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Fire engineering and fire safety issues – for building designers, constructors and users.
Fire is on the most amazing and helpful thing ever discovered by the human being. But it also can be disastrous and devastating. Without fire, the civilization would be totally different, or it might not even exist. Fire can tear apart close people, and once started, it can destroy everything on its way. The statistics say that from 2 to 3 people in UK die in fire every day. By neglecting the fire safety regulation, people expose themselves and the one around them to the risk of burns, or in the worst case, to death.
Nowadays it is accepted that complete fire safety in buildings is an impossible goal, that’s why fire safety is not only in terms of the design of the buildings, but also in the behaviour of the people in the particular building. The inherent fire risk in different buildings types is normally highlighted only in serious and fatal cases as the King’s Cross fire in November 1987, when 31 people were killed. Such disastrous cases does not only remind architects and engineers about the importance and responsibility of a proper buildings design, but also about the responsibility of everyone that is involved in the usage of the building.
Understanding fire. Basics.
What is fire?
Knowing the nature of fire and understanding the way it can spread will help designers to plan necessary ways to protect the actual building, and the people within. So what exactly is fire? The “Oxford English Dictionary describes fire as “the active principal operative in combustion”. Combustion is a chemical process; the oxidation of organic material with the development of heat and light. When a combustible material such as paper or any fuel reacts with oxygen in the air, it produces heat. Normally this process will only begin when an external source of heat – for example a lit match – is applied to the “fuel”. Once started, the reaction generates its own heat, often enough to make the process self-sustaining. The well know fire triangle (fig. X) shows the three factors that are needed to keep a fire going. The absence of any of this three components will break the triangle and stop the reaction.
The triangle in action. The evidence and progress of fire.
Flames are the visible manifestation of this reaction between fuel and oxygen. Therefore it is not the fuel itself that burns, but the vapours given off as the fuel is heated. Once ignition has begun and the vapours are ignited, these flames will in turn further heating heat the fuel and increase the rate of production of flammable vapours. In diffusion flames the rate of burning is determined by the rate of mixing of the fuel and oxygen and this in normally controlled by the degree of ventilation, the amount of fuel and the configuration of the room – all factors which the building designers can influence.
Heat – thermal energy – is transferred from hot materials to the cooler ones by the three basics mechanisms of heat transfer: radiation, conduction and convention.
- Thermal radiation – just like light, can travel across empty space and the amount which is transferred from one place to another has the same geometrical rules as light. Therefore radiation does not require not require an intervening medium between the source and receiver.
- Conduction is the direct transfer of energy due to the proximity of molecules. It is dependent on the material properties of conductivity, density and specific heat of solids, still fluids or gases.
- Convention involves the movement of medium and is therefore restricted to liquid and gases. From high temperature sources, the convective type of heat transfer is usually not so important as the radiation. It is however an important factor in cooling low temperature surfaces.
At the typical fire temperature in a building, the main mechanism of heat transfer through still air will be radiation.
Thus, a rise of temperature of any cause like heat from a chemical action, space heating, human action, computers etc, may lead to the earliest stages of fire.
The structure of a building.
A major for fires is with no doubt provided by the building contents, as statistic say this is one of the reasons most fires start. This contents are defined by materials used in the building construction, as well as the furniture and the fittings used in the dwellings of the building. The main problem with the fittings and furniture is the burning characteristics of the materials this objects are made, smoke and toxic gases they release. This is the cause of the majority of the fatalities from fire in UK.
Among the building materials and internal finishes, the greatest risk of spread of flame is leaded by plastic, because of its chemical compositions.
Danger from fire.
As smoke can be a severe factor to damage a building, is not the main reason to cause a total collapse; however, extreme heat can totally destroy a building. Steel will lose 2/3 of its strength by the time it has been heated to 600oC – by no means an uncommon temperature in a domestic fire. Concrete is a more fire resistant materials, but as it’s usually steel reinforced with steel, it should have sufficient insulation to prevent reaching critical temperatures. Timber on the other hand, burns but is a good structural material as burning occurs a at a constant so structural timbers could be oversized to provide a specific measure of fire resistance.
One of the best structural resistances is provided by bricks, as they have already been kiln-fired at high temperature during the manufacture process.
