Structures Materials and Fire - Where Sandwich Panels Are Used in the Construction of Buildings - Term Paper Example

FV2207 Assignment Brief Sandwich panels are and where they are used in the construction of buildings Sandwich panels refers to thermally insulated composite elements used in construction of buildings for both wall and roof cladding, consequently creating a building envelope. Sandwich panels consist of outer and inner galvanized sheet cover with a matrix material in between. In most cases sandwich panels are used in industrial construction for instance in the building of cold rooms and industrial halls (Brunner, 2008). They are now commonly applied in creation of building envelopes as well as in partition. Sandwich panel falls under broad category of composite materials since they consist of insulating form layer sandwiched between two thin layers of GI sheets. In most cases the core matrix is mostly polyurethane form (PUR), although in certain cases mineral fibre wool (MIWO). The key advantage of a sandwich panel is its high load- bearing capacity and high rigidity, due to the bonding or adhesion of the outer layers as well as the shear resistance of the core (Fricke, , et al., 2008). The core gives the building envelope superior fire resistance capabilities, as well as insulation. Other key merit of this panels, is the ease of installation as they are widely used for walls and roofing for industrial, and commercial offices. Being modern, sandwich panels are flexible, versatile, durable and ecologically friendly. Sandwich can be classified based on their applications as shown in figure 1 below. Figure 1: Sandwich Panel Classification Different types of sandwich panel available and their material properties Sandwich panels with mineral fibre wool insulation This type of sandwich panels consists of compressed mineral wool sandwiched between two sheets of galvanised steel. This is achieved at a temperature greater than 100 degree Celsius, upon feeding into a machine. To bond with the sheets the bottom sheets is sprayed with a special PU adhesive, which allows the bonding of the rock wool strips. The advantage of the rook wool panels is the high specific weight of between 80 and 160kg/m3. This gives these panels a relatively high compressive strength. One notable demerit of the high weight ratio is that these panels are prone to collapsing leading to the peeling off of the sheet metal from the fibre wool core (Brunner, 2008). PUR Formed panels This type of panels consists of the following primary components: these are expanding agents, fire protection agents, activating catalysts and fibre matrix, leading to the formation of a rigid form element with high heat insulation properties. In the manufacturing of the PUR panels, the top and bottom coils are fed into a forming machine which heats up the form contained in between the sheets, thereby binding the sheets together leading to the creation of a robust, composite structural veneer, cable to resist shear forces. The created PUR foam has a specific weight of between 35 and 50kg/m3. The realised homogeneity ensures high adhesion between the sheet metal and foam. PUR panels have the following properties (Ghazi, et al., 2009). Weather resistance Dimensional stability (good load bearing properties) Appealing surface finance, colours and profiling Superb thermal insulation although variable as per the panel thickness Wall Cladding – wall panel and façade panel Both wall and façade panels are created for the wall cladding applications. Although both can be used for both horizontal and vertical mounting. The difference the two is that, wall panels are fastened through the side body of the panel, with the screw and the sealing washer remaining visible. This ensures that these screws are uniformly arranged thereby becoming part of the architectural aesthetic. On the other hand, in façade panels, the fastening screws used are positioned inside the panel in a longitudinal seam, thereby remaining concealed. The disadvantage of such fastening arrangement is that, if too much torque is applied, especially close to the ends, the sheet seams can be pulled inwards. Such depressions appear unattractive and conspicuously visible on the installed façades (Künzel,, 2005). Installation of the wall panels Since they are not grooved to conceal the fasteners, sufficiently long screws are then used through the panel. It is advisable to use screws with EPDM seals as well as large washers. The large diameter washer helps in withstanding the pull over of the panels. Walls are also butt joined, accurately with both screws and supporting thread. Fastening of the façade panels Due to their shape, the panels are fastened to the structure frame through a specially designed beam. The panels helps in concealing the screw. During installation, the panels are joined properly to remove unwanted and unpleasant gaps which interfere with wind proofing thus encouraging the formation of the condensate. It is also crucial that all panels need to be adjoined with the smallest possible gap, to ensure that the inner sealing is enough to windproof the joint thus preventing the formation of the condensate (Künzel,, 2005). - Ruukki Sandwich panels This typical example of sandwich panels, comes in diverse materials as well as thickness, coating and profile shapes. This versatility provides a customer with a variety of materials to choose for both roof and wall cladding, based on the project specifications. As a building cladding material, Ruukki sandwich panels provide superb fire safety properties, especially Ruukki panels with mineral wool core, which present brilliant fire safety properties. Ruukki panels offer superb thermal insulation capacities, As airtight cladding