Structures Materials and Fire Article Example | Topics and Well Written Essays

Part 1: A review of Sandwich Panel construction Explain what sandwich panels are and where they are used in the construction of buildings A sandwich panel is a laminated composite structure made of three layers: light weight, thick and weak core inserted between two thin, rigid, strong facesheets layers (Thomsen et al., 2005). This arrangement enables the structure to achieve minimum weight while it has excellent mechanical performance. As a result, a sandwich pane has lightweight, strong and stiff structure (Mortensen, 2007). Sandwich panels are used where high strength and light weight structure are required. They are normally used aeronautic, automotive, and marine, as well as in building and construction engineering applications like building roof and wall cladding and bridges due to their thermal performance (Mortensen, 2007). The various types of applications in construction include: External claddings – They are often used for single buildings, but it can also be used in multistory buildings. The common insulation core used for this application is polyurethane. The panel structure has to be strong to carry wind loads. The external envelopes rarely cause major fire spread compared to insulated claddings like the one used to enclose food processing areas (Davies, 2008). Internal partitions and envelopes – These commonly found in food processing plants, pharmaceuticals and other places where the temperature needs to be controlled. The most commonly used material for this insulation is Expanded Polystyrene. Fire resisting compartment walls – sandwich can be used in compartment walls. Rock-fibre mineral wool with high density is normally used as it provides panels which have up to 240 mins resistance to fire. It is ensured that the panel can provide insulation such that the flammable materials that are being protected are not ignited. It is also necessary to design the junction between the buildings envelop and the compartment wall to prevent the spread of fire around the fire resistant compartment wall (Davies, 2008). Discuss the different types of sandwich panel available and their material properties Sandwich structures can be classified into various forms depending on the materials used to manufacture, but the main classification is dependent on the core configuration. The core can be made of varied architecture or materials, but they can be put into four categories that include: (a) foam core (b) corrugated core (c) honeycomb core and (d) web core. In general the sandwich core materials are light and soft (Meyers, 1992). Foam cores construction Foam cores construction is less expensive and is made of balsa wood, and a large amount of plastic/foam materials. Foam core construction can be divided into polyurethane foam and vinyl sheet foam. Vinyl sheet foam is more popular. It is closed, rigid and can resist diesel oil, gasoline, hydrocarbons and sea water. It is mainly used in the construction of aircraft and in automotive structures, but it can be used where easy handling and high properties are required. Vinyl foam is made in an oven with small pressure. Polyurethane foam is a closed cell, rigid material with exceptional flotation and insulation properties. It has been used in marine over the years and is cheap in case lower core is required (Meyers, 1992). The advantages of foam core structures are: It remains watertight after a damaging impact. This is because they have isolated and closed cells, making it suitable to be used in marine applications. A damaging impact on the surface will not cause any significant deformation to the core It can also withstand high temperature of upto 1760C. The surface finish is resistant to harsh environment It is easy to maintain It has excellent damping properties It also has good electrical insulation Disadvantages include: With its rigid plastic cores, it is not resistant to fire Low thermal capacity It can deform if one side of the factsheet is exposed to heat (Meyers, 1992). Honeycomb core Honeycomb core are widely used. Its name comes from honeycomb structure built by bees. Square shell shaped and hexagonally shaped cell structures are the common types. This structure consist of a series of cells put together to form a structure with a cross section similar to a cross-sectional slice of beehive. It has a lot of spaces in its core. It is flexible, resistant to fire, light and resistant to impact (Bitzer, 1997). The main advantage for this type of structure is that its core has the best strength to weight ratio. Honeycomb is mainly used aerospace structural applications. They are manufactured varied range of materials depending on the required features and intended use. The characteristics low stiffness and strength used in less load applications or stiff and high strength characteristics used in high performance purposes. The strength of the laminated sandwich structure is dependent on the panel size, the material used as facesheet and the density or the number of cells it has (Bitzer, 1997; Meyers, 1992). Advantages The cores provide greater stiffness and shear strength with its light weight. Its flexibility and low weight enable innovative designs and easy forming processing. They are good in absorbing impact, and they are used to make structure that can absorb energy like bumpers Disadvantage If the core resistance is increased, by increasing the density, the weight and cost are increased The major disadvantage of honeycomb cores is its price; the material is very expensive. Furthermore, corrosion can be another pitfall of honeycombs. Web core structures Web core structures are similar to I-beams but with flanges welded together. The figure below shows the triangular core construction for web core construction (Bitzer, 1997). Advantages It has spaces in its core that can be used in heat exchanger or for storing liquid. Corrugated core Corrugated core sandwich consist of corrugated core welded between two facesheets. As shown below. Corrugated core is made of materials with high strength with more resistance. Compared to honeycomb, corrugated core sandwich panel resist both twist and bending, and it is normally used in applications which require flexural rigidity. The advantage of this type of structure is large stiffness