Fire Resistance of Timber

As a general rule. structural elements made of timber ignite and get rapidly destroyed in case-of a fire. Further, they add to the intensity of a fire.  But timber used in heavy sections may attain high degree of fire-resistance because timber is a very bad conductor of heat. This is the reason why time is required to build up sufficient heat so as to cause a flame in timber. 

With respect to fire resistance, timber is classified as refractory timber and non-refractory timber. The refractory timber is non-resinous and it does not catch fire easily.  Examples of refractory timbers are sal, teak, etc. The non-refractory timber is resinous and it catches fire easily.  Examples of non- refractory timbers are chirdeodar, fir, etc. 

Although combustible, timber is not easy to ignite in the sizes normally encountered in buildings.  Once ignited, timber  bums very slowly and builds up a protective layer of charcoal on its surface, which insulates the remainder of the section from the worst effects of fire.  This protective shield plus the excellent Athena insulation property of timber combined a very low co-efficient of thermal expansion and properties largely unaffected by elevated temperatures allowed a calculation method to be &rived which can be used to exposure; that the residual section after charring can perform its structural function even if fully exposed and subject to fire.  Depletion of timber in a fire is slow and predictable and has been widely researched over many years. It has been established for example than a soft wood section will deplete around 20m in thickness and in width in half an hour during  a standard fire test when fully exposed. 

Armed with facts about the charring rate and knowledge that the strength of a residual section relatively unaffected by exposure to heat it is possible to design a section for a Fie safety. 

Although timber  members can be designed for full exposure in a tire situation it is a much more common condition of the timber to be concealed by other building materials like plaster board in a timber stud wall, where these materials have fire resistance in themselves, they may be taken  into account in determining the fire resistance of the element as a whole. 

Fire resistance is not the only fire safety requirement in a modem building.  Many wall and ceiling surfaces are required to provide resistance to surface spread of flame. For fire proofing timber, the method recommended is the pressure impregnation of the timber with large quantities of chemicals, the most common of which are ammonium diphosphate, sodium arsenate, sodiunl letraborate. Fire resistant paints are also available. White washing is effective to some extent in retarding the action of fire. not possible to make timber Sire proof.  Chemicals and points only retard the action of fire. 

To make timber more fire-resistant, the following methods are adopted: 

Application of Special Chemicals 

Timber surface is coated with the solution of certain chemicals. It is found that two coats of solution of borax or sodium arsenate with strength of 2 per cent are quite effective in rendering the timber fire-resistant. 

These special chemicals are known as fire protection compounds or antipyrines and they are more reliable. When the temperature rises, they either melt or give off gases which  hinder or forbid combustion.  When the wood is treated with antipyrine, it does not inflame even at high temperature, but it merely smoulders, i.c, burns slowly without flame. The antipyrines containing salts of ammonium  or boric and phosphoric acids are considered to he the best in making the timber fire- resistant. 

Sir Abel's Process 

In this process, timber surface is cleaned and it is coated with a dilute solution of sodium silicate. A cream-like paste of slaked fat lime is then applied and finally, a concentrated solution of silicate of soda is applied on the timber surface.  This process is quite satisfactory in making the timber fire- resistant.