properties minimizes the maintenance costs of a building such as air conditioning, heating and refrigeration. Ruukki panels are durable as they are constructed using high quality cladding and core material, this coupled with innovative gluing system, the resultant building possesses high thermal insulation and mechanical properties. The advantages of Ruukki sandwich panels: The core is constructed using rigid, firm environmentally friendly mineral wool which offers excellent fire resistance The panels are constructed with facing profiling, this leads to the creation of an apparent façade appearance. Also the profiled grooves (auxiliary) are carried out to permit precise assembly of panels To increase panel tightness, Ruukki panel have double lock from inner and outer sizes thereby facilitating assembly The panels also possess concealed panel fixings which help in ensuring the façade appearance of the panel assembly. Figure 2: Typical Ruukki Panel Sandwich panel fabrication technology A number of benefits can be designed and incorporated into a sandwich, however fundamentally all sandwich panels provide outstanding stiffness accompanied with high strength and low weight. Thin, high strength skins are bonded to thick lightweight cores, higher stiffness and strength is achieved through when more layers are bonded together. Structural effects of fire on Concrete Framed buildings compared to the steel framed structures Concrete Framed buildings One of the benefits of concrete over other building materials is its inbuilt fire resistive properties; however to achieve optimum performance, concrete structures need to be designed to for certain magnitude of fire effects. The key objective being that structural elements within the building should withstand both dead and live loads without collapse even at elevated temperatures. Such high temperatures greatly reduces both lean and reinforced concrete strength and modulus of elasticity. Additionally, fully developed fires causes the creation of undue stresses and strains within the structural steel elements. Effects of fires on concrete material Concrete properties changes as a result of high temperatures is based on the type of course aggregate used in the making of the concrete material. Based on this fact, concrete can be classified into three main categories i.e. siliceous, carbonate, and lightweight. Basically, the fire behaviour of concrete is interlinked to the temperature dependent of materials. Due to the low thermal diffusivity, of concrete compared to steel, advanced temperature gradients are generated within fire exposed sections of concrete. Coupled with the high thermal, the core region of the concrete element may take up heat for an extended duration. Thus, although the compressive strength of concrete is highly compromised, beyond the critical temperature, the overall structural effectiveness is not diminished until the bulk of the structural element reaches similar temperatures. Another phenomena which occurs when concrete is heated beyond certain temperatures is spalling- the explosive ejection of flakes of concrete from the surface of the structural element. This results from the breakdown of the existing surface tensile forces. This also arises from the mechanical forces that created within the structural element as a result of sudden rise or fall in temperatures i.e. thermal stresses or through rapid expansion of moisture present within the contracture thereby elevating the pore water pressure. Strong temperature gradients present both in heating and cooling phases also leads to spalling. Physical and chemical response of concrete under fire Fire and concrete have an intricate interaction, arising from both composition of concrete and the extreme conditions present in fire. Concrete is rarely homogenous material comprising of aggregate, cement slurry, and in some cases steel reinforcement bars. Each of this constituent elements have varied reaction to different thermal levels. This makes it difficult to fully define the behaviour of the composite system under fire. In addition, the low thermal conductivity of concrete impedes the adoption of “lumped parameter” frequently used in thermal analysis of other structural material such as steel (where thermal gradients is assumed) There are a myriad of chemical and physical changes which occur when concrete is subjected to extreme temperatures. A number of these changes are however reversible upon cooling, while majority are irreversible and thus significantly weaken the structures after fire. Most of concrete structures contain water pores, which starts to vaporise at temperatures above 1000C, leading to the build-up of pressure within the concrete. In a fire scenario, due to pressure effects within the structure, the boiling temperatures within the pores may extend to about 1400C. After that the structure experiences a moisture plateau to about 4000C, from where calcium hydroxide begins to degenerate, disintegrating into more water vapour ultimately leading to the significant reduction in the physical strength of the concrete material. Other changes that occur at higher temperatures include mineral transformation which leads to the volume increase of the quartz based aggregates. This changes occur at 5750C while at 8000C, limestone aggregates begin to decompose. In isolation, thermal response of aggregates within concrete may be direct however the overall response of the concrete mix as a result of changes occurring within aggregate elements present is a lot different. For instance, differential expansion between aggregate and the cement matrix may lead to spalling and cracking (Gabriel , 2010). In combination, the resultant physical and chemical dynamism within concrete have a