Structure Materials and Failure Modes - Assignment Example

Name Course Date Part 3 - Failure Modes The main aspects of behaviour of plastics, steel, concrete and wood under normal conditions and under fire conditions Steel – steel is an alloy of carbon and iron. It is a strong material that has been used widely as a building material. It has high compression and tensile strength. They can be produced in different sizes and shapes, ranging from thin sheets to heavy columns and beams. They are used for roofing, in floor or on walls. They neither age nor rot, but they can undergo rusting process when they are exposed to moisture and air. Steel can melt at very high temperatures, which may not be attained easily in a structural building. It is a good conductor of electricity and they can expand and lose strength at high temperatures. Therefore, other materials like concrete are used to protect steel from fire heat. Coating with cement-like materials protects steel from rusting and insulate it from the effect of fire. The amount of heat absorbed by steel depends on the amount of protection on steel and mass of the material, such that lighter materials absorb heat faster than heavy and larger pieces (Schottke et al., 2014, 169). Steel expands and loses strength in fire conditions. Thus, the roof beams made of steel elongate in case of fire resulting in the collapse of the supporting walls. Heated steel beam usually twist and sag, but steel columns buckles when they lose strength. The distortion and bending are caused by non-uniform heating of steel beam in a fire situation. Failure of steel material is dependent on three factors that include the mass of the steel material, the load that is carried by the component, and the type of connection used to connect steel pieces (Schottke, 2014 et al., 169). Concrete - Concrete is a mixture of cement aggregate that consist of gravel and sand. Concrete is cheap and can easily form different shapes. The ratio of the mixture depends on the purpose a specific building. They can be used to on the floors, roofs, walls, columns and foundation. The concrete is under compressive forces, and can be able to support large weight, but it is weak under tensional forces. Therefore, in a building, steel reinforcement is included in the concrete to increase its strength under tension. On the other hand, the concrete is a poor conductor of heat, it thus insulate steel from heat. Although the concrete is fire resistive material, due to the fact that it does not conduct heat or burn, it can be damaged when exposed to fire. Moisture in the concrete can be converted to steam by fire. The steam creates internal pressure as it expands, causing some section of the concrete surface to break through spalling process. Spalling can cause exposure of steel reinforcing steel rods to fire, which might weaken the steel, leading to the collapse of the structure (Schottke et al., 2014, 170). Wood - Wood widely used as a building material in modern environment. It is less expensive, easy to use and can be made into different shapes, that include thin strips used in external siding to heavy structural support beams. Wood can be made from a soft wood or a hard wood. The engineered wood products are prone to warping when they are exposed to humidity. They may also contain toxic products like formaldehyde. In fire conditions, the materials burn and fail quickly and they produce large void spaces (Schottke et al., 2014, 171). The amount of moisture content also affect the rate of ignite in wood. Damp wood takes long to ignite and burn in a fire condition. Usually, new lumber has high moisture content, and it takes longer to ignite compared to old lumber which has been in a building for a long time. In addition, high humidity in the atmosphere makes it more difficult for wood to ignite (Schottke et al., 2014, 172). Wood that has been treated with fire retardant is difficult to ignite and may burn at lower rate. Thus, the wood treated with fire retardant has less fire hazards in a construction. The disadvantage which may arise is that the strength of the wood may be reduced after treatment. In addition, wood which has been treated with fire retardant great danger to the fire fighters as it may cause collapse of the roofing as it cause wood to weaken and deteriorate faster. The roofing may collapse when extra weight is added (Schottke et al., 2014, 172). Plastics - Plastics are synthetic materials which have different properties, and they can be combined with other building materials during construction. They can either be flexible or stiff, opaque or transparent, brittle or tough. Although they are rarely used as a building material, plastics can be found throughout the building. Plastic combustibility varies where some plastics burn quickly, while others burn with the presence of an external source. Others can