The Inevitable Risks Out of the Many Accidental Fire Risks - Essay Example

FIRE RESISTANT DESIGN HIGH RISE BUILDING Name Professor Institution Course Date Introduction There has been a growing concern about the fire safety of the occupants and property in high rise buildings. Fire cases such as, World Trade Center fire that was caused by an aircraft collisions and ultimately led to collapse of the building has magnified this concern. In order to mitigate the risks associated with fire, design procedures have to be incorporated to ensure that high rise buildings meet the threshold of fire safety. Some of the design considerations have been well defined by a number of building legislations across different countries. The fire cases of the past have also taught lessons which are crucial in engineering considerations of materials to use in construction for increased fire safety. As far as high rise buildings are concerned, a lot of activity has been going on about fire safety. This entails coming up with sufficient measures on fire prevention and protection. A lot of planning at the engineers and architects’ tables has led to high rise building designs that are safe from fire or that resist fire in a significant way. High rise buildings are on the increase due to increasing population and restrictions to expand horizontally. In the use of concrete and steel materials for construction of buildings, primary objectives such as safety of life, protection of property, protection of environment and continued operation have to be taken into consideration (Kathy, 2010). Therefore, a fire control plan for high rise buildings must start with the construction process. Concrete and steel can be used in the construction of fire resistant barriers such as floors, walls, and partition. These barriers stop or delay the propagation of fire from one room to another, and they are crucial features which can dictate the size of fires. Both concrete and steel have got inherent fire resistance and have been used and tested right from the historical times ( Phillips, 2006). For instance, concrete has low thermal conductivity whereas its heat capacity is high. Naturally, the elements of concrete are capable of resisting temperature rise when exposed to fire. The technology in the use of concrete and steel has been advancing at a very high rate. One of the driving forces has been the desire to construct high rise buildings so as to properly utilize the land footprint. However, as the endeavour to construct more high- rise buildings increase, fire safety also becomes of great concern. The fire incidents of the past, and which have attacked high rise buildings cannot be ignored at all. Therefore, this research addresses fire resistant design for high rise buildings through the use of materials such as concrete and steel. The Use of Concrete Material in High-Rise Constructions to Offer Fire Resistance High rise buildings support a lot of load. The escape distance from the higher storeys makes it long, thus increasing the risk when there is fire. In the event of fire, the building should remain standing without collapse, and pre-installed mechanisms should be put in place to help the fire-fighters. In order for the building not to collapse, fire resistance material such as concrete should be considered at the design stage. Therefore, in the construction of high rise buildings, the overall structure should reduce fire development, ensure stability, limit the spread of fire and smoke, assist the evacuation of occupants, and support the intervention of the fire-fighters. In order to meet this standard, the required material for floors, walls, and ceiling should be non-combustible, have high resistance to fire and allow no burning droplets. Effectively, these requirements can be achieved by the use of concrete. Concrete material is inert, non-combustible, has low thermal conductivity, can reduce susceptibility of fire, is extremely robust, and the load elements constructed out of concrete can retain their integrity for quite a long time (Frank, 2010). Further, concrete does not become molten when fire has attacked a building. Looking at its relevance to high rise buildings, most designers would prefer it. The main advantage being that concrete is capable of supporting the building stability for some time because it has higher capability of resisting fire for reasonable time. Unlike other materials, concrete has an inherent property of resisting fires at elevated temperatures. Besides, the structures of concrete have to be designed in order to withstand the fire effects. In other words, its structural members must withstand the live and dead loads without collapsing even if temperature rise leads to decrease of the modulus of elasticity and the strength of the concrete and the reinforced steel. Also, infernos in high rise buildings can lead to the expansion of the structural components. Therefore, the resulting strains and stresses have to be resisted by the concrete structural members. When the temperature rises, it causes the free water contained in the concrete to change from liquid form to gaseous form. This change of state increases the rate of transfer of heat from the surface to the interior components of the concrete. The good thing about concrete is that it does not burn. Further, the changes in the properties of concrete with temperature increase depend mostly on the kind of coarse aggregate used. The aggregate used is categorized into siliceous, carbonate, and lightweight. Siliceous aggregate consists of material that has silica, such as sandstone and granite. Carbonate aggregate consists of limestone and dolomite. Lightweight aggregate is normally manufactured out of heating slate, shale, or clay. A simple illustration on the effect of temperature increase on Compressive Strength of Concrete is as shown in Fig 1, whereas the effect of temperature increase on the modulus of elasticity of the concrete materials is as shown in Fig 2. Fig 1: Effect of Temperature Increase on Compressive Strength of Concrete Fig 2: Effect of Higher Temperatures on the Modulus of Elasticity of the Concrete Material Why Concrete Structures are Appropriate for High-Rise Buildings In most building projects, concrete is preferred for several reasons based on cost, speed of construction, and architectural