profound effect of minimizing the compressive strength of the concrete structure. In practice, the optimal temperatures for significant reduction in strength is strongly dependent on the aggregate type used for instance siliceous (4300C), carbonate (6600C), and sand light weight (6500C).in lower temperatures, the influence of strength as a result of temperature is very variable, based on both the environmental condition as well as the composition factors, such the degree in which sealing is done to prevent the infiltration of moisture (Gabriel , 2010). Some of the notable failures that occur in concrete structures as a result of fire include cracking and spalling, as discussed below. Spalling This is one of most poorly understood behavioural characteristic in the reaction of concrete to fire. Spalling is often assumed to occur at high temperature, however it has been seen to occur to occur at lower temperatures. If severe, spalling can have a damaging impact on the strength of reinforced concrete structures, due to accelerated heating of the steel reinforcement. Spalling eliminates or at bests minimizes the concrete layer cover along the reinforcement bars. Spalling greatly impacts upon the physical strength of concrete structures occurring through the decline of the cross section of concrete available to shore up the remaining weight of the structures. The mechanism believed to cause spalling is high thermal stresses arising from rapid heating as well as the rapid pressure build-up as a result of evaporation of moisture trapped within the porous concrete structure. Cracking It is widely believed that cracking in concrete structure arises in a similar manner to spalling. Dehydration of concrete structures and the thermal expansion arising from the heating leading to the creation of fissures in the concrete culminating in explosive spalling. The created fissures provide an avenue where the internal reinforcement bars .The resultant fissures may offer conduits for heating of the steel bars, feasibly inducing more thermal stresses leading to more cracking. In some instances the cracks may offer routes for fire to spread to adjacent compartments. Steel framed structures On majority occasion, some structural members remain intact after the building has been subjected to extreme temperatures of fires. Often, engineers are concerned that the fire action leads to permanent metallurgical impairment with subsequent loss in both mechanical integrity and properties of the structural members (Steel construction encyclopoedia, , 2014). Properties of steel The principal properties of steel that influence the temperature distribution and rise are its specific heat, density, and thermal conductivity. This together represent the thermal properties. The mechanical properties of steel that affect the performance of steel structural members under fire are modulus of elasticity, strength, coefficient of thermal expansion, and creep of steel when subjected to elevated temperatures. Steel as a building material is formidable for construction. Due to its relatively less weight, and high load bearing capacities, it is a favourite choice for designers. Without steel the modern high-rise buildings would not exist. However, without proper fire resisting materials, performance of steel as a building material is highly severed. The steel strength and performance remains unchanged to about 3160C, however about 50% of the strength is diminished at 5930C, with entire strength depleted at 14820C. Though, for design purposes, it is considered that entire load bearing capacity of steel is lost at 12040C ( Continuing Education Units, 2012, December 11). In addition to losing all its load bearing capacity, steel structures also suffer considerate expansion when heated sufficiently. The magnitude of this is dependent of the “coefficient of expansion” which varies based on the material composition of steel. The thermal expansions due to fire can lead in the steel beams and columns pushing the supporting walls and slabs off the alignment, thereby increasing the risk of structural collapse. Steel framing connections, where two steel elements are joined to form different geometry for instance column and beam intersection, are prone to other consideration. Fire protection techniques for steel Given the significant reduction of thermal and mechanical properties of steel at elevated temperatures. A variety of methods are available to control the rise in temperature of steel structural members, including the capacitive method and insulation method. Insulation method involves the attaching the spray applied insulating material. This include asbestos, mineral fibre, gypsum wall board, masonry wall, and cements spray. Works Cited Continuing Education Units, 2012, December 11. Building Construction: Fire Effects on Steel Structures. [Online] Available at: http://www.usfa.fema.gov/downloads/pdf/coffee-break/cb_fp_2012_50.pdf [Accessed 10 Dec 2014]. Brunner, S. a. S. H., 2008. In situ performance assessment of vacuum. Vacuum. Fricke, , J., Heinemann,, . U. & Ebert, , H., 2008. Vacuum insulation panels - From research to market.. s.l.:s.n. Gabriel , A. K., 2010. Effect of fire on concrete and concrete structures. Queensland: University of Queensland. Ghazi, R., Bundi, R. & Binder,, B., 2009. Effective thermal conductivity of vacuum insulation panels vacuum insulation panels.. Building Research and Information. Künzel,, H., 2005. Effect of interior and exterior insulation on the hygrothermal behaviour of exposed walls.. Materials and Structures. Steel construction encyclopoedia, , 2014. Fire and steel construction. [Online] Available at: http://www.steelconstruction.info/Fire_and_steel_construction [Accessed 10 December 2014]. Read More