Fire Resistant Design - Term Paper Example

FIRE RESISTANT DESIGN Traditional aspects of designing buildings did not see any relationship between the structure of a design and fire protection systems that it offered. Both were largely seen as two different entities independent of each other. But ever since a great deal of research went into fires from last decades of 20th century till date, the art of fire began to be seen as science of fire. This is because understanding to the deeper causes of fire gave a realization that the same laws of nature are applicable to fires as any other chemical or physical phenomenon. This led to a mature evolution in modern architecture that stressed the need for spatial function based on users’ perception of how did they want to use a premises or a building for commercial or private use. Traditionally building were seen as an arrangement of multiple rooms connected by a simple passageway and a circulation route, but contemporarily they are seen more as structures that breathe and offer a breathtaking feel and ergonomics. Modern buildings offer better horizontal and vertical spaces spanning full extent of structures and logistic thing behind fire resistance stands at the very core of designing them. This is a change in approach towards fire resistance. Traditionally fire resistance was provided by and mainly focused on passive and static elements used in the design. Contemporary design focuses more on using measures that provide active fire protection options when there are real fires, which might wreak havoc even on the traditional measures that have been previously used. Such active fire protection measures are being seen as a rational engineering approach for designing structures that stand greater chances of withstanding fires and providing safety. In evolving such measures principles of common sense, reason, engineering, science, and practicability are used in order to identify fire safety objectives clearly and succinctly; considering all likely fire scenarios (Bukowski & Meacham, 1996). These fire resistant measures offer a number of advantages that include a reliable and better safety in buildings, multiple choices in favor of fire protection and effective in terms of cost and convenience, and proper communication between different teams involved in building right from its design to inception and then construction. Change of Approach Offered by Fire Resistant Designs Given the research that has gone into fires and their prevention, the strengthened view on fire protection has had twofold impact on design and construction of buildings. One, fire protection as a term has been understood on a broader note; and two, in-depth knowledge has been gained on different fire protection methods and systems. It has been a paradigm shift in the way buildings have begun to be looked at subsequently designing them as fire resistant. Consequently the definition of fire protection covers multitude of elements, which include the type of construction undertaken, measures that offer fixed fire protection options, fire detection and suppression systems, emergency measures in place during fire, fire water systems, safety controls installed within a building, fire prevention and most importantly fire protection engineering. Fire protection doesn’t only concern indirect or direct consequences of fire on a structure but also on people occupying it, programs being run within the same and property on the whole. Fire resistant designs are a scientific outcome on such elements as fire protection systems, life safety systems, maximum credible fire loss (MCFL), maximum possible fire loss (MPFL), redundant fire protection, and performance-based design, which is based on agreed-upon benchmarks set for design elements of a building and encompassing engineering analysis, performance objectives and goals, and a thorough assessment of alternatives in a quantitative manner. In creating fire resistant designs accepted engineering methodologies, tools, and performance criteria are used, that include but are not limited to building code analysis, type of occupancy, assessment of fire-rated walls, and doors, smoke barriers and compartments, and fire dampers with ratings on fire, egress means, automatic sprinkler systems analysis, water inlets and fire hydrants, methods for smoke control, fire detection and alarm systems, fire extinguishers, ratings of interior finish, assessment of hazardous areas, antiterrorism requirements and security coordination, and access to fire department. Modern Approaches in Fire Prevention Methods Modern approaches to fire prevention are, in fact, a synergy established between several methodologies required to accomplish effective fire prevention and also safety to life and property. Modern approaches rely chiefly on comprehensive and coordinated efforts rather than one particular method in fire prevention. Such methods go beyond the usual plan reviews and code enforcements and offer cutting-edge solutions to preventing fires. This is done through combined measures offered by engineering elements needed for fire prevention, well-designed logistics, development and implementation of quality codes in designing and construction, having prevention programs in place and training personnel for the same. Once these are in place, evaluation is carried out periodically. Generally, two types of fire prevention methods have been relied so far, and have become a standard for fighting fires. While one is private fire prevention system, another is the public one. These have been age-old and time-tested methods for fighting fires. But recently emphasis has been laid on preventing the conditions that are responsible for giving rise to fires. These codes ensure that that the buildings are constructed fire-resistant from the very