withstand fire exposure and high temperatures without igniting (Schottke et al., 2014, 173). Plastics become weak, melt and drip as the temperature increase. The materials can be shaped into different forms, but dripping can spread fire rapidly. When plastics burn, they produce dark, dense, heavy smoke and produce high concentration of toxic gases. The kind of smoke produced by plastics resembles those of petroleum products (Schottke et al., 2014, 173). Types of failure modes that can occur within structures Structural failure occurs when a structure or a component in a structure loses the load carrying capacity. It begins when the material experience stress beyond its strength limit, thus causing fracture. Structural components undergo general bending, in-plane loading, and shearing display different failure modes that include compression and tensile facing failure, core failure, indentation of failure, wrinkling of facing and deponding at the facing (Gdoutos, 2003). Facing failure- this failure occurs in form of tensile and compression failure of facings. It is caused by insufficient thickness of the facing or facing strength. (Bitzer, 1997, 59). Face wrinkling – this is a localized buckling of the facing due to compression forces. It occurs when facing buckles like a cover on an elastic foundation. The buckle can be outward or inward, depending on the strength of the compressive forces. Indentation of failure - this failure occurs when the construction components undergo highly localised external forces, and is related to local bending of the facing into the core. Debonding at the facing- this failure occurs due to weak core due to overloads. General bulking - This occur in structural components with large core, which may cause failure through propagation across the material which induce face wrinkling. It occurs in thin walls or if the wall is subjected to shearing forces. This failure may result in shear crimping (Bitzer, 1997, 59). Signs of collapse and collapse hazards of different types of construction Weakening of the structure can be caused by fires, building renovations, earthquake, mining, earthquake etc. It can also be caused by excess loading. Some of the signs that may show signs of collapse include the following. Downward bowing of the roof or floors, which indicate weakening of the column, load bearing of the walls and other supports. Distortion of windows and doors frames or the glass window panes may break. These may indicate the floor or wall movement or twisting of the structure due to torsional forces. V-shaped cracks- this indicate excess pressure on the components in the structure that result in a collapse. A V-shaped crack may indicate the presence of upward pressure that cause the floor or roof to fail if the support ends in a line with it. An inverted V-shape crack indicates downward pressure applied on the component or structure, for example the floors that have fallen away pulling the wall with them. Stepped cracks that lie along the masonry materials may indicate that there is deflection on the floor, foundation, or other supports. Rusting of the external wall and cracks in pad stone support indicate excessive force applied. Iron wall brackets which have been damaged can fail due to extra weight due to movement of the people or movement of equipment or fire fighters. Sliding bricks, plaster, or plaster dust indicate a moving structure, floor or wall. Failure of roof building materials indicates that the connections can damage, or that there is excessive load or the walls may have been damaged. A leaning wall may indicate that the structure has been exposed to forces or loads that exceed the design strength. Therefore, a wall that that is out of plumb may indicate that the connections are damaged or the integrity of the wall has been compromised. Water or a smoke coming from a crack in the floor or wall in a building may indicate internal pressure or heat which may split the wall and may lead to collapse. It may also show that the slabs or the connections are damaged. (Collins, 2004, 62, 63) References Bitzer, T. (1997). Honeycomb technology: Materials, design, manufacturing, applications and testing. London: Chapman & Hall, p59 Collins, L. (2004). Technical rescue operations. Tulsa, Okla: PennWell Corp., p 62-63 Gdoutos, E. E., Marioli-Riga, Z. P., Paipetis, S. P., & Symposium on Recent Advances in Composite Materials. (2003). Recent advances in composite materials: In honor of S. P. Paipetis ; [papers presented at the Symposium on "Recent Advances in Composite Materials" at the Democritus University of Thrace, in Xanthi, Greece on June 12 - 13, 2003]. Dordrecht [u.a.: Kluwer Acad. Publ. Schottke, D., National Fire Protection Association., & International Association of Fire Chiefs. (2014). Fundamentals of fire fighter skills. Jones & Bartlett Publishers, p 169-173 Read More