Fire Resistant Building Techniques

The fire load in a building should be kept to the minimum possible. The term fire load indicates the amount of heat liberated in kilo joules per square metre (kJ/m2) of floor area of any compartment by the combustion of the content of the building including its own combustible part. It is determined by multiplying the weights of all combustible materials by their respective calorific values and dividing that with floor area. The single word that describes the material, financial, and emotional damage caused by fires in structures. We rely on buildings to provide safety and protection, but how can buildings themselves be protected from the ravages of incineration?

A building may be made more fire resistant by:

1. Using suitable materials.

2. Taking precautions in building construction

3. By providing fire alarm systems and fire extinguishers.

Reasonable Resistance

While it is difficult and costly to construct a completely fireproof structure, it is reasonable to construct buildings that are significantly fire resistant. Firefighters will readily agree that no two fires are exactly the same even in seemingly identical structures. But there are similarities in certain aspects of how fires start, spread, and extinguish in buildings and structures. Therefore techniques used for fire resistant construction attempt to take advantage of these similar processes.

Built-In Prevention

Fire typically requires four elements: fuel, oxygen, heat, and a chemical chain reaction. To start a fire there needs to be a source of heat in the presence of oxygen (or an oxidizer of some kind) and a bit of fuel. These must combine to begin the chemical chain reaction needed to generate additional heat, which consumes more fuel and oxygen, generating more heat, and so on. Removing any one or more of these elements will prevent and/or suppress a fire. Creating fire resistant structures then can be as simple as using non-combustible construction materials such as metals, Concrete, masonry, clay, sand, glass, cork, ceramics, etc. However, as developed as these materials have become in terms of aesthetics, combustible materials are still typically desirable for interior and exterior design, finish work, furniture, floor coverings, and so on.

An additional prevention step is to exclude oxygen, but for obvious reasons this technique is limited to applying gas impermeable coatings to a limited selection of combustible materials. In doing so care must be taken to avoid generation of poisonous gases and fumes if combustion does manage to start, as they may be more hazardous than the fire itself.

Which leaves the final prevention step of eliminating sources of excess heat, sparks, and exposed flame. Electronic ignition used in gas appliances, spark arrestors, appropriate attention to electrical wiring codes and power usage, and less reliance on flame based heating appliances like fireplaces and wood stoves have significantly reduced the related incidence of fires started by these sources.

The fire resisting material is having the following characters:

(a) It should not disintegrate under the effect of heat

(b) It should not expand under heat so as to introduce unnecessary stresses in the building

(c) The material should not catch fire easily

(d) It should not lose its strength when subjected to fire.

Fire resisting characteristics of some of the commonly used building materials are:

Stone: It is a bad conductor of heat. Sand stones with fire grains can resist fire moderately. Granite disintegrate under fire. Lime stone crumbles easily. Most of the stones disintegrate during cooling period after heated by fire.

Brick: Bricks can resist heat up to 1200°C. At the time of construction, if good quality mortar is used, fire resistance is extremely good.

Timber: Any structure made of timbers is rapidly destroyed in fire. Timber enhances the intensity of fire. Use of heavy sections of timber in buildings is not desirable. To make timber more fire resistant the surface of timber is coated with chemicals such as ammonium phosphate and sulphate, boric acid and borax. Sometimes fire resistant paint is applied to timber used in the building.

Concrete: Concrete has got very good fire resistance. The actual behaviour of concrete in case of fire depends upon the quality of cement and aggregates used. In case of reinforced concrete and prestressed concrete, it also depends upon the position of steel. Larger the concrete cover, better is the fire resistance of the member. There is no loss in strength in concrete when it is heated up to 250°C. The reduction in strength starts if the temperature goes beyond 250°C. Normally reinforced concrete structures can resist fire for about one hour at a temperature of 1000°C. Hence cement concrete is ideally used fire resistant material.

Steel: It is a good conductor of heat. Steel bars lose tensile strength. Steel yields at 600°C. They melt at 1400°C. Steel columns become unsafe during fire. Steel reinforcement weaken the reinforced concrete structures. Hence steel columns are usually protected with brick works or by encasing in concrete. Reinforcement in concrete are protected by concrete cover. Steel grills and beams are applied with fire resistant paints.

Glass: It is a poor conductor of heat. It expands little during heating. After heating when it cools, cracks are formed in glass. Reinforced glass with steel wire is more resistant to fire and during cooling process, even if it breaks, fractured glasses are in their original position.

Aluminium: It is good conductor of heat. It has got higher resistance to fire.

Asbestos Cement: It is non-combustible material. It possesses high fire resistance.

Planned Passivity

However much care is taken to prevent structure fires from starting, they will almost certainly continue to ignite due to accidents, negligence, arson, carelessness, and equipment failures. The next best fire defense then is to prevent it from spreading once started. Several construction techniques are used to achieve this goal. Passive measures such as spray applied fire proofing coatings can be used in inaccessible, non-finished areas. Fire stops are required by most building codes and consist of blocks, putty, sleeves, caulking, sealants, and/or in tumescent materials. They function by preventing a fire from spreading through joints, penetrations, and framing gaps between and through walls, floors, and ceilings. Controlled openings for access and ventilation along with blocking vacant vertical shafts (such as chutes, elevator runs, stairwells, etc.) prevent them from acting as chimneys during a fire. This can effectively oxygen-starve combustion and help with suppression. Used in conjunction with rated fire walls, ceilings, floors, and doors these techniques effectively compartmentalize a fire and slows or prevents its spread.

Active Automation

Built-in active measures are also used to suppress fires in progress. These include automated sprinkler systems and chemical extinguishers, automated door closer, and alarm systems. While typically more expensive to incorporate than passive measures, active measures have been shown to be extremely effective in slowing and even suppressing fires that would have otherwise become uncontrollable. And with a structure fire occurring almost every minute in the United States alone, it is clear that all available prevention and suppression measures will continue to be needed in building construction for many years to come.

Consequently, it's not a question of whether a fire can damage a structure, but a question of when. It simply takes longer for fire to affect fire-resistant materials. The key is to construct a building in which a fire would take effect slowly, allowing the occupants plenty of time to escape. This is also why materials themselves are rated in respect to how long it would take fire to affect its structural abilities. Even heavy timber can be considered fire-resistant. It's combustible, however, while metals like aluminum or steel aren't combustible -instead, they tend to buckle under intense heat.

We'll explore some of the best building materials for preventing and impeding a raging fire.

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