Fire Resistant Properties of Different Structural Materials - Essay Example

FIRE RESISTANT DESIGN by [Enter your own presented to [Enter your [Enter your [Enter the of [Enter the name of the city, Enter the name of state] [Enter the due date] 1. Introduction: In this modern age of skyscrapers, complex domes and intricate architectural designs focusing more and more on beauty and outlook, engineers are being pushed to the limit of development, especially in terms of materials. Materials have to sustain impossible loads and be stand under unheard of structural complications while still providing stability not only in structural terms but also when the building is faced with hazards such as earthquakes, storms and hurricanes. All these external hazards are highly destructive yet seldom happen, but such is not the case with fires. Fires are arguably the biggest hazard in terms of engineering difficulty. With winds, earthquakes and storms, the change is only in terms of the loads upon a structure. The behavior of the structure and the materials in it is mostly the same, thus precautionary measures are easier to engineer and hazard predictions can be made. In case of fires, the material properties themselves change. The major structural materials – steel, concrete, wood and masonry – all lose their structural integrity and strength as temperatures are raised higher and higher and beyond a certain limit, each of these materials is no longer able to sustain the loads that they need to bear. Another important feature that needs to be focused on is that these materials are usually not used by themselves; rather they work in conjunction with each other, e.g. steel reinforcement of concrete pillars. When such a structure is subjected to higher temperatures, the steel and the concrete might be, by themselves, within their usable limit, but the bonding interface between the two materials may cause failure. As more and more complex designs are being made, joining of varied materials is becoming a very important field in itself. Although the joining materials might be stronger than the materials joined at room temperatures, yet they might lose their joining ability at lower temperatures than the temperature at which the joined materials lose their integrity. With higher and higher focus on cost reduction, designers and engineers are being pushed to replace fire emergency equipment and repair costs with nothing i.e. they are being asked to use materials which can resist fires without damages. Thus fire resistant design is very important. This paper focuses on the different methods of assessing the high temperature properties of a certain structure, setting up experiments for the assessment, and it also focuses on different case studies shedding light on the properties of different structural materials at high temperatures. 2. Fire Resistant Properties of Different Structural Materials: Structural materials can be many, though this paper discusses only the most common structural materials – steels, wood, concrete and masonry. 2.1. Steels: Steels are the most produced materials in the world in terms of tonnage. Construction too has an immense demand of steel. Steel can be used to make a whole structure of to provide a skeletal frame for it. It can also be used as reinforcement in wooden and concrete structures. Steels retain their yield strength up to around 215ºC (Kumar and Kumar, 2008). Their elastic modulus is also somewhat retained up to around 400 ºC (Kumar and Kumar, 2008). Although the loss of strength is not very significant instantly above these limits, but the gradual loss of mechanical strength is quiet noticeable when we reach up above 700 ºC. The figure above (Kumar and Kumar, 2008) compares the heating rate of unprotected steel with fire protected steel. It gives a certain heating pattern of a furnace which was used to test both the materials. The test indicates that there is only a mild difference of heating rates between the furnace and the unprotected steel, which causes that steel to lose its major properties within the first half hour of heating. When we compare this to the fire protected steel, we notice that it heats very gradually, allowing time for emergency measures to be taken before there is any structural damage to the steel. The figure above (Kumar and Kumar, 2008) shows how the properties of steel degrade as temperature is increased. The ratio on the y-axis indicates the ratio of the properties at elevated temperature to the properties at room temperature. This indicates the need for fire resistant steels, as they have a very low heating rate. Thus fire resistant steel (FRS) is a very useful way of preventing disaster. This is achieved simply by adding a small amount of Molybdenum and Chromium in mild steel, maintaining the ferrite and pearlite structure, though it is prepared through thermo-mechanical treatment. The following table indicates the composition of the FRS in comparison to mild steel (Kumar and Kumar, 2008). C Mn Si S P Mo + Cr FRS 0.20% 1.50% 0.50% 0.040% 0.040% 1.00% Mild Steel 0.23% 1.50% 0.40% 0.050% 0.050% - Table 1: Extracted from Kumar and Kumar, 2008 The fire resistant properties of steels are not limited to composition only, but also encompass the design of the components. A large amount of research has been done in this field as well. The following table shows a combination of beams and columns that can be used to provide fire protection in terms of minutes of exposure (Bailey et. al., 1999). Further details on the structures can be consulted from the source. Unprotected Column Blocked-in Column Partially Encased un-reinforced Partially Encased reinforced Concrete filled hollow sections Protected Columns Unprotected beam 15 minutes 15 minutes 15 minutes 15 minutes 15 minutes 15 minutes Slim floor systems 15 minutes 30 minutes 60 minutes 60 minutes 60 minutes 60 minutes Shelf angle floor 15 minutes 30 minutes 60 minutes 60 minutes 60 minutes 60 minutes