Plastic Behaviour in Fire - Essay Example

Failure Modes Name Institution Course Date Introduction A fire inspector needs to know the construction materials used in a building and the different types of building construction. This information is vital because the way fire spreads is affected by the manner in which building construction was done. It also assists in determining the required special safety features needed in controlling the growth of fire and protecting the occupants of the building. 1Moreover, the building codes compliance is determined by the building construction. In order for one to understand the building construction, it starts with having the knowledge of the materials used. Construction of a building is accomplished by use of building components which in most cases contains different materials. Basic fire characteristics that the building itself exhibits is therefore determined by these materials properties and the construction details. In this essay, the behaviour of plastics, steel, concrete and wood in fire conditions is discussed. Moreover, failure modes, signs of collapse and collapse hazards occurring in different construction structures are also discussed. The behaviour of plastics, steel, concrete and wood in fire conditions are affected by combustibility, thermal conductivity, temperature levels and thermal expansion when subjected to heat. 2A material capability of burning or not determines its combustibility. Wood burns when ignited and it releases smoke, light and heat until fire consumed it completely. Non-combustible materials such as concrete and steel are neither ignited by fire nor consumed by it. Steel enables heat to flow through it while concrete, wood and plastics conducts heat poorly. Concrete can withstand higher temperatures for a longer time and by contrast, steel bend and loose strength at the exposure of fire temperatures. Some materials expand when they are exposed to heat and steel in particular will elongate when fire is exposed to them. Plastics, steel, concrete and wood behave differently when they are exposed to fire conditions. Plastic Behaviour in Fire Plastics are used in buildings for structural support rarely, but they are often present throughout a building. They may be tough or brittle, transparent or opaque, stiff or flexible. The plastics materials behaviour in fire conditions is dependent on basic plastic chemical composition, material’s shape and size and the additives kind used. 3Almost all kind of plastic materials contribute fuel to the fire and are therefore combustible. Thermoplastics melt, behave and burn just like combustible liquids in a fire. 4Plastics are capable of melting and flowing away inhibiting further burning, or produce tar-like dripping and flaming when heated. Flame propagation rate and ignition point vary widely in burning plastics. Plastics combustibility varies and some are easily ignited and therefore burn quickly, while others burn only with external presence of heat. Furthermore, some plastics can fail to ignite after withstanding fire exposure and high temperatures. When plastics are exposed to fire conditions, they produce heavy quantities of dark, dense smoke and it releases toxic gases with high concentration. Steel Behaviour in Fire Steel is strong in both compression and tension and is used in supporting roof and floor assemblies in the structural framework. It is not a fire resistive building material when it is considered by itself. Steel melts at exceptionally high temperatures. 5Steel is a good conductor of heat and so it expands and loses its strength with an increase in temperature. Therefore, a steel roof beam which is unprotected by either concrete, layers of gypsum, or masonry and exposed directly to fire sometimes elongates sufficiently causing a collapse to the supporting wall. Heated steel beams twist and sag, while steel columns buckle when they are losing strength. 6Uneven heating that is occurring in the actual fire conditions causes bending and distortion in the steel. Concrete Behaviour in Fire Concrete is often an insulation material from fire for other building materials. In most cases, it is used for floors, walls, exterior pavement, columns, roofs and foundations. It is fire resistive, does not conduct heat or burn well. When it is exposed to fire, concrete does not expand to a large extent nor loses strength under exposure of high temperatures. 7Although concrete is fire resistive, it can be damaged in fire conditions. Concrete trapped moisture may be converted to steam by fire. The concrete surface sections breaks off when steam expands and creates internal pressure in a process called spalling. 8Formation of fissures due to heating as a result of concrete dehydration and thermal expansion may provide avenues for reinforcement bars heating causing further thermal stress and hence cracking in building made of concrete. Wood Behaviour in Fire Wood is a commonly used building material for flooring, interior finish, sheathing and framing and in case there is fire, it provides a source of fuel. It is ignited faster and this depends on the ignition source and the exposure length. The rate of burning and ease of ignition of wood is dependent on its physical form. A small spark ignites wood dust which explodes violently; ignition of a log or beam is hard; and wood shavings pile ignites easily and burns rapidly. 9 As a poor conductor of heat, wood inhibits heat from penetrating readily into it when in beam or log form. Moreover, a char that is developed on the wood surface provide an insulating effect when it is exposed to fire. Moisture content also influence the ignition rate and burning of the wood. Wet wood burn slowly and harder to ignite compared to dry wood. Wood produces large quantities of smoke at the onset of a fire and the flame spreads quickly across a wood surface. Large quantities of hot gases and heat are generated by the burning process of wood, and what remains in the end is residual ash in small quantity. 