Materials Used for Structural Purposes - Assignment Example

BUILDING MATERIALS ASSIGNMENT PART 3-FAILURE MODES Name: Course: Professor: Institution: Date: a) Behaviour of building materials under normal conditions and under fire conditions Materials behave differently when used for structural purposes, under different environments. Plastics Under normal conditions Deformation: Plastic materials deform in an elastic manner up to the yield point under normal loading. Strength: The strength of plastics vary with the type of plastic. But generally, strength decreases with age and temperature. The ultimate tensile strength of plastics ranges from 40 to 110 mPa. The decomposition temperature of a plastic material has to be considered before using it in structural capabilities. Under fire conditions Ignition: Plastics will ignite easily and burn to contribute to fuel since they are combustible materials. Plastics begin decomposing at temperatures above 140 oC. Different plastics have different ignition temperatures and rate of flame propagation and this means they can exhibit different behavior under fire. Some continue to burn even when the ignition flame is removed to enhance the spread of fire as it poses a flame-spread hazard. Melting point: They have a relatively low melting point, which make it easier to burn since after melting the fuel surface area is increased. On melting, the plastic materials behave like combustible liquid fuels. Toxic emissions: The behavior of plastics when exposed to fire also depends on the chemical composition of the material, size and shape, and the additives used. They burn to release smoke and toxic fire gases such as sulfur dioxide, oxides of nitrogen, polycyclic organic matter and volatile organic matter into the atmosphere (Chalivendra & Bo Song, 2012). i. Steel Normal conditions Strength: Under normal conditions, steel behaves elastically under load until it reaches its yield point. Structural steel yields at around 250mPa. At yield point, the material becomes plastic and fails in a ductile phenomenon. In both compression and tension, steel is equally strong. The ultimate tensile strength ranges between 400mPa to 550mPa. Corrosion and melting point: Steel is also resistant to corrosion and has high melting point of around 1370 oC. Under fire Conditions Strength: At temperatures beyond 300°C hot finished carbon steel starts to lose strength and steadily continues to lose strength up to a temperature of 800°C. As temperature increases towards the melting point of 1500°C, there is gradual decrease in residual strength. Hot rolled steels used in reinforcement also experience a similar behaviour. For cold worked steels that, including reinforcement, the strength of the steel rapidly decreases beyond 300°C. Creep: In addition to decrease in stiffness and material strength, steel displays creep phenomena above 450°C. These creep phenomenon causes an increase of strain or deformation with time even when the applied stress and temperature remain constant. Creep is an indication of temperature and stress considerations that have to be taken into account when estimating the deformation behaviour and the strength of steel structures under fire conditions (Chalivendra & Bo Song, 2012). ii. Concrete Under normal conditions: Compressive strength: Under normal conditions, concrete is exceptionally strong and durable and structurally performs well. The strength increases with age and can be modified by varying the mix ratios. The strength varies from 17 mPa for normal use to 70 mPa for commercial structures. Thermal conductivity and shrinkage: It has a low thermal conductivity and continues to shrink as a result of ongoing reactions. Excessive shrinkage can lead to cracking of concrete. Under long duration loading, concrete can experience creep, a behaviour that causes a permanent deformation to release internal stresses. Under fire conditions: The behaviour of concrete under fire is partially dependent on the mechanical, deformation and thermal properties of concrete. These properties change significantly when a structural member is exposed to fire. Concrete has a low thermal conductivity, lower rates of loss of stiffness and strength properties and high thermal capacity. Concrete exhibits a reduction in lining when exposed to fire, an effect described as explosive spalling. This effect causes reduction in the thickness of concrete and occurs at temperatures between 400 oC and 600oC. The compressive strength of concrete gradually starts to reduce at temperatures between 200 oC and 500oC. Thermal conductivity reduces as temperatures increase, a behaviour that can be attributed to variation of moisture as temperature increases. Transient strains and creep deformations are highly improved at elevated temperatures when the concrete is under compressive loading (Ward-Harvey, 2009). iii. Wood Under normal conditions, wood (whether soft or hard), is relatively stiff, tough and strong. The density of wood correlates with the stiffness and strength of the material. Denser wood materials are stiffer and stronger than lighter woods. Under loading deformation, wood exhibits viscoelastic behaviour, which dampens vibrations and dissipate loading energy. Viscoelasticity also protects wood from excessive deflection. Under uniaxial tension, wood normally has a good performance. But in compression, it is weaker and collapses under excess loads. Gradual crushing is likely to occur on a compressed member, transferring the compression load to the tensioned side. Most of the time, wood fails by propagation of cracks on the side that is under tension. Wood is a combustible material, but when used in a properly designed structure, it can perform well under fire conditions. When wood is exposed to fire, the temperature of the wood rises to near the temperature of fire. When the outermost layer of wood