Partially encased 15 minutes 30 minutes 60 minutes >60 minutes >60 minutes >60 minutes Protected beam 15 minutes 30 minutes 60 minutes >60 minutes >60 minutes >60 minutes Another very important structural method of decreasing heating rate of a structure is to increase the volume to surface area ratio. The greater is the ratio, the lesser is the heat absorption of the element. Bailey et. al. (1999) have worked on this giving a very expansive research on fire resistant design of steel structures. A very common way of protecting steel from fire is spraying different fire retardants. This has its disadvantages, including a tarnished look. Other coatings, categorized as Intumescent coatings, can increase fire resistance of steels by up to 2 hours. An older method of protection was concrete encasement, although it is not preferred now (Kumar and Kumar, 2008). The table below gives specifications of different steels used in fire protection and fire resistant research in steels. The compositions furnished in the table have been extracted from the research done by a Turkish company CEPA (2010). 2.2. Wood: There are many classes and types of wood used in construction. Their fire resistance is dependent both on composition as well as thickness. The most common types of woods used in construction are fiberboard, plywood and gypsum board. Wood frames are generally covered with different woods or boards to give some sort of fire protection. The fire resistance rating, a measure of the sustenance of a wall in a fire, of different wood coverings has been compiled in the table below (University of Missouri, 1998), in comparison with masonry: Construction Fire resistance rating (minutes) Wood frame covered (both sides) with: 1/2-inch fiberboard 10 1/2-inch fiberboard, flame proofed 10 1/4-inch plywood 10 3/4-inch T&G boards 20 3/8-inch gypsum wallboard 25 1/2-inch gypsum wallboard 40 5/8-inch gypsum wallboard (type X) 60 Cement asbestos board 3/16-inch thick 10 3/16-inch cement asbestos board over 3/8-inch gypsum board 60 Masonry construction 4-inch blocks plastered both sides 60 6-inch blocks 60 6-inch concrete 240 As can be observed from the table above, gypsum board has quiet good resistance for fire. It is a well-known fact that woods generally have a poor fire protection. Thus many coatings have been developed for protecting woods from fires. The following materials (University of Missouri, 1998) are regarded to have satisfactory levels of fire protection: (i) ½ inch thickness of cement plaster (ii) Fire-rated gypsum board (iii) ¼ - ½ inch thickness of sprayed-on magnesium oxychloride (iv) Asbestos cement-board ¼ inch thick. Apart from fire resistant coatings, woods can also be treated to provide good fire retardation. Two methods are generally applied for this: (i) Pressure treatment with certain chemicals (ii) Painting with fire resistant coatings For the pressure treatment, special waterborne salts are used that reduce the amount of combustible material released from woods on burning. This causes fire retardation. Some of the chemicals used for this include mono-ammonium and di-ammonium phosphate, ammonium sulfate, zinc chloride, sodium tetra-borate and boric acid. Fire resistant paints, when exposed to fire, expand like foam over the wood, insulating the wood and stopping the fire from spreading. This treatment can drastically reduce the fire spreading on the wood if done properly. These two treatments do not prevent the wood from catching fire; rather they inhibit the spreading rate and allow emergency measures to be taken in time. A research done by Robert H. White on different fire resistant and fire retardant coatings revealed the improvement of fire resistance of wood through different coatings (1984). The following table describes the coating designations used and the tables following that indicate drastic improvements in the fire resistance in terms of fire resistant rating improvements. Another recent research by the same author (2009) indicates that coatings can be used to increase the fire resistance rating ranging from half an hour to over an hour. Also, the thickness of coating has a lot of effect on the fire resistance of the woods as indicated below: 2.3. Concrete: Concrete has been the backbone of high strength construction for ages in combination with masonry as well as steel. One great advantage of concrete is its excellent fire resistance. It is inherently able to sustain temperatures up to 600°C. Another major usage of concrete is in protecting reinforcing steel bars. It rarely falls off the steel it is coated upon, a problem observed with other coatings (Erlin and Hime, 2004). The excellent thermal properties of concrete are owing to its high specific heat capacity. This causes the temperature of concrete to increase very slowly during a fire (Gosain, 2006). The effects of fire on concrete are greatly influenced by the type of coarse aggregate used. Concrete containing carbonate aggregates, including limestone and dolomite, and lightweight aggregates, like naturally occurring or manufactured by expanding shale, clay, or slag, retain most of their compressive strength up to 650 °C. However, concrete containing siliceous aggregates, such as granite, quartzite, schists, and other materials consisting largely of silica, retain only about 55% of their compressive strength at 650 °C (Abrams, 1971 and Neville, 1995). Also, tests have shown that reinforcing bars heated to temperatures beyond 500 °C undergo a significant reduction in both yield strength and ultimate strength (Edwards and Gamble, 1986). The defects that fires may cause in concrete have been listed as under: 2.3.1. External cracking: Fire events with extended life and intensity will create high temperatures in the concrete mass, causing free water inside the concrete to vaporize. Because the concrete will generally have insufficient continuous pores to relieve the vapor pressures, the tensile stresses created will result in cracks that extend to the surface (Chiang and Tsai, 2003). 