Vulnerability of Buildings to Blast Damage and Blast-Induced Fire Damage

Vulnerability of Buildings to Blast Damage and Blast-Induced Fire Damage

The popular conception of an explosion, as depicted in TV action movies, generally is more dramatic than accurate. The big fireball, resulting fire, and apparent chaos at the blast scene do not accurately characterize the high-explosive detonations of materials such as TNT or C-4.

First, building fires usually are not a dominant effect of high-explosive detonations. If the detonation occurs outdoors, in a well-ventilated area, the hot gases from the initial detonation rarely ignite anything, although they can cause charring of nearby materials. If an explosive is detonated outside but in a car (i.e., a car bomb), the result will depend on the size of the explosive. The expanding hot gases from smaller (lower charge weight) explosive detonations will ignite flammable materials in the car, such as upholstery, fuel, and tires. The smaller explosive charges will do less mechanical damage to the car and often will result in a concentrated fire within the confines of the vehicle. As the charge becomes larger, more mechanical damage will be done to the car, and fire effects will be less concentrated. For example, expanding gas pressure will propel the engine and transaxle apart from the rear axle and wheels, and relatively small fires will appear as a number of the car’s components come to rest at (perhaps) widely separated locations.

Unless the detonation is specifically designed to be incendiary, only locally available combustible materials will be ignited. The particular difficulty with explosion-induced fires is that ignition can occur at almost the same time at widely separated locations because the hot gas cloud expands and propels ignited or ignitablc objects away from the center of the blast.

Building fires generally are not an issue when explosive detonations occur outside of buildings. And except in cases where detonations are incendiary in nature, building fires generally are not a major issue when detonations occur inside of buildings. Interior explosion-induced fires generally are caused when something within the building is ignited by the explosion. The problem is like the ignition of gas lines in an earthquake —the earthquake’s mechanical effects themselves do not directly cause fires.

When the detonation occurs inside a building and in a confined and fuelrich space, such as in a parking garage, the hot gases from the detonation cannot expand freely to mix with an ever-increasing volume of cooler air. The shock effects and mechanical damage from the explosion will overturn vehicles, fracture gas tanks, break pipes, and breach walls, exposing a variety of materials to the hot gas cloud. Many of these w ill ignite, generally on the periphery of the affected space, where the expanding gas causes the least local oxygen deprivation.1 Thus, suddenly the detonation w ill spawn many separate fires. From that point on, the building will respond as it would had each of the fires been set with a match. How ever, if the building were severely damaged by the blast, life safety systems may be incapacitated, gas lines severed, and electrical systems disturbed, all of which will increase the fire vulnerability of the building.

All things considered, while severe explosion-induced fires are unlikely from exterior detonations, interior detonations, particularly those in confined, fuel-rich spaces, frequently result in fires that are exacerbated by damaged building systems. Whether inside or outside, blast effects such as shock waves, expanding gas, and heat frequently dissipate within a second.2 Discreet fires ignited in this interval generally will require a much longer period (typically several minutes) to become serious building fires.

Unfortunately, there are other adverse factors to consider. Ignition at many locations simultaneously and damaged life safety systems already have been mentioned, But the blast also may blow out windows, providing undesirable venting to the fire. If the detonation is a “dirty” one, using a low explosive or smoke-producing components, toxic smoke can be rapidly propelled throughout the structure, creating an instant, additional hazard to building occupants at locations where they are safe from the direct effects of the blast.

The explosion collapsed a portion of ceiling on the B-S level onto a PATH train concourse.

(Photos courtesy of Port Authority Risk Management.)

PRECAUTIONS AND PREPLANNING

All of these issues make it desirable to assess the expected performance, in response to both blasts and blastinduced fires, of major habitable structures before such incidents actually occur. The tools to sort out the apparent chaos of a blast scene and determine specific damage to buildings arising from credible—but hypothetical-attacks exist. BombCAD for example, is a computer software that permits the user to model a building or complex of buildings and then expose the computer model to a specific interior or exterior detonation. BombCAD makes detailed predictions of blast damage and human injury.

These estimates can be graphically presented on a window-by-window, wall-by-wall, space-by-space basis. Special computation routines concentrate on specific damage aspects, such as glass damage from exterior bombs, the dominant source of damage, and injury from terrorist bombs. Analyses made with BombCAD will define the precise circumstances (explosive type, weight, and location) that result in no damage, specific glass damage, specific surface damage (wall, floor, or roof), or potential structural collapse from a particular bomb detonation.

Generally, building structure types can be classified in terms of vulnerability to a bomb blast. These types, from most vulnerable to least vulnerable, are the following:

  • wood-frame,
  • ordinary construction,
  • reinforced concrete, and
  • steel frame.

BombCAD’s gas expansion routines also predict smoke transport potential for blast products as well as initial conditions for stack-effect calculations and other conventional fire and smoke transport modeling tools. Ultimately, BombCAD will incorporate fire-related analysis routines. The resulting resource, FireCAD. will permit both explosive and fire effects to be modeled separately and collectively (when they arise from the same source).

Today, the hopeless chaos routinely depicted on TV at blast sites is only a characteristic of the rubble. The ability to analyze and minimize the consequences of such events is neither hopeless nor chaotic.

Endnotes

1. Local oxygen deprivation caused by high-explosive detonation actually is a method commonly used to extinguish oil well fires and other fires that cannot be effectively extinguished with water and chemical suppressants.

2. Shock waves travel typically at several times the speed of sound (about 1,100 feet/second). Thus, an unconfined shock wave is about 1,000 feet from the detonation point in a second. The expanding gas moves more slowly, at about the speed of sound, but both its temperature and pressure drop with the volume of ambient air into which it expands. A 10:1 volume increase decreases the maximum gas pressure by about 10:1 as well.

Midair plane collision Arizona

Two Dead After Midair Plane Collision at AZ Airport

A midair collision involving two small planes in southern Arizona killed two people Wednesday morning, authorities said.
Rick Lasky, Scott Thompson, Curtis Birt, and John Salka

Humpday Hangout: The Right Firehouse Attitude

In this Humpday Hangout, Rick Lasky and the rest of the crew talk putting the firefighter back in the firefighter and the firehouse back in…