10 Failure Modes in Structures Structures can fail in a number of ways. It can occur when a building collapses or fails physically in some other similar way through either man-made or natural causes. Natural disasters like earthquakes can cause structures to fail even though the occurrence of the phenomenon was factored out in construction of a structure. Overloading, natural disaster, negligence, design or the building actual construction are some of the causes of structures to fail. Materials and the mechanical stress that a building will be subjected into are the factors considered into the design of a building. Fracture is one of the failure modes that can occur within structures. It is caused by the cracks rapid extension. 11Fracture can occur either as a rupture, ductile or brittle fracture. Brittle fracture takes place in materials of low toughness or when the steel ultimate tensile strength diminishes sharply. Temperature and loading rate affects the tendency of steel to fracture. Buckling is a failure mode in structures where it occurs when a structure’s load is not supported due to connection snaps, bending, cracks that occur in a structural member. Cracking is a failure mode in structures which can be caused by temperature differences, foundations differential settlements or shrinkage.12 Hydration of cement generates heat that results in movement due to the cooling of structural members. Concrete setting and hardening sets free cement hydration heat which is not passed rapidly enough by the surface of the concrete into the surrounding air. Settlements of foundations cracking affect mainly non-structural elements like windows and doors, infill panels and partitions. Constructions Signs of Collapse In fire fighting, it is imperative for fire fighters to understand the different types of buildings construction. The structures combustibility nature of buildings and its components fire resistance are used in classifying types of construction. 13Type I and II constructions primarily have non-combustible materials and wood in limited amounts. In buildings using the construction types III, IV or V, wood and its products are mainly used and therefore the building contents and structural components will burn. The signs of collapse of different types of construction are also important to fire-fighters in saving and rescuing people’s lives. Although collapse of different types of construction occurs suddenly, there are always imminent warning signs of collapse. Constructions sign of collapse includes bulging walls, cracks in walls, sagging roofs and floors, and separation of ceilings and walls. 14 A bulging wall is dangerous and any other addition of shift in bulging may result in the occurrence of major collapse. A leaning and bulging walls are good signs of collapse in a building because these walls are loaded by forces not earlier designed to support it causing lose of strength in it. 15 Cracks in the walls indicate impeding collapse because this is a sign of increasing load in the construction that is continuously pushing it towards its failure. Sagging of a roof or floor is a structural damage sign caused by factors such as lack of maintenance, age, weather exposure or overloaded conditions. 16 The potentiality of collapse due to a sagging floor is as a result of undue stress placed on walls or columns connections points that are supporting them. A separation of ceilings and walls is an indication of exterior walls or floors movement and the structure bearing members excessive stress which may cause a building’s failure. Collapse Hazards Collapse hazards in different types of constructions include damaged infrastructure; fire; explosions; vibration, aftershock or gravity causing secondary collapse. Unstable structures pose potential hazard which can cause serious injuries to the rescuing team stemming from secondary collapse. Flying objects and falling materials from sections of unstable roofs and walls are also collapse hazards that need to be taken into consideration. Exposure to hazmat, dust and smoke can pose serious potential hazard in case of a construction collapse. 17Risk of explosion and occurrence of fire is often imminent in most construction collapses. Electrical hazard is commonly inherent in almost any construction collapse because of the live electricity potential in the debris of the collapsed building. Electrocution owing from the damaged utilities can occur at the early stages of the construction collapse incident. Collapse hazards can be caused by concrete floor collapse, crane collapse and any other type of construction structure.18 References Cote, E., and Bugbee, P., Principles of Fire Protection, Quincy, MA: National Fire Protection Association, 1988. Dalton, J.et al., ‘Structural Collapse: The Hidden Dangers of Residential Fires’ Fire Engineering, vol. 162, no. 10, 2009, p. 101-114. Dunn, V., Collapse of Burning Buildings: A Guide to Fireground Safety, Tulsa, Okla: PennWell, 2010. Fletcher, I. et al., ‘Behaviour of concrete structures in fire’, Thermal Science, vol. 11, no. 2, 2007, p. 37-52. Jenaway, W., Fire Inspector: Principles and Practice, Burlington, MA: Jones & Bartlett Learning, 2012. Marti, P., Theory of Structures: Fundamentals, Framed, Structures, Plates and Shells, Berlin: Ernst, Wilhelm & Sohn, 2013. O'Connell, P., Collapse Operations for First Responders, Tulsa, Okla: Penn Well Corp, 2012. Perlman, A. et al., ‘Hazard recognition and risk perception in construction’, Safety science, vol. 64, 2014, p. 22-31. Read More