appearance (Zadrack & Larson, 2008). Even so, one of the principle advantages of concrete is its good performance when subjected to fire. This has really influenced most decision makers to use it in constructions. The following are some of the favourite characteristics that are particularly present in the concrete structures: Concrete is a good load-bearing material that is capable of resisting fire is a remarkable way without need for any additional protective material, such as intumescing paint or the coating of plaster. Concrete remains permanent at no additional maintenance cost. Concrete gives the required resistance to fire in a manner that is economic. Concrete elements have got extremely high resistance to fire. They can withstand high temperature for a long period of time and with minimum deformation. The reinforced concrete slabs reach the critical temperature of about 500oC after two hours, and at a depth of only 3.5 cm. The lightweight concrete is capable of meeting higher demands because it constitutes effective barriers against spreading of fire. Concrete doe not burn in fire. Therefore, when in fire, does not melt, does not give off toxic smoke or fumes, and does not foster propagation of fire even when the temperatures are extreme. The structural elements in concrete have got reserves that offer protection against fire and property insurers value the concrete properties. Further, concrete that is used in compartmentalisation prevents the spreading of fire. This reduces the associated environmental impacts. Concrete does not produce a lot of toxic residues which can be attacked by fire. Even the large quantity of water that is used for putting out fires does not affect the concrete structure. Fire Resistance of the Structural Elements The structural elements of concrete should have the ability to withstand fire effects ( Colin, 2005). For instance, the load-bearing members should resist collapse, resist penetration of fire, and resist the transfer of excessive heat. The structural elements upon which concrete may be incorporated include the main load-bearing structures such as the floors, the frames, and load-bearing walls. Even the concrete compartment walls play a significant role of preventing the spread of fire in high rise buildings. High rise buildings should be provided with load-bearing elements in order to meet the minimum standard of fire resistance. This prevents premature failure of the concrete structures. The purpose of such a structure is to reduce risk to trapped occupants, fire fighters, and reduce danger to the individuals in the vicinity. Overlooking the building legislations on fire resistances is a mistake that can be very costly and result to dangerous high rise buildings. A concrete strength and thickness that can stand a two-hour fire resistance is appropriate. For safe and sound designs, fire considerations have to be part and parcel of the preliminary design stages. One suitable example that illustrates how concrete frames play a core role in fire is that of The Windsor Tower, in Madrid Spain, 2005 (Gray & Larson, 2008). A total direct property value of 122 million Euro was destroyed in the fire. The tower consisted of 29 storeys, 5 basement levels, and 2 technical floors. This tower was constructed between 1974 and 1978. When the tower was being designed, sprinklers had not been included in the building codes of Spain but after subsequent amendments in the building codes, the tower was refurbished. The refurbishment entailed making the entire steel perimeter columns to be fireproof, add new escape routes and new external stairs, and also upgrade the fire alarm systems. The shape of the building was rectangular and the structural frames were made out of the normal concrete. Two years later after the refurbishment and when the building was unoccupied, fire broke out in the building. It started from the 21st floor and spread upwards through openings and downwards through some burning debris that entered the windows below. The fire raged the building for over 26 hours, and engulfed almost all the floors. Finally, when the building was extinguished, everything had been burnt from the 5th floor upwards and there were fears that the building would collapse. However, the building never collapsed until it was demolished. The main idea here is that concrete columns and the core provided passive resistance to fire and that is why the building did not collapse. This reconfirmed the building legislations that had been put in place, and gave a clear evident that, if concrete floors are situated at regular intervals, the risk of fire will be minimized. Figure 4 shows the remaining structure after the fire incident. Fig 3: A Picture of The Windsor Tower after it had been Raged Completely by the Fire. The Use of Steel Material in High-Rise Constructions to Offer Fire Resistance If a slab is subject to severe fire, the strength of both the reinforcing steel and concrete decrease due to temperature increase. However, for high rise buildings, the material should be able to stand the load above it before collapsing. The point at which the reinforced steel reaches before the critical point is attained should be noted. This point depends largely on the reinforcement that is offered by the concrete cover Steel is normally used in reinforcing concrete. Steel is more sensitive to very high temperatures as compared to concrete. The impact of high temperature on steel’s yield strength is as shown if Fig 4, whereas Fig 5 indicates the effect of temperature on the modulus of elasticity of steel. From these figures, it shows that up to 800oC, reinforced bars retain a lot of their yield strength while the cold-drawn steels begin to lose their strength even at about 500oC. Fig 4: The Effect of Increase of Temperature on Yield Strength of Steel Fig 5: The Impact of High Temperature on Modulus of Elasticity of Steel The main materials used to offer fire protection for the structural steel include the thin-film intumescents, sprayed fire-resistive material, epoxy based intumescents, intumescent mat wrap material, and board type products. Thin-film intumescents are applied to structural steel to add some thin thickness to steel. Therefore, when steel is exposed to heat, this film undergoes a chemical change thus forming an insulating layer