America has become convinced that security is no longer someone else's issue. Government agencies, businesses, and institutions are increasingly concerned about protecting their facilities and the people who occupy them. In addition to the threat from hostile forces, blasts can be caused by natural gas and chemical leaks, tanker crashes, boiler explosions, and other types of industrial accidents. Strong demand for blast resistant structures obligates design and construction professionals to acquire the resources to answer this “protective” need.
It is necessary to understand the fundamentals of explosive threats, and the strategic concepts that are used to protect against them, to construct or retrofit for blast security.
Experts separate blast attacks into two basic categories. Standoff explosions are blasts that take place at some significant distance from the target. Contact detonations involve an explosive device placed in physical contact, or very close proximity, to the structure. Each type has a different mechanism of damage and requires a different form of protection.
All common forms of explosives work by the same basic mechanism. On detonation, the material undergoes an extremely rapid and powerful chemical transformation into a gas that occupies thousands of times the volume of the liquid or solid that generated it. This volume-change occurs in milliseconds and has tremendous force. Air surrounding the explosion is violently compressed, forming a pressure wave that moves outwards in excess of 700 mph.
“The shock front moves very fast,” explained Peter DiMaggio, a principal of Weidlinger Associates, New York, a structural engineering firm whose specialties include blast resistance design. “It hits a structure in an impulsive manner, like a hammer blow.”
The resulting “blast load” is directly related to the distance between the detonation and the place at which the load is induced. As the pressure wave moves outwards, the gas must occupy a larger volume, so the pressure diminishes. The pressure decreases by a factor related to the cube of distance. For instance, a 25-lb charge exploding 30-ft away produces a pressure wave of 824 lbs/ sq ft (psf). Increase the distance to 50 ft and the pressure decreases to 365 psf.
The blast shock wave creates two kinds of hazards. The pressure itself can destroy objects in its path. People can be killed directly by the pressure: 30 - 40 psi can cause lung collapse in an adult, and 100 - 120 psi is almost certainly fatal. Death can occur at much lower pressures with vulnerable individuals, such as children and the elderly.
Objects broken by the shock wave are set in motion at high velocity. The greatest hazard in many blasts is flying fragments – shrapnel –that effectively become a part of the weapon. Anything encasing the explosive is either disintegrated or broken into small projectiles, referred to as “primary fragments.” These fragments can be lethal to people directly in their path, but are usually too small to pose a hazard to buildings.
Other objects very close to the detonation may be disintegrated or broken into shrapnel, called “secondary fragments.” When a building wall is hit by the shock wave, and even if the wall is not completely destroyed, the opposite side often spalls and the fast-flying pieces have deadly consequences. This action can include the very materials, such as concrete, that are used to resist blast loads. Fragmentation is often considered more deadly than the shock wave, and will be discussed after the effects of blast pressure have been explained.
Taking the pressure
The most effective protection against the pressure wave is standoff distance –space between the detonation and the target. For this reason, the first principle of blast resistance is to limit access to the target. Even a few feet of standoff are valuable in some situations.
Large explosions always require significant quantities of explosives. Although some explosive materials are more powerful than others, the differences in power are measured in fractions.
Blast force is generally measured with reference to the explosive energy of TNT. A fertilizer/fuel oil bomb has approximately 0.8 times the energy of the same weight of TNT. C4 plastic explosive has 1.3 times the energy of TNT.
It's safe to assume that the quantity of explosive a human being could carry on his person in a backpack or suitcase – perhaps 50 lbs – is not enough to blow off the entire side of a large building at a distance. The amount of explosive for such a broad attack requires a large conveyance, such as a car or truck.
Therefore, the first element of defensive strategy is to create a secure perimeter that cannot be penetrated by any large conveyance. The amount of guaranteed standoff that can be achieved depends on many factors, including the location, size and configuration of the site, and available budget. Careful placement of access roads, driveways and gates, building set-backs, walls, and bollards all contribute to this effort.