Jane Embury looks at fire containment
It’s 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.
Definitions aside, fire is also dangerous, although it can be tamed or contained.
The fire regulations on which building safety depend are themselves based on an understanding of fire dynamics – the fundamental relationship between fuel, oxygen and heat.
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 fire in a building, how it spreads depends on a whole range of factors. These include the type of fuel (everything from ceiling tiles to furniture) to building construction and ventilation.
Taming fire generally involves the removal of heat, in most cases using water to soak up heat generated by the fire. That’s what a building’s sprinkler system is there to do.
Without energy in the form of heat, the fire cannot heat unburned fuel to ignition temperature. The fire will eventually go out. In addition, water acts to smother the flames and suffocate the fire.
But what is really needed is containment – to prevent the fire 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.
That’s where fire resistant glass and glazing systems are so important. Modern steel systems are so technically advanced that they have overcome the limitations inherent in the glass itself.
The biggest limitation is that glass softens over a range of 500˚c to 1500˚c. To put that in perspective, a candle flame burns at between 800˚c and 1200˚c. In a typical flashover fire inside a building, temperatures can reach between 1000˚c and 1400˚c.
These temperatures can disrupt the integrity of conventional panes of glass. They can then 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.
That can, incidentally, be a problem that a sprinkler system actually causes. There have been several notable cases where cold water from a sprinkler system has come into contact with heated non-fire rated glass. This has caused the glass to break and allow more oxygen to the seat of the fire.
As a fire escalates, the amount of heat produced can grow quickly, spreading like a predator from one fuel source to another. It devours materials that, in turn, will produce gases that are both highly toxic and flammable.
To make things worse, due to thermal expansion, these flammable gases are usually under pressure. It means that they are able to pass through relatively small holes and gaps in ducts and walls. This will also spread the fire to other parts of the building. Heat will also be transmitted through internal walls by conduction.
As the fire worsens, and when unburned flammable gases reach auto ignition temperature, or are provided with an additional source of oxygen – for example, from a fractured window – an explosive effect called ‘flashover’ takes place.
Flashover is the most feared phenomenon for any fire fighter. It signals several major changes in the fire and the response to it. First, it brings to an end all attempts at search and rescue in the area of the flashover. 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. That then means a change in fire-fighting response because it marks the start of a worse danger – the risk of structural collapse.
Most fires with only a minimum of real danger – a dropped cigarette, a spark from a faulty wire. If dealt with quickly it poses no real threat.
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The risk of fire is a clear and present danger in any building. Best to guard against it with the best steel glazing systems on the market.