Explosive Events

We were sitting in the station house when the place was rocked by what sounded like a thunderous sonic boom. No way, I thought. That was too powerful.

I ran outside and saw a big cloud of smoke coming from downtown. "Something bad's happened," I announced, not even bothering to be officially dispatched. "Let's go!"

Two minutes later, we were dodging chunks of debris, wire and broken glass in the street. Parked cars were burning and exploding all around us, their tires bursting like bombs. This is just too weird, I thought. It was like we had entered some futuristic, post-nuclear holocaust scene.

This account is from Jana Knox, a paramedic in Oklahoma City, OK, talking about her response to the 1995 bombing of the Alfred P. Murrah Federal Building. Her account, detailed in the book The Heart Behind the Hero, by Curt and Karen Yoder, talks about the immediacy of the response to the Murrah Building attack. Two years earlier, the World Trade Center's north tower was rocked by an underground explosion that missed its mark in crippling the skyline sculpture. These bombings, while not as devastating as the attacks of September 11, 2001, were some of the first indications that Americans weren't safe anymore.

Explosives

Annually, bombings and explosives incidents account for the majority of terrorist activities both domestically and abroad. The Federal Bureau of Investigation's 1999 Report on Terrorism lists 321 separate bombing events in the United States as part of 457 total terrorism events from 1980¡V99. The availability of explosive components and bomb-making information that can be found in high schools, on the Internet and probably at your local library makes these weapons of mass destruction a favorite of persons or groups intent on causing fear and intimidation and injuring both civilians and rescuers.

While many reading this will think I've never had to respond to a bombing and probably won't, it is important to remember that bombings are increasingly being used not only for terrorism purposes, but also for criminal activity. Every community in America has faced bomb threats at some point. While they may only occur here and there, this low-frequency event carries with it a high risk, and risk managers will tell you that high-risk/low-frequency events cause the most trouble. To take any reported bomb or bombing event lightly is dangerous for you, your crew and the citizens you protect.

Explosions are caused by a chemical reaction that happens nearly instantly, changing solids, liquids or gases into superheated gases that expand into a greater volume. This greater volume of gas travels outward away from the explosion, following a path of least resistance.

Explosives are categorized as high or low explosives based on their speed of combustion. Whereas low explosives are designed to burn, or deflagrate, producing a gas output (such as gunpowder propelling a bullet out of a barrel), high explosives are designed for a much quicker energy release. Typically, low explosives will combust in milliseconds, whereas high explosives detonate thousands of times faster, in microseconds, producing the characteristic supersonic boom we associate with explosions. This movement of gas is known as the shock front or shock wave.

Blast waves can reach speeds ranging from 3,600 feet per second to more than 27,000 feet per second.1 For comparison purposes, a hollow-point bullet shot from a 110-grain Winchester .357 Magnum jacket will achieve a velocity of approximately 1,300 feet per second.2 This explosion speed, also known as detonation velocity, will help determine if the explosive has a shattering or pushing effect. The former is known as brisance, the latter as heave. An explosive with a higher detonation velocity will have a higher likelihood of causing a shattering blast-a blast that cuts through an object.