentrations of oxygen reduce it.
3. The presence of an ignition source with energy output sufficient to initiate deflagration. Ignition can result from a hot surface, flame or spark. Location of the ignition source at the geometric center of a confined fuel-oxidant mixture results in development of the highest pressure and rate of pressure rise.
4. The combustion of a gas must generate a pressure greater than the structural capability (strength) of the confining structure. An explosion occurs when the enclosing structure ruptures.

Conventional structures, in particular those above grade, are susceptible to damage from explosions, because the magnitudes of design loads are significantly lower than those produced by most explosions. For example, design snow loads in the Midwest range from about 20 psf to about 50 psf. The peak pressure in the blast pulse produced by 10 lb of TNT at a range of about 50’ is approximately 2.4 psi (which is 348 psf!) with a duration of the positive phase of 7.7 ms. Conventional structures are not normally designed to resist blast loads.

Terrorist attacks demonstrate the types of damage that can be produced. The 1993 terrorist attack on the World Trade Center in New York City removed several thousand square feet of concrete floor slabs in the general area of the explosion and severely damaged several buildings’ communication, transportation and utility systems. Due to the inherent redundancy of the steel frames, the structures did not collapse.

The 1995 attack on the Alfred P. Murrah Federal Building in Oklahoma City revealed the vulnerability of conventional structural designs when subjected to blast loads. When a weapon is located at street level, the blast shock wave acts up against the underside of the floor slabs at upper stories. Floor slabs are not designed for this magnitude and direction of load—for this direction of load, the reinforcement is in the wrong place.

See Reference 1 for more information

[1] Longinow, A., “The Threat of Terrorism – Can Buildings be Protected?” Building Operating Management, July 1995