Compartmentation key to containing fires
LEE COATES, Wrightstyle’s technical manager, explains the mechanics of fire and fire safety and indicates how compartmentation can prevent the spread of a fire.
01 March 2009
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.
Definitions aside, fire is also dangerous, although it can be tamed or contained.
Each year, fire kills an estimated 166,000 people worldwide — that’s nearly 19 fatalities every hour. Some estimates suggest nearly 300,000 deaths per year. The International Technical Committee for the Prevention and Extinction of Fire (CTIF) estimates that the annual cost of fire damage adds up to a whopping one per cent of international GDP or €400 billion ($513 billion).
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 — the so-called fire triangle on which all fires depend. Get those three elements together 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 fire in a building, its spread depends on a whole range of factors – from the type of fuel (everything from ceiling tiles to furniture) to building construction and ventilation.
Taming a fire generally involves the removal of heat, in most cases using water to soak up heat generated by the fire. This turns the water into steam, thereby robbing the fire of the heat used. 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 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. What is also 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, because 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 deg C to 1,500 deg C. To put that in perspective, a candle flame burns at between 800 deg C and 1,200 deg C. In a typical flashover fire inside a building, temperatures can reach between 1,000 deg C and 1,400 deg C.
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 increase 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, due to thermal expansion, these flammable gases are usually under pressure and able to pass through relatively small holes and gaps in ducts and walls, causing the fire to spread 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 of any fire fighter and 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.
Secondly, 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 signals a change in fire-fighting response because it marks the start of a worse danger – the risk of structural collapse.
However, most fires start with only a minimum of real danger — a dropped cigarette, a spark from a faulty wire — and, if dealt with quickly or adequately contained, pose no real threat. That’s where Wrightstyle’s advanced systems come in.
Wrightstyle is one of the UK’s most innovative steel glazing specialists with an international client base. Its internal and external steel and glass systems have been tested together to furnace temperatures of well over 1,000 deg C, testing the strength of the glass, the protective level of the glazing system, and its overall capability to maintain compartmentation in a fire situation.
The company has also added US-testing to its armoury because, in the US, immediately after fire exposure, the testing regime also requires the glazing system to then be subjected to a high-pressure fire-hose test, aimed directly onto the super-heated steel and glass assembly and generating a water stream in the region of 30 psi.
In the hose stream test, the longer the fire resistance being applied for, the longer and more severe is the high-pressure water exposure. This tests the glass for the thermal shock of being deluged and suddenly cooled by the fire-fighting 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 because the safety features built into the buildings have worked effectively to reduce the impact of fires on their fabric and human occupants.
Not so everywhere. In 1993, near Bangkok, Thailand, a fire in a toy factory killed 188 people and injured 500 others. The fire quickly spread through several interlinked buildings that had no fire-rated internal compartments — the main reason for the large loss of life.
It’s still ranked as the world’s worst-ever factory fire. The tragedy is that they could have been contained and a great many lives saved.
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