reaches its burning temperature, which is about 300°C, it ignites and starts burning rapidly. A layer of char is formed around the burned wood, making it to loose strength. The layer also acts as an insulating layer that resists heat transfer into the inner layers because of its low thermal conductivity. Under this layer, lies another layer called the pyrolysis layer with temperatures of about 200°C. As the wood burns, this layer is chemically decomposed, a process that is irreversible. Chemical decomposition is accompanied by discoloration and loss of weight. The innermost core is also affected by temperature, which leads to lose of stiffness and strength properties as a result of moisture evaporation and exposure to heat. The performance of wood under fire depends on the rate of charring, modulus of elasticity and loss of strength. The stiffness and strength properties of wood depend on the moisture content and temperature. b) Failure modes of structures Structural failures can occur as a result of several types of failure. Good engineering structural designs require a better understanding of different types of failure modes that can occur within real structures. Generally, the term failure means a partial or total collapse of a structure. The following are failure modes likely to be experienced in structures. i. Design failure mode This mode of failure occurs when the structure’s performance and functionality falls below the anticipated minimum design requirements. Some components of the structure, or the whole structure will fail if they cannot support loads that are below the anticipated design loads. ii. Failure due to load Exceedance This mode of failure is also known as catastrophic failure. When the design loads are exceeded, the structure cannot be strong enough to support the excess loads and failure occurs when the overstressed structural component reaches a point of critical stress. iii. Construction failure This failure mode occurs due to poor construction practices, workmanship and defective materials. Improper selection of building materials or use of defective materials, shoddy workmanship and failure to conform to design requirements can lead to unpredictable structural failure. iv. Deterioration failure This mode of failure results from lack of proper maintenance, structural fatigue and corrosion. This mode of failure normally occurs at stress points; bolt holes and squared corners. Structural fatigue may result to development of cracks that slowly progress to a critical length, leading to breakage even under normal load conditions. v. Unexpected failure This type of failure occurs when unexpected problems such as natural disasters, earth quakes, sabotage and vandalism occurs. Such problems can overstress a structure to a point of failure (LLC Books, 2010). c) Signs of collapse and collapse hazards of different types of construction. i. The age of the structure Age is one of the main factors that show a sure sign of collapse of a construction. Older buildings have increased risks of foundation cracks and failure in load supporting beams. The materials that were used to build the structure also weaken with age and therefore, need for renovations that may be substandard or done by contractors who are not licensed and qualified by the local municipality. Such a building cannot handle natural disasters or fire stress. ii. Wall and beam conditions Bulges or cracks on the walls and beams of a structure is an indication of imminent collapse. If walls become unable to support the load of the floors and the roof, cracks will start to develop due to the load stress. Penetration of smoke or water through the walls with solid masonry are clear signs of structural fatigue. iii. Sagging roofs, ceilings and floors Sagging roof rafters, ceilings and floors is another sign of building collapse. These signs are often unnoticed or sometimes they may produce popping and/or creaking sounds. iv. Structural deficiency rating Structures such as bridges have to be inspected on annual basis to evaluate whether they still have the design functionality. If the structure is found to have foreseen problems such as fatigue cracking on the floor deck truss or the main deck truss system, then it is rated as structurally deficient and cannot handle traffic loads in such a condition. Therefore, this should be a sign of structural collapse. v. Huge floods Frequent huge floods hitting a bridge can lead to the collapse of the structure if mitigation measures are not taken. High waters and load carried by a river over-stresses the bridge and may be a probable sign of bridge collapse. vi. Leaning or bowing Leaning of beams and walls is an indication of an impending structural collapse. This occurs due to excessive loading, use of defective building materials, poor design and/or geometry and poor workmanship. Other factors of structural collapse include deteriorated motor joints, sustained multiple fire attacks on the structure and building movements detected by a groaning sound (Cronn-Mills, 2009). Collapse Hazards i. Collapse of a building can result to burns form heated fire gasses and steam. This will cause injuries and subsequent deaths. ii. Electrocution from exposed electric cables iii. Gas explosions and leaks that pose a danger of fire break out and gas explosions. iv. Sharp objects such as steel, glass and roofing tin may cause cuts and serious injuries. v. Building materials such as asbestos are hazardous to human health. vi. Injuries and deaths to structural users or building occupants in case of a residential or commercial building. vii. Fire explosions from gas leaks may occur after building collapse, which poses a danger of a flammable environment and release of toxic gases of burning materials and pollution. viii. The water system may cause flooding after breaking (Cronn-Mills, 2009). References Read More