of char. This provides fire resistance for about 2 to 3 hours more. Intumescent paints have been known for long, for over 10 years. They are commonly used in the UK and United States. These paints have two main components: resin binders and a mixture of other chemicals which easily decompose and release some gas when ignited. Therefore, when a high rise building is on fire, the intumescent paint used in construction of the building melts. This triggers a gas-producing reaction at a temperature which corresponds to the appropriate melt viscosity of the resin. The release of the gas makes the resin to melt and the form developed acts as an insulating layer. Consequently, a thick layer of char is produced and insulates the steel material from fire. The Twin Towers of Petronas consist of about 88 storeys in total, with each side soaring up to 452 meters above the street level. The towers also have concourse levels, concourse mezzanine levels, and four basement levels. These twin towers were certified as the tallest high rise building in 1996 by the Council of Tall Buildings & Urban Habitat. The construction of these towers made use of several local materials, among them concrete and steel. In fact, the towers we supported by barrette piles together with a 4.5 m thick raft foundation of reinforced concrete. The core columns and walls were framed with concrete, steel trusses and beams, and the entire structure was finished with stainless steel and vision glass, making the building to glisten towards the skyline. A number of high rise buildings have suffered severe fires in the past, and the lessons learnt have been applied in some design plans to prevent the impacts of future fire cases. The First Interstate Bank Building in Los Angeles is an example of a high rise building that was attacked by fire in 1988 (Robert & Arthur, 2009). This skyscraper building had 62 storeys and the fire that caught it lasted for about three and half hours. More than 64 fire-fighting companies battled with the blaze. The fire resulted in a lot of windows’ damage and this complicated the efforts of fighting the fire. The flames that jutted out of the storeys, from 12th to 16th floor were large and led to more smoke damage on the floors that were above. An estimated property value of about $200 million was destroyed. However, according o reports carried out about this fire damage, it is reported that there was no damage caused on the principle structural members that were made of reinforced steel. Only a small number o floors and a single secondary beam were destroyed by the fire. The fire protection technologies involving steel are numerous. These technologies have also been applied in the construction of high rise buildings in order to protect the structural elements when there is fire. Some of these technologies include direct application of insulating material on steel, and using membrane protection alongside others systems meant to circulate and discharge water so as to cool the structure. The performance criteria for steel must also be determined before applying it for construction of high rise buildings. Such criteria may include the cost, maintenance, duration, aesthetics and structural changes when exposed to fire. Insulating Technologies used In Steel High rise buildings involve a lot of steel material. Therefore, in the design process, there is a dire need to insulate both steel and other structural members (like the pillars) from fire. Insulation shields the structure from direct exposure to fire because heat insulators are poor in conduction of heat. In other words, insulators increase the overall time for transfer of heat to the structural elements. In high rise buildings, materials such as concrete, bricks, tiles, and asbestos perform excellent at elevated temperatures, and are therefore, used to insulate steel (Zamril, 2010). Gypsum is also a good insulator because it has higher percentages of water which is chemically combined. A lot of energy must be used to evaporate this water. Encasement of Steel Both steel and concrete work together and play different roles in ensuring fire safety in high rise buildings. Steel structures can be encased in concrete in order to protect them from direct exposure to fire. Since concrete is a good thermal insulator, it will delay the transmission of heat to the encased structural members made of steel and other materials. Sufficient insulation can be provided by increasing the thickness of the concrete. In addition to encasing steel, concrete may also be used to carry some of the load. This protection technology is employed in construction of high rise buildings in the UK, USA and Japan. Conclusion The rise of populations has raised concern on management of the available land. This in turn has called for construction of high-rise buildings. In order to mitigate the inevitable risks out of the many accidental and non-accidental fire risks, concrete and steel materials have been considered in the design stages. The two materials easily work together toward the primary objective of resisting fire. They do not only support the structural load of the high-rise buildings, but can also resist the fire effect such as falling off of debris from the structural members and subsequent collapse. A number of engineers and fire experts advocate for this. In some legislation, the fire ratings for different materials are already pre-defined. Further, the fire incidents of the past reconfirm these building legislations that are meant to ensure fire safety. References Robert, A, & Arthur, I, F 2009, Fire Safety for High-Rise Buildings: The Role of Communications, Van Haren, Zaltbommel. Colin, B 2005, High-Rise Security and Fire Life Safety, TSO, Norwich. Frank, T 2010, Fire Officers Handbook of Tactics, APB Group, Norwich. Gray, CF, & Larson, EW 2008, Fundamentals of Fire Protection. 4th Edition, McGraw-Hill Educations, Singapore. Kathy, S 2010, Tall Building: Criteria and Loading, Course Technology Ltd, Boston. Phillips, J 2006, Fire Safety in High Rise Buildings, 2nd Edition, McGraw Hill, California. Zadrack, CF, & Larson, EW 2008, Fighting High-Rise Building Fires-Tactics and Logistics McGraw-Hill Educations, Singapore. Zamril, S 2010, Building Systems for Interior Designers for High Rise Buildings, Course Technology Ltd, Boston. Read More