stage of their conception. The enforcement of these codes vests in the fire department and these regulations are created, enforced and followed differently in different nations. The codes take into account almost everything that can cause or help prevent fires, and include anything from installation of fire protection systems or their uninstallation if they are outdated; and give directions on fire escape and overcrowding in case of fires. Furthermore, what has been seen as a part of fire prevention is full preparedness and emergency planning incase a fire takes place. Importance of such an activity has been realized following 9/11 terrorist attacks on the World Trade Center in 2001 and the devastating Hurricane Katrina that struck in 2005. Recently, with regard to high rise buildings, solid stream jets and water fog have been debated as two effective methods to control fire and prevent their spread. This is being seen as a capable solution for countering nozzle pressures that are rendered ineffective when dousing high-rise fires. Other fire-fighting techniques being used concurrently get compromised because nozzles are not able to create fine water droplets that help fight fires effectively. Fire Properties of Concrete, Steel, Timber and Masonry When buildings are on fire, the burning materials exhibit typical behavior of losing mass and releasing energy. Subsequently fire develops and deploys in stages aided by rising temperature, ventilation and duration of the process. The burning material acts as a fuel to further spread of the fire; more combustible the material is rapid is the spread. Material orientation, quantum of air supply to the burning portions, surface to mass ratio and incident heat further influence the fire and its characteristic spread determined by latent heat of vaporization and combustion. Different materials exhibit their structural response to the fire, and buildings depict fire resistance according based on the design and capability of its components to withstand fire. Each building is supposed to have a fire resistance rating, which is determined on a collective function of applied structural load intensity. structure of a member (whether it is a beam, a wall or a column), incident heat flux, and construction material used. Three primary construction material i.e., wood, steel and concrete get influenced by fire. Concrete Concrete is known as loadbearing or Group L construction material, and is capable of withstanding high pressures. Even though concrete is composed of a number of aggregates, the strength or weakness that it attains is chiefly because of the hydration induced in cement, which is the chief component. When concrete catches fire, it gradually loses its stiffness and strength with each degree rise in temperature. As a result of this, it spalls exposing rest of the construction assembly to laps of fire. Fires that grow rapidly tend to spall concrete quickly. Hardened cement paste and other aggregates of concrete show different behaviors at different temperatures. The evaporable water starts getting lost at around 105 degree C, followed by C-S-H starts decomposition and at 500 - 600 degree C CH starts dehydration. This increases porosity and at around 900 degree C C-S-H starts destructing. Primarily there are three causes because of which concrete starts cracking, like hardened cement undergoing different expansion rates, differing thermal gradients between outer and inner concrete layers, and accumulation of vapor pressure within the mass on account of porosity. Overall concrete's fire resistance depends on a number of factors, which are permeability, moisture content, density, thickness, aggregate type and tensile strength (Grau, nd). Steel Steel withstands high temperatures better than concrete. Generally two types of steel materials are used in construction. One of these, reinforcing steel or hot-rolled steel withstands temperatures to as high as 800 degrees C, while the other, prestressing strands or cold-drawn steel starts breaking down at 500 degrees C. This results in varying fire resistance ratings between the two. In case of fire steel loses its strength to the point that it no more resists deformation. This is explained by the modulus of elasticity. Both types of steel are used in construction and building design and carbon content in each determines its strength. Certain constructions warrant use of steel that is passed through a process called as tempering ort quenching, from which they derive certain amount of extra strength. Whether or not fire will buckle or dislodge concrete determines the extent of loading roofs and floors. Response of steel to fire depends on its principal properties mainly density, specific heat and thermal conductivity. The mechanical properties determining its strength are coefficient of thermal expansion, modulus of elasticity and component material's creep that it attains at high temperatures. Different types of steel exhibit varying degrees of mechanical and thermal properties (Lie 1992). Fire melts steel if the temperatures reach around 1400 degrees C. Recent advances in fire prevention have lead to widespread use of insulating material on the outer surface of steel so as to prevent it from losing strength when temperatures reach 500-540 degrees C. Insulating materials used are asbestos, gypsum wallboard, mineral-fiber sprays, Portland cement concrete, Portland cement plaster, ceramic tiles, intumescent coatings, and masonry materials. For long-lasting protection cementitious materials or spray-applied mineral fiber are the methods of choice. In case of full-loaded structures strength of steel is assessed not by the temperatures steel losing strength but by critical temperature reached by the structures in case of fire. That is around 538 degrees C for columns, 593 