in the transverse section. It possesses greater stiffness and shear strength if the facesheets are bonded together adequately (Meyers, 1992). Advantages It is resilient to impact damage over other sandwich panels It is less costly, has less weight, easy to manufacture and resistance to impact compared to other panels It rigidity help to facilitate endurance from damage due to impacts Corrugated core like web core types have spaces in their core, thus can be used in heat exchanger or for storing liquid. The load in wed and corrugated core structures is carried by the core, but in foam and honeycomb core the load is carried by the facesheets (Meyers, 1992). Disadvantages It is resistant to bending In general, sandwich materials are durable, efficient, light in weight and less costly. Construction details Sandwich structure is made by welding two strong, thin, and stiff skins to a thick and light weight core. They are glued with adhesives as shown below. They are similar to I-beam due to the fact that the flanges carry the load in case it is subjected to bending. The core often carries shear force. Thicker core produce high stiffness. Sandwich structures mainly depend on the core configuration (Davies, 2008). There are different types of core materials and facesheet depending on the desire mechanical properties of the sandwich structure. Various conditions are met in order to employ the correct components that include: Determining the required weight for a particular loading and geometry of the amaterial Comparing one type of sandwich with others. Comparing the types of structure Selecting the best core and facesheet material to reduce the weight ratio Choosing the bets stacking sequence for composie materials Comparing the structural optimum weight with the weights required from some restriction (Allen, 1969; Davies, 2008). Example of building made from sandwich materials Some of the example of building made from sandwich materials include: Woolworth State office in South Australia, Epcot’s Spaceship Earth, VanDusen botanical garden and Asalon sculpture at saint George cathedral in Australia. Part 2 The structural effects of fire on a concrete framed building with those of a fire on a steel framed building The structural effects of fire on the concrete The structural effect of fire on the concrete frame building is different from that of steel framed building. Due to the poor thermal conductivity of concrete, the concrete does not respond immediately to physical changes like expansion, but it can go through a number of chemical changes. The changes are made complicated because the structure is not uniform (Cobb, 2009). The concrete structure consists of aggregate and cement, that react to heat in different ways. Cement undergo through chemical ad physical changes, that are reversible and those that are irreversible. This can weaken the structure of concrete in case of fire. The concrete has specific amount of moisture in their structures. When the temperature is raise significantly, the moisture vapourize and build up pressure in the concrete. The temperature above 4000C dehydrates calcium hydroxide in cement. This produces more water vapour as well as weakens the concrete structure (Grantham et al., 2011). The aggregate also experience physical changes at high temperatures. The aggregate made of quartz increase in volume at high temperature of over 4000C because the mineral alterations, while the aggregate that contains limestone decompose at 8000C. Cracking and spalling occurs due to the different expansion rate between the cement matrix and the aggregate. In general, the heat response of aggregate (Cobb, 2009). On the other hand, the chemical; and physical changes in the concrete reduce the compressive strength of concrete structure. The concrete maintains its compressive strength until a critical temperature of 6000C is attained, after which it fall. The critical temperature for concrete is higher than that of steel, but due to its low conductivity, the heat does not penetrate far into the material structure. Thus, the structure usually retains a greater percentage of its strength (Fédération internationale du béton, 2013). Spalling in concrete is a process where chunks of concrete are ejected from the concrete surface. It can occur at early stage of fire, usually at 2000C. Spalling is formed due to build up of pressure within the material structure, of which the structure cannot be able to hold, thus causing fracture and sudden outward ejection of the materials. It can cause more harm on the strength of reinforced concrete, because of heating of the steel. It can remove the layer of concrete over the reinforcement steel. This exposes steel to further heating and thus reducing the strength of steel, which reduces the overall strength of the whole structure (Cobb, 2009; Grantham et al., 2011). Modulus of elasticity of concrete, which also influence its compressive strength, depends on water ratio, age, and the nature and amount of aggregate. It decreases rapidly with increase in temperature. The strength decreases with increase in temperature as shown in the figure below. The compressive strength for two concretes at high temperatures Therefore, concrete is affected by fire and lose stiffness and strength at high temperatures. Spalling is common in fast growing fires. Steel frame after fire The strength of materials made of steel reduces as the temperature increase. However, the advantage of steel is its incombustibility and ability to recover its strength after being exposed to fire. Steel is a good conductor and can absorb most of the thermal energy, and usual conduct it away from the hotspot. The heat in the frame is therefore kept below fire temperatures so long as the fire does not spread to other parts of the structure or the entire structure. Steel reinforced concrete does poorly in this respect, because the concrete is not a good conductor of heat. The conductivity of steel bars is limited by their mass and the entrenched concrete matrix (Jármai & Farkas, 2008; Craighead, 2009). Steel returns to its stable state after cooling. Individual steel frame may be damaged or bent after heating and cooling, but stability in other parts of the structure that is protected is maintained. There is no doubt that the performance of steel during fire