Taming the Fire Triangle

By LEE COATES

Fire is defined as a rapid, persistent chemical change that releases heat and light and is accompanied by flame, especially the exothermic oxidation of a combustible substance. Annually, it kills an estimated 166,000 people worldwide—nearly 19 fatalities per hour. Some estimates suggest nearly 300,000 deaths per year. The International Technical Committee for the Prevention and Extinction of Fire estimates that the annual cost of fire damage adds up to one percent of international gross domestic product, or $640 billion.

The fire regulations on which building safety depends are themselves based on understanding fire dynamics—the fundamental relationship between fuel, oxygen, and heat—the so-called fire triangle on which all fires, intentional or otherwise, depend.

Combine those three elements, and the fire triangle is joined by a fourth element—the chemical chain reaction that is actually the fire. In technical jargon, the triangle of combustion then becomes a tetrahedron.

It’s a geometry that can either be friend or foe, as fuel and oxygen molecules gain energy and become active. This molecular energy is then transferred to other fuel and oxygen molecules to create and sustain the chain reaction. In an uncontrolled building fire, its spread depends on a whole range of factors, from the fuel type (ceiling tiles, furniture, etc.) to building construction and ventilation.

Taming fire generally involves removing heat—in most cases, by using water to soak up heat generated by the fire. This turns the water into steam, thereby robbing the fire of the heat used. It’s what a building’s sprinkler system is there for.

Without energy in the form of heat, the fire cannot heat unburned fuel to its ignition temperature and the fire will eventually go out. In addition, water acts to smother the flames and suffocate the fire.

But sprinkler systems can only suppress. Containment is also needed to prevent the fire’s spreading from its original location. Those protective barriers, often external curtain walling or internal glass screens, must also provide escape routes for the building’s occupants. This is the reason fire-resistant glass and glazing systems are so important. Modern steel systems are so technically advanced that they have overcome the glass’ inherent limitations.

The biggest limitation is that glass softens over a range of 932˚F to 2,732˚F. To put that in perspective, a candle flame burns at between 872˚F and 2,192˚F. For a typical flashover fire inside a building, temperatures can reach between 1,832˚F and 2,552˚F. These temperatures can disrupt the integrity of conventional panes of glass, which can crack and break because of thermal shock and temperature differentials across the exposed face. This will compromise the compartmentation of the building’s interior, allowing fire to spread from room to room.

As a fire escalates, the amount of heat produced can grow quickly, spreading like a predator from one fuel source to another, devouring materials that, in turn, will produce gases that are both highly toxic and flammable.

To make things worse, because of thermal expansion, these flammable gases are usually under pressure and able to pass through relatively small holes and gaps in ducts and walls, spreading the fire to other parts of the building. Heat will also be transmitted through internal walls by conduction. As the fire worsens and unburned flammable gases reach autoignition temperature, or are provided with an additional source of oxygen (i.e., from a fractured window), an explosive effect called a “flashover” results.

Flashover is the most feared phenomenon to any firefighter and signals several major changes to the fire and the response to it. First, it brings to an end all attempts at search and rescue in the flashover’s area. Simply, there won’t be anybody alive to rescue. Second, it signals that the fire has reached the end of its growth stage and that it is now fully developed as an inferno. This signals a change in firefighting response because it marks the risk of a worse danger: structural collapse.

However, most fires start with only a minimum of real danger—from a dropped cigarette, a spark from a faulty wire, and so on—and, if dealt with quickly or adequately contained, pose no real threat. That’s where our advanced systems come in.

Wrightstyle’s internal and external steel and glass systems have been tested together at furnace temperatures of more than 1,832˚F, testing the strength of the glass, the protective level of the glazing system, and its overall capability to maintain compartmentation in a fire.

We’ve also added U.S. testing to our armory because, in America, immediately after fire exposure, its testing regime also requires subjecting the glazing system to a high-pressure hose test aimed directly onto the superheated steel and glass assembly and generating a water stream around 30 pounds per square inch.

In the hose stream test, the longer the fire resistance is applied, then the longer and more severe the high-pressure water exposure; this tests the glass for the thermal shock of being deluged and suddenly cooled by the firefighting services, as well as the building’s own sprinkler system.

Thankfully, in the developed world, large and lethal fires are rare. However, that’s not to say that they don’t happen—merely that a building’s fire safety features have worked effectively to reduce the fire’s impact on its fabric and human occupants.

LEE COATES is the technical director for Wrightstyle Limited and leads research, development, and testing for its steel glazing systems.

 

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