Understanding High Rise Building Fires
One of the most difficult things for security managers, security officers and the general office population of a building to appreciate is the nature of high-rise building fire.
AN understanding of the nature of fire is vital to the development of an effective fire prevention and control capability in high-rise buildings. It’s a mistake for security managers to rely solely on systems that may have been installed years before. Understanding fire gives security managers a greater ability to recognise the weaknesses in their systems and work on improving evacuation procedures.
The fact most people are familiar with fires in open or controlled spaces, such as campfires, fireplaces and even bushfires, means they’re hampered when it comes to appreciating the dangers of high-rise and large building fires. In buildings, fire has a totally different temperament to that which it exhibits in open spaces.
In a worst-case scenario, superheated gases may blast up stairwells at hundreds of kilometres an hour, incinerating everything in their path. High-rise fires can advance with explosive force as pressures and oxygen levels change throughout a building.
Each building, thanks to its design, ventilation, fire control systems and the type of combustible materials that may be available, breeds a completely different type of fire the nature of which may vary depending on the day that fire starts – a few fire doors propped open on a lower floor can change everything.
Phases of Fire
Fire has 3 distinct phases when it breaks out in a room inside a building, with the first phase being the incipient phase. Depending on the nature of the fire and what has pre-combusted before the initial outbreak of a fire, this incipient stage may see flame temperatures between 400-600 degrees celsius but there may be very little impact in the way of increased room temperature. There may also be only the slightest amount of gas given off.
At the earliest stages of combustion, the fire will produce water vapour and carbon dioxide gas but there’ll be little impact on the levels of oxygen present in the room. The next stage of fire is the flaming phase when oxygen-rich air is sucked into the fire, contributing to its growth. While this is taking place, increasingly hot convection currents create a thick gaseous layer under the ceiling, which spreads out across the ceiling of a room, spilling up into higher cavities. This layer will also move through doorways as it thickens and deepens. Depending on sparks and other debris floating in this layer, the fire can spread to adjoining rooms during this stage.
The layer of gases and hot air will quickly become hot enough burn the lungs of a person who enters the room standing erect and anyone seeking egress will need to crawl on hands and knees. While this process is going on, smoke will travel up through a building via lift shafts, staircases and through openings or gaps in the floor. This can make it difficult to establish the location of the seat of the fire in a multi-story building. Smoke can get above hung ceilings, get between wall panels and sneak through roof voids.
Any air currents in the building will also facilitate smoke’s spread. Ventilation ducts can allow passage of smoke to locations a long way from the fire. Another way in which smoke can move about is through the air conditioning system. Depending on the setup in your building, the air conditioning system may have turned itself off, or been turned off as soon as the first fire alarm sounded. If the air conditioning system is not turned off it will pump smoke and dangerous carbon monoxide through the building.
Anyone with experience of open fires will know that leaving logs beside a fire to heat up to 100 degrees or more before they’re placed in the flames guarantees a very quick ignition. That’s because the wood has already been heated closer to its ignition point before it’s put on the fire. In an enclosed space during a building fire, this ignition point phenomenon is known as flashover.
In flashover, the superheated gases pump an entire room up to ignition temperature and when it’s reached all the room’s combustible contents and surfaces will instantaneously burst into flames. The room temperature required to stimulate flashover is between 500-600C degrees but flashover may not take place till a room’s contents have reached 800-1000C in some cases.
Prior to flashover, as the room’s temperature has increased, all the contents of the room will have spilled out flammable gases, particularly carbon monoxide, in a process called pyrolysis. The process of total combustion takes place when flaming begins and it’s visible as tiny sparks of flame in the gaseous cloud. As more gas ignites, rollover will occur. Rollover describes a bank of flame that rolls out of a gaseous cloud across a ceiling.Once a flaming rollover has occurred, the situation is extremely dangerous, and the fire can spread far and fast just as long as there’s oxygen to feed it. Flashover’s final phase before total room involvement is thermal collapse as scorching temperatures come down to floor level.
The third and final phase of enclosed fires is the hot smouldering phase. The nature of this phase will be impacted on by certain factors, including the supply of oxygen available to the fire – in many cases there may be no visible flames. This is an extremely dangerous time for building occupants and fire fighters. Should a smouldering fire in an enclosed space be delivered oxygen, it will erupt into life with explosive force as a backdraft event.
Danger signs include yellow-grey smoke, lots of heat radiating from the area and wisps of smoke coming from under doors. The smoke you can see is mostly carbon monoxide, which is explosive even in quite small volumes – down to 12 per cent and up to 74 per cent. Other warning signs include inward-moving air streams pulling smoke back under a doorway, black smoke squeezing out around a closed door or window frames. You could also see glass windows, marked with smoke condensation and flexing from the pressure of the fire.
Essentially backdraft is a chemical smoke explosion that can blast through walls and shatter windows. The defining feature of a backdraft is the presence of shockwaves that will break the enclosure surrounding the fire and spew flame into the surrounding area. Interestingly, while backdraft is a smoke explosion, its trigger is oxygen.
Simulations have showed that a room with as little as 25 per cent smoke volume inside can experience an area-wide backdraft explosion. What’s most frightening about these explosions is that they can kill even at comparatively low pressures. An entire masonry wall may collapse at pressures of just 7 pounds per square inch, humans will sustain lung damage at 15 pounds per square inch, can die at 35psi and will die in 99 per cent of cases, at 65psi.
Because backdraft can occur in either stage 1 or stage 3 of a fire, it’s hard for security officers clearing a building that may be filling with smoke, to ascertain the risks before they open the door to an enclosed office. For this reason, it’s important that security officers understand fire. This doesn’t mean they’ll be fighting blazes with the big boys, but they will have responsibility for quenching or containing spot fires and they’ll certainly be playing a big part in evacuations. Things security managers should ensure their security officers do before opening any doors in the event of a fire include:
* Checking for raised temperatures
* Listening for burning sounds
* Looking for blistering paint
* Checking for discolouration of plaster or timber
* Touching the door with the back of their hand to feel for heat
* Using their nose: Wood, plastic, paper all burn with a distinct odour
* Looking for smoke
* Checking for discolouration of window glass
* Listening for the sound of water coming from sprinkler heads.
Another significant threat in building fires is building collapse caused by the weakening of structural members being consumed by flames and/or heat. It goes without saying that security officers should be well away from the site long before there’s any hint of building collapse. But should they ever be in that position signs of building collapse to look out for include:
* Buckled columns
* Walls that are bulging
* Fallen arches
* Floors sagging
* Displaced columns
* Cracking in walls
* Movement of the building
* Strange building noises
When moving around buildings that are structurally suspect it’s vital to:
* Move cautiously
* Check footing before putting weight on floors
* Walk near walls or over structural supports where possible
* Check ceiling/roof before walking under.
Modern fire control systems incorporating sprinklers are the perfect way to control building fires at all levels so long as there are no exceptional fuels involved. These fire suppression systems need to be supported by an ability to reduce ventilation, close air conditioning systems, and get occupants out of buildings fast. Security managers should also be aware that smoke and gases are likely to cause more damage to life and property than flames themselves. And there are significant dangers when conducting last minute evacuations in buildings that are on fire.
Major fires caused by accident or deliberate action like arson, or a bomb blast, can lead to the loss of thousands of lives and have the capacity to destroy businesses totally. Under certain circumstances fire suppression systems may be too badly damaged to function. Security departments need be able to rely on their suppression systems, but they also need to understand the nature of unsuppressed fires and have contingency plans in place to try to ensure the safety of the people they’re protecting in all possible circumstances.