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Fire engineering for buildings
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Examining and thinking through problems associated with aspects of fire safety is essential
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Slow death: Fire of certain intensity and duration will nearly eventually affect every building component.
Many people, directly or indirectly involved in the design, construction or management of buildings view compliance with safety regulations as an awkward problem to be overcome with minimum cost. Seldom is it understood that fire safety objectives that underpin these provisions are limited.
Typically, also, the safety of fire fighters and protection of property are merely given token consideration. And, even if comprehensive legislation existed at the national level, it may not be effectively monitored or controlled on the ground. A practical introduction to fire resistance performance in buildings in a format that is intended to appeal not only to experts, specialists, practitioners, regulators and controllers in the fire community, but, additionally, to other disciplines working in the mainstream building sector.
Fire of certain intensity and duration will nearly eventually affect every building component. However, it is necessary to have building components to retain their stability and integrity at least for a sufficient period to ensure safe evacuation of all the occupants.
Moreover, a fire should be prevented from spreading from one building to another and to prevent the sudden collapse of assemblies for the safety of fire fighting personnel. A practical fire engineering approach involves examining and thinking through problems associated with aspects of fire safety and protection in buildings, in this case fire resistance performance, and to base solutions on principles of reason, common sense, science, mainstream engineering, practicality and cost-effectiveness.
The benefits accrued by such an approach include the provision of better and more reliable fire safety and protection in buildings, more cost-effective safety and protection measures and more options with regard to their choice and operation, and better communication with other disciplines involved in the construction sector. Most important is that the fire resistance has to be designed as per codal regulation and cannot be assumed.
Fire-related issues
The building components should have the required fire resistance level.
This is specified by the user or city ordinances or based on National Building Code regulations.
The fire resistance level is specified in terms of duration (in minutes) that the element can stand when subjected to standard fire load. It will depend on:
• The purpose for which the structure is used
• The time taken to evacuate occupants in case of fire
The members and connections in a structure should be such that they give a period of structural adequacy larger than the required fire resistance level.
Calculation of the period of structural adequacy involves:
• Calculation of the strength of the element as a function of temperature of the element and the determination of limiting temperature
• Calculation of the thermal response of the element, i.e. calculation of the variation of temperature of the element or part of the element with time when exposed to fire
• Determination of the period of structural adequacy at which the element or part of the element reaches the limiting temperature
The period of structural adequacy can be determined either analytically or through tests.
Fire resistance ratings
The fire resistance ratings of various structural components such as walls, columns, beams and floors are given in ARE Codes.
The ratings could be improved to the required level by protective coatings. The fire resistance ratings vary generally from one hour to about four. In codes, the stability criterion is the ability of a specimen of a load bearing element to support its test load, where appropriate, without exceeding the specified criteria with respect to either the extent of or rate of deformation or both. The test specimen is considered to have be failed if it:
• Exceeds a deflection of L/20
• Exceeds the rate of deflection of L2/9000d in 1 minute interval,
where L = the clear span of the specimen and d = the distance from the top of the structural section to the bottom of the design tension zone (in mm).
Integrity is the ability of a specimen of a separating element to contain a fire to the specified criteria for collapse, freedom from holes, cracks and fissures and sustained flaming on the unexposed face.
Insulation
Insulation is the ability of a specimen of a separating element to restrict the temperature rise of the unexposed face to below specified levels.
By adopting proper guidelines, with regard to both material and structural levels, the loss of life and property in the case of a fire can be reduced.
The author is Emeritus Professor, Department of Civil Engineering, IIT-Madras and former Dean, Anna University
A.R. SANTHAKUMAR
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