2.3.2. Delamination and spalling: High vapor pressures will also cause internal delaminations that are normally associated with external cracking, but can be present even when there are no visible external cracks. Spalling will occur when the surface layer at a delamination falls away and exposes the internal concrete. Depending on the extent and depth over which they occur, delaminated areas may be repaired by rebonding, but a spall must be replaced by patching. 2.3.3. Internal micro-cracking Severe fires may cause dehydration and chemical changes in the concrete. These changes will result in micro-cracking, sometimes deep inside concrete elements. Extensive micro-cracks may reduce the load-bearing capacity of the structure. 2.3.4. Chemical changes Severe fires may also create intense heat causing the compounds in concrete to undergo chemical changes and often forming crystals or changes in color that can be used as indicators of the extent of damage (Yuzer, Akoz and Ozturk, 2004). In the late 1990s the rapid rise in temperature from fires in concrete-lined tunnels was causing explosive spalling of large areas of concrete. The large, falling concrete chunks were nearly as dangerous to trapped motorists as the smoke and fumes. The problem had to be addressed. There are many different methods of improving the fire resistant properties of concrete. Some of these are discussed as under. UGC International, a Zurich, Switzerland-based division of MBT International, a Degussa company, developed a cementitious-based passive fire protection barrier that shield underground concrete structures from heat up to 1350°C. The mortar product, known as Fireshield 1350, is based on standard concrete technology and replaces the normal aggregate with another natural resource. The mixture consists of a mineral/organic main component, Portland cement, water, and admixtures. It has relatively high compressive strength – up to 4350 psi – and bonds well to most substrates (Hanley-Wood, 2004 and Gale Group, 2004). Another method is to add substances to concrete, such as calcium aluminate, to increase the fire resistance of the concrete. Also, hydrated cement is used in places such as concrete vaults. In the occasion of a fire, this water starts to absorb most of the heat, allowing the structure more strength and longevity so that emergency actions can be taken. Fire proofing is also done through addition of mortar. 2.4. Masonry: Prepared by baking in fire themselves, bricks have a good fire resistance. The fire resistance rating of some brick walls is tabulated (Brick Industry Association, 2008) below: Different finish materials are coated on bricks, which further enhances their fire resistance. The following tabulation (Anon., 2008) comments on the fire resistance properties of these: Other measures of providing fire resistance to masonry include coating of masonry with concrete. The table below indicates the thickness of different types of masonry required for certain fire resistance ratings (Anon., 2007): 3. Fire Design of Buildings – Assessment: Research keeps on getting done in the field of fire resistant design of buildings. One very important part of this research is the assessment of the theoretical research done. For this, a certain “performance based structural fire engineering” PBFSE method has been developed by NIST, which works according to the following flowchart: 4. Conclusion: Fires are an everyday engineering problem, and the research on them is still at its early stages. This paper summarizes different researches done on the fire resistant properties, and methodologies of inducing fire resistance of the major construction materials. Thus it sets forth different guidelines on which future engineering should be based and focused upon. References Kumar, S. R. Satish and Kumar, A. R. Santha, 2008. Design of Steel Structures. Madras, Indian Institute of Technology Madras. Bailey, C. G., Newman, G. M. and Simms, W. I. 1999. Design of Steel Framed Buildings without Applied Fire Protection. Berkshire, The Steel Construction Institute. CEPA. 2010. Chemical Composition. Available at [Accessed 09 December 2010] University of Missouri. Improving Fire Resistance of Farm Building. Available at < http://nasdonline.org/document/1654/d001529/improving-fire-resistance-of-farm-building.html> [Accessed 03 December 2010] White, Robert H. 1984. Use of Coatings to Improve Fire Resistance of Wood. Standard Technical Publication 826. Philadelphia, American Society of Testing Materials. Available at < http://www.fpl.fs.fed.us/documnts/pdf1984/white84a.pdf> [Accessed 08 December 2010] White, Robert H. 2009. Fire Resistance of Members with Directly Applied Protection. Wood Design Focus. Available at < http://www.awc.org/pdf/WDF19-2_White.pdf> [Accessed 08 December 2010] Abrams, M.S. 1971. Compressive Strength of Concrete at Temperatures up to 1600 F. Temperature and Concrete. Michigan, American Concrete Institute. Neville, A.M. 1995. Properties of Concrete (4th Ed). Pearson Education Limited Edwards, W.T., and Gamble, W.L. 1986. Strength of Grade 60 Reinforcing Bars After Exposure to Fire Temperatures. Concrete International, V. 8, No. 10, Oct. 1986. Chiang, C.-H., and Tsai, C.-L. 2003. Time-Temperature Analysis of Bond Strength of a Rebar after Fire Exposure. Cement and Concrete Research, V. 33, No. 10, Oct. 2003. Yüzer, N.; Aköz, F.; and Öztürk, L.D. 2004. Compressive Strength-Color Change Relation in Mortars at High Temperature. Cement and Concrete Research, V. 34, No. 10, Oct. 2004. Hanley-Wood Inc. and Gale Group. 2004. High-temperature fire resistance for concrete. Concrete Construction. Available at: [Accessed 17 December 2010] Anon. 2008. Fire Resistance of Brick Masonry. Technical Notes on Brick Construction. Virginia, Brick Industry Association. Anon. 2007. Masonry Fire-Resistance-Rated Construction Code References. Angelus Block Co. Incorporation. Read More