is high than that of the concrete. It has large load capacities and light in weight. However, when exposed too much heat its performance reduces (Craighead, 2009). Thermal expansion of steel in steel frames pushing out of the alignment, and may lead to the collapse of the structure. The steel frame connections joints normally have more materials like bolts than the beam. The connections are less exposed to heat and more capacity to dissipate heat because of their contact to other members. Therefore, the temperature is more likely to rise in columns and beams than in connections (Jármai & Farkas, 2008). Effects of the reinforced concrete The steel and concrete has similar thermal expansion upto to the temperatures of 4000C, after which the steel experience more expansion compared to concrete. If the temperature increases to 7000C, the capacity to bear load by steel bars is reduced significantly. The physical changes on the concrete due to dehydration and thermal expansion because of heat may cause cracks and fissures in the concrete. The cracks may expose heat to the reinforcement steel, creating more thermal stress and cracks in reinforced concrete(Jármai & Farkas, 2008). The reinforcement affects the movement of vapour within the concrete, creating an impermeable section which does not allow water vapour to pass through. The results in water flowing around the steel bars, creating pressure in some parts of the concrete and thus more risk of spalling. But also these regions of trapped water provide insulation against the heat flow near steel, and therefore reducing the temperature of the inner parts of the concrete structure (Craighead, 2009). After fire exposure, the concrete do not fully recover its original state owing to the structural property changes, as compared to steel structure where its original state is restored on cooling. The change is because of the chemical and physical properties of cement. In some cases, the concrete structure may not have visible damage, but it is considered weaker after fire (Federal Emergency Management Agency, 2003). The temperature in building fires may rise to 10000C or more. At this temperature, steel lose stiffness and strength. In other words it becomes weaker and softer. On the other hand, the fire performance of steel is dependent on the mechanical and thermal properties on the material. Thus, as the temperature increase due to exposure to fire, the strength of steel reduces as well as its property to resist deformation that can be characterize by modulus of elasticity. Prolonged exposure to fire causes more deformation other property change (Harmathy, 2007). Trusses and beams react in different ways to fire exposure, and are dependent on the fabrication. Thus, the members that are not connected may collapse when the stress due to applied load go beyond the strength limit for trusses and beams. The connected members experience deflection due to reduced modulus, but the integrity of the structure is preserved. The properties which influence the rise in temperature in a steel frame are the density, specific heat and thermal conductivity (Federal Emergency Management Agency, 2003). Mechanical properties that influence the performance of steel include: the coefficience of expansion, the creep of component, modulus of elasticity and the strength of the material at high temperatures. The stress strin behavior of steel at different temperature is summarized below. Stress-strain curves for steel structure (Federal Emergency Management Agency, 2003). The ultimate strength and yield decreases with temperature rise. The variation of strength for steel at higher temperatures is shown in the graph below. Strength of structural steel at different temperatures As shown in the figure above, at a temperature of 5000C, the strength of steel is about half the value at ambient temperature. In addition, steel structure expands with increase in temperature, and it has coefficient of thermal expansion of 11x10-6 mm/mm-0C (Federal Emergency Management Agency, 2003). References Allen, H. G. (1969). Analysis and design of structural sandwich panels. Oxford: Pergamon Press. Bitzer, T. (1997). Honeycomb technology: Materials, design, manufacturing, applications and testing. London: Chapman & Hall. Cobb, F. (2009). Structural engineer's pocket book. Amsterdam: Butterworth-Heinemann. Craighead, G. (2009). High-rise security and fire life safety. Amsterdam: Butterworth- Heinemann/Elsevier. Davies, J. M. (2008). Lightweight Sandwich Construction. Chichester: John Wiley & Sons. Fédération internationale du béton. (2013). Fib model code for concrete structures 2010. Federal Emergency Management Agency, Federal Insurance and Mitigation Administration, (2003). World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, Government Printing Office Grantham M., Mechtcherine V., Schneck U., (2011) Concrete Solutions 2011, CRC Press, Jármai, K., & Farkas, J. (2008). Design, fabrication and economy of welded structures: International conference proceedings 2008 Miskolc, Hungary, April 24-26, 2008. Chichester: Horwood Pub. Meyers R. A., (1992) Encyclopedia of Physical Science and Technology, Volume 14, Encyclopedia of Physical Science and Technology, Robert Allen Meyers, Academic Press, ISBN 0122269306, 9780122269301 Mortensen, A. (2007). Concise encyclopedia of composite materials. Amsterdam: Elsevier, 2007. Harmathy, T. Z., ASTM Committee E-5 on Fire Standards., Society of Fire Protection Engineers., & Symposium on Application of Fire Science to Fire Engineering. (1985). Fire safety, science and engineering: A symposium. Philadelphia, Pa: ASTM. Structures, (2005). Sandwich structures 7: Advancing with sandwich structures and materials : proceedings of the 7th International Conference on Sandwich Structures, Aalborg University, Aalborg, Denmark, 29-31 August 2005. Dordrecht: Springer. Thomsen, O. T., Bozhevolnaya, E., & Lyckegaard, A., International Conference on Sandwich Zingoni, A & International Conference on Structural Engineering, Mechanics, and Computation, (2001). Structural engineering, mechanics, and computation: Proceedings of the International Conference on Structural Engineering, Mechanics, and Computation, 2-4 April 2001, Cape Town, South Africa. Amsterdam: Elsevier. Read More

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