each for beams, reinforcing steel, and open web steel joists, and 426 degrees C for prestressing steel. Apart from the type of steel used, there are a number of factors responsible for steel's performance during fires. Two important ones being loading and connections (Fitzgerald 1998). If loading exceeds the limit beyond which a structure cannot hold, it weakens steel's capability to bear excessive temperatures further. Same is true with connections which are either wielded or bolted, or both used together. If a connection shares load meant otherwise for steel, the structure has a tendency to hold strong for long. Technically, this is called end restraint (Gewain and Troup 2001). Experiments have demonstrated that for the same fire and loading condition two different restraints deflect differently. For example, rotational restraint would deflect less severely than a simple restraint. Simple restraints are generally free to expand and thus their tendency to go berserk during fires. Timber Timber is combustible in nature, but in case of fire its very quality of being combustible acts as its protection after its initial exposure to fire. This holds true for timber that is designed properly for building structures. Light timber has a cladding material on the outer layer that makes it fire resistant and heavy timber is inherently fire resistance on account of its outer layer turning to char during fire, which later prevents further penetration of heat to its inner core (Benichou N., et al 2001). Outer layer of heavy timber starts burning at 300 degrees C and rapidly turns into char which, though, is devoid of any strength but is a bad conductor of heat. As a result of this the inner core stays protected for long. At this point the layer beneath the char heats up to around 200 degrees C and gets discolored due to a process called pyrolysis, which weakens it constantly (Buchanan 2001; Purkiss 1996). Masonry Masonry refers to material like concrete, stones and bricks held compositely with each other by mortar. Considered to be excellent in compression, masonry is actually weak in tension. Engineering does not allow their use in structures which must exhibit some level of tensility and bending. Different types of masonry include clay, concrete, AAC or autoclaved aerated concrete, light weight concrete aggregate and calicium silicate. It has been seen that masonry walls show a lot of endurance if the supporting foundation or adjoining structures are good enough to hold the walls during fire. However, these walls cannot hold in place if the fire is bowing on them on one of the two sides (British Standards Institution (2001). Case Studies There have been certain historical fires and investigation into these fires revealed that either faulty, inept or absent fire protection system lead to heavy loss of life and property. One of these fires was that of The Windsor Tower in Madrid, Spain in 2005, which was a concrete 32-storey building the fire of which lasted 18-20 hours engulfing the whole of it 21st storey above and down to the 2nd one. The lab collapsed on the 17th floor, the reason for which as cited by investigations was that the perimeter steel columns above this floor were not protected. The commercial building had no fire sprinklers and had passive fire protection. The building had been built by following 1970s Spanish building codes, which had no provision for fire sprinklers or protection for steel. Open plan floor was another factor responsible for the massive fire along with outdated vertical compartmentation measures (Dave, 2005). Similarly in 1988 First Interstate Bank fire in Los Angeles was attributed to the lack of fire protection systems, floor area of 1600 meter square on a open floor plan, the failure of floor openings and façade system (Federal Emergency Management Agency, 1988). References Bukowski R.W. Fire Safety Engineering in the Pursuit of Performance-based Codes: Collected Papers, NISTIR 5878, National Institute of Standards and Technology, Gaithersburg, MD, USA, 106 p Buchanan A.H. (2001). Structural Design for Fire Safety, John Wiley & Sons, Chichester. Benichou N., Sultan M.A., MacCallum C. and Hum J. (2001). 'Thermal properties of wood, gypsum and insulation at elevated temperatures.' Internal Report IRC IR-710, Institute for Research in Construction, National Research Council of Canada, Ottawa British Standards Institution (2001). BS 5928: Code of Practice for Use of Masonry – Part 3: Materials and Components, Design and Workmanship, London. Dave, P. (2005). “Madrid tower designer blames missing fire protection for collapse”. New Civil Engineer, 2 June 2005. Federal Emergency Management Agency. (1988). Interstate Bank Building Fire – Los Angeles, California (May 4, 1988). United States Fire Administration Technical Report Series. Fire Chiefs Reference Manual, Published by the N.C. Fire Chiefs Association in Raleigh, North Carolina, Available at http://www.ncafc.com/firemanual.htm Fitzgerald, R.W. (1998). Structural Integrity During Fire, Fire Protection Handbook, 18th Edition. A.E. Cote, National Fire Protection Association, Quincy, MA Gewain, R. G., and Troup, E.W.J. (2001). Restrained Fire Resistance Ratings in Structural Steel Buildings, AISC Engineering Journal Grau E., (nd). The Effect of Fire on Concrete. Available at http://www.ce.berkeley.edu/~paulmont/241/Reports_04/Fire_pp.pdf Jim A. Crawford, James Crawford, Prentice Hall, 2002 - Technology & Engineering - 131 pages Lie, T.T. (1992). Structural Fire Protection: Manual of Practice, No. 78, ASCE, New York Meacham, B. J., The Evolution of Performance-Based Codes & Fire Safety Design methods, Society of Fire Protection Engineers, Boston, MA, USA, 1996, 87 p. Purkiss J.A. (1996). Fire Safety Engineering Design of Structures, Butterworth-Heinemann, Oxford. Read More