Therefore, in the construction of high rise buildings, the overall structure should reduce fire development, ensure stability, limit the spread of fire and smoke, assist the evacuation of occupants, and support the intervention of the fire-fighters. In order to meet this standard, the required material for floors, walls, and ceiling should be non-combustible, have high resistance to fire and allow no burning droplets. Effectively, these requirements can be achieved by the use of concrete. Concrete material is inert, non-combustible, has low thermal conductivity, can reduce susceptibility of fire, is extremely robust, and the load elements constructed out of concrete can retain their integrity for quite a long time (Frank, 2010).

Further, concrete does not become molten when fire has attacked a building. Looking at its relevance to high rise buildings, most designers would prefer it. The main advantage being that concrete is capable of supporting the building stability for some time because it has higher capability of resisting fire for reasonable time. Unlike other materials, concrete has an inherent property of resisting fires at elevated temperatures. Besides, the structures of concrete have to be designed in order to withstand the fire effects.

In other words, its structural members must withstand the live and dead loads without collapsing even if temperature rise leads to decrease of the modulus of elasticity and the strength of the concrete and the reinforced steel. Also, infernos in high rise buildings can lead to the expansion of the structural components. Therefore, the resulting strains and stresses have to be resisted by the concrete structural members. When the temperature rises, it causes the free water contained in the concrete to change from liquid form to gaseous form.

This change of state increases the rate of transfer of heat from the surface to the interior components of the concrete. The good thing about concrete is that it does not burn. Further, the changes in the properties of concrete with temperature increase depend mostly on the kind of coarse aggregate used. The aggregate used is categorized into siliceous, carbonate, and lightweight. Siliceous aggregate consists of material that has silica, such as sandstone and granite. Carbonate aggregate consists of limestone and dolomite.

Lightweight aggregate is normally manufactured out of heating slate, shale, or clay. A simple illustration on the effect of temperature increase on Compressive Strength of Concrete is as shown in Fig 1, whereas the effect of temperature increase on the modulus of elasticity of the concrete materials is as shown in Fig 2. Fig 1: Effect of Temperature Increase on Compressive Strength of Concrete Fig 2: Effect of Higher Temperatures on the Modulus of Elasticity of the Concrete Material Why Concrete Structures are Appropriate for High-Rise Buildings In most building projects, concrete is preferred for several reasons based on cost, speed of construction, and architectural appearance (Zadrack & Larson, 2008).

Even so, one of the principle advantages of concrete is its good performance when subjected to fire. This has really influenced most decision makers to use it in constructions. The following are some of the favourite characteristics that are particularly present in the concrete structures: Concrete is a good load-bearing material that is capable of resisting fire is a remarkable way without need for any additional protective material, such as intumescing paint or the coating of plaster. Concrete remains permanent at no additional maintenance cost.

Concrete gives the required resistance to fire in a manner that is economic. Concrete elements have got extremely high resistance to fire. They can withstand high temperature for a long period of time and with minimum deformation. The reinforced concrete slabs reach the critical temperature of about 500oC after two hours, and at a depth of only 3.5 cm. The lightweight concrete is capable of meeting higher demands because it constitutes effective barriers against spreading of fire.

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