[ Page 5 of 12 ]

Advertising supplement provided by Solutia Inc. and Viracon

Properties of glazing to resist bomb blast attack

The primary cause of glazing damage is the shock, or blast wave, that expands in all directions from the explosive charge. A secondary cause of damage is debris. The two main sources of debris are 1) the intentional missiles in the composition of the weapon such as nails, and 2) fragments from the surrounding environment such as rocks or pieces of the transport vehicle. To resist the damaging effects of a bomb attack, the glazing must be able to withstand both the primary blast event and the accompanying debris.

Eagleton Federal Building, St. Louis Photography: Tim Parker

Basic principles of a bomb blast

An enormous amount of energy is released when a bomb is detonated. After the detonation, ambient pressure increases almost instantaneously, and promptly begins to decay, forming a nearly triangular overpressure pulse. The highest load is called the peak positive overpressure, which represents the pressure seen at a point in space when the shock wave is unimpeded in its motion. The duration of the positive overpressure is called the positive phase duration.

The peak overpressure and positive phase duration determine the specific impulse of the blast wave, and both factors influence the property damage and injury that the blast wave can cause. These two parameters must be addressed, as some materials can resist rapid high level blast, but will fail as the duration is extended.

Some of the necessary information for designing glazing for bomb-blast mitigation are the peak overpressure (psi), duration of the load (msec) or the impulse (psi*msec) if the blast properties are known. If only the threat is established, a load can be calculated from the size of the anticipated bomb (TNT equivalent lbs), the distance away from the target (stand off distance) and the site altitude.