The amount of the heat produced in a fire defines a measurement tool of the severity of a fire. An understanding of the material properties and would allow to estimate the level of heat production in case of fires, and therefore estimate the potential of the actual fire to destroy property. The rate of fire will be identified as being dependent of the fuel and ventilation provided. This two factors determine the heat which will be produced.
Not only the type and amount of the fuel will influence the heat output; the arrangement of the fuel is also significant. The bigger the area of fuel exposed, the bigger is the potential of fire development. The ventilation is a critical factor that determines the fire severity and heat output. Both the air supply and air extraction are significant. If more oxygen is supplied than needed, the burning rate will be controlled by the available fuel. The last factor which determines the fire severity and heat output is the size of the room in which the fire occurred. While a larger area contains a bigger amount of fuel, the distance between the ceilings and the walls will significantly slow down the fires in the beginning stages.
A small percentage of victims from fire is due to the heat in fire that causes the building to collapse. The majority of deaths are because of smoke, either by inhalation of toxic gases. The frequent burning of bodies after death due to toxic gas inhalation could give a false impression of the relative dangers of different products. In the building designing phase all smoke should be considered as potentially lethal, though the toxicity level will vary depending of the type of the fuel.
“On December 5, 2009, a fire occurred in a nightclub in the city if Perm, Russia. Sparks from firework ignited the low ceiling made of plastic material which was covered by willow twig. The fire quickly spread to the walls damaging the electrical system, which cause the lights to fail. More than 150 people lost their lives that day, and up to 160 were injured with serious burns. This was the deadliest fire in Russia since fall of the Soviet Union in 1991. It was stated that most victims died from smoke inhalation and carbon monoxide poisoning. Moscow Fire Safety Authority motioned suspending activity of 54 nightclubs and 450 facilities were warned for safety violations. Some of the typical violations were lack of fire alarms, bars installed on windows, construction materials blocking walkways etc. The fireworks that caused the ignitions appeared to be outdoor pyrotechnics, prohibited for use indoors.
By initial plans, the building was designed to have large windows which would have allowed the crowd to escape, but the final built plan had critically important differences, like bricked small windows, and in some new extensions, no windows at ll.
This disaster has some similarities to the 2003 Station fire in United States, that was caused by the improper use of indoor pyrotechnics. The flammable sound insulation was ignited which led to a very fast burning of the stage. 100 persons died.”
The case study above might seem in some points “ridiculous” and to “exaggerated” in terms of neglecting the authority safety regulations and basic building design rules, but it shows again how important is every factor in building construction design, from material nature used in construction, to the layout of every window and door and to the responsibility of every individual involved in the building usage. Despite the fact that the fire in the cases above occurred due to an improper usage of flammable products, all the consequences mentioned earlier could be significantly minimized by a proper design of the building itself. Fire safety is considered to cover not only the safety of the people, but also the safety of the property – in the building itself and in the surrounding area. Thus the fire safety are divided in two categories – life safety and property salvation. The architect’s objective is to ensure that the building should remain safe for enough time to evacuate the people safely, an also to ensure that as much as possible of the building can continue to function so it can be repaired and reused.
There are normally five tactics for the fulfilment of the life safety and property salvation requirements.
- Prevention – controlling in advance the ignition and fuel sources to avoid a fire start.
- Communication – in case of ignition, the users are informed and any fire safety system is triggered.
- Escape – the occupants of the building have a safe and quick access to safe areas before they are affected by the heat or smoke.
- Containment – ensuring that the fire is retained in the smallest area possible to limit the amount of the construction to be damaged and to keep the people safer.
- Extinguishment – the fire is extinguished quickly with minimum damage to the property
Above is a logical sequence of the five tactics, and of the key one is the prevention tactic. If the prevention tactic fails, next tactics are attempted. Prevention is the simplest and most effective tactic available. There are two ways of preventing fires which are related to the “fire triangle” we spoke about earlier. This two ways are prevention of ignition and limitation of fuel available. The third one, which is oxygen, is almost impossible and undesirable to be excluded from the building, therefore is not considered as a way of prevention. First two things that should be done in the fire safety designing phase is: 1. – design out the foreseen ignition sources; 2. – to manage the building layout in such a way so the risk of ignition is excluded.