Fire Resistant Materials and Structures - Essay Example

FAILURE MODES by name Course Professor’s Name Institutional Affiliation City and State Date of Submission Failure Modes Construction Materials and their Characteristics It is important for firefighters to have an understanding of the basic types of construction materials and recognize how these materials affect fire growth and spread. The most frequently used construction materials include plastics, gypsum board, glass, aluminium, steel, concrete, masonry, engineered wood products, and wood[daS05]. The main factors that impact the behaviour of all these materials under fire conditions includes thermal expansion once heated, decrease in strength at higher temperatures, thermal conductivity, and combustibility[Gil10]. Masonry building materials include brick, concrete blocks, and stones. These building materials are normally bonded together with the use of mortar. Masonry building materials are fire resistive and poor conductors of heat. However, masonry structures may collapse under fire conditions if the floor or roof assembly collapses. Concrete is also a fire resistive construction material that has poor heat conductivity. It is usually used to insulate other construction materials from fire. The main benefit of concrete as a construction material is that it does not lose strength when it is exposed to elevated temperatures[daS05]. The strength of concrete building can be enhanced by embedding steel reinforcing rods within it. Nevertheless, high heat from the fire has the capability of converting the moisture trapped in the concrete to stream, and this will cause spalling[daS05]. A severe spalling may expose the steel reinforcing rods to heat, which will affect the strength of the structure. Concrete thermal conductivity depends on the type of aggregate used to make it. They are three types of aggregate used to make concrete, and each has different thermal conductivity: siliceous aggregate, calcareous aggregated, and lightweight aggregates[Gil10]. Lightweight concrete has the best conductivity, calcareous concrete is a bit stable, but siliceous aggregates have a tendency of spalling due to high thermal conductivity. Steel is usually used in the structural framework of buildings and is considered to be the most valuable structural material in modern construction[Gil10]. Steel building material is resistant to aging and very strong, but it is affected by rusting. Structural steel is more vulnerable to fire as compared to reinforcing steels which are usually encased in concrete, providing them with good insulating properties that protect them from significant losses in strength. One of the major weaknesses of steel is that it not fire resistive and has the ability of conducting heat. Therefore, it can expand and lose strength when exposed to heat. The risk of failure associated with a steel structure depends on many factors such as the technique used to connect the steel components, the load placed on the components, and the mass of the steel components. If one notices a sign of stretching, sagging or bending in a steel structure, this should be taken as a warning of impending collapse[daS05]. Engineering wood products are also referred to as composite wood, manmade wood, and manufactured board[Gil10]. Engineering wood products are manufactured from slender pieces of wood joined with adhesives or glues such as polyurethane resins, melamine-formaldehyde resins, phenol-formaldehyde resins, and urea-formaldehyde resins[Gil10]. These products may release toxic fumes when in fire and warp under high humidity. The most significant characteristic of engineered wood products and wood is their high combustibility. The rate at which wood decomposes, burns and ignites depends on a number of factors such as the size and form, preheating, density, moisture and ignition[Gil10]. The strength of wood is reduced whenever it is heated. Since the wood does not expand when heated, it does not threaten adjoining masonry like concrete and steel. It is not possible to treat wood to make it non-combustible; however, it is possible to make it more difficult to burn and ignite by applying a fire-retardant treatment[Gil10]. Plastics are not commonly used for structural support, but they can be found all over the building. The combustibility of plastics significantly differs as it depends on the material of the plastic. Most of the plastic produce high concentrations of toxic gases and release quantities of dense, heavy, dark smoke[Gil10]. The key disadvantage of plastics is that they are all combustible. Some treatments can be used on plastics to inhibit flame spread and increase ignition temperature. Unfortunately, nothing may be added to plastics to make the non-combustible[Gil10]. Failure Modes that Occurs within Structures An important element of structural hazard evaluation is the assessment of the probable failure modes of the remaining debris and the probability of their occurrence. In this assessment, the following failure modes should be considered: toppling or overturning, story mechanism, buckling/crushing of columns/walls, flexural/shear failure of slabs/beams, dropping/sliding/shifting of elevated failed components, shifting debris pile, and falling of loose debris[FEM09]. The types of failure modes provided above indicate that failure of a structure may occur from a number of problems. Most of the problems that cause failure of a structure are unique to the different industries or the type of structure. Nonetheless, most of them can be associated with several causes as discussed below. A failure may occur because of the choice of material, its shape or its size, which might make it not strong enough to support the structure load. If a structure is not firm enough to support the load, it can cause a catastrophic failure when the overstressed structure attains a critical stress level. Instability as a result of material choice, design or geometry may also cause failure of the structure from corrosion or fatigue[Cha05]. This kind of failure happens at stress points, which cause cracks to form slowly and then develop through cyclic loading. Failure normally takes place when these cracks achieve a critical length and cause breakage to occur unexpectedly under normal loading conditions. Another cause of failures is manufacturing errors. Manufacturing errors can be caused by careless workmanship, failure to adhere to the design, improper heat treating, incorrect sizing, or improper selection of materials[Cha05]. Failures caused by manufacturing errors may occur suddenly and are normally unpredictable. Using defective materials may also cause an unpredictable failure. Such materials could be damaged from previous use or improperly manufactured. Finally, failure of structures can also be caused failure to consider unexpected problems such as natural disasters, sabotage, and vandalism which can overstress a structure to the point of failure[FEM09]. The Signs of Collapse and Collapse Hazards The main sign of collapse in masonry and reinforced concrete is cracks. Other signs of collapse include leaning or badly cracked walls; offset residence from foundation; leaning 1st story in multi-story structures; leaning, cracked masonry veneer or chimney; separated porches, split level roof or floors; peeled walls (split thickness); loose, broken ornamentation and parapets; and partly collapsed and unsupported floors[FEM00]. Cracks are normal in concrete structures, but there are those that should cause an alarm. Cracks in concrete structures are caused by predictable structural behaviour, temperature change, and shrinkage. There are several cracks that can occur in masonry and reinforced concrete. They include diagonal tension cracks in walls, diagonal tension cracks in beams, tension cracks, temperature cracks, and shrinkage cracks[FEM00]. Temperature and shrinkage cracks occur in columns, beams, walls, and slabs. Diagonal cracks originate in re-entrant corners in walls and slabs. Significant diagonal tension cracks in masonry walls are likely to cause sudden, brittle failure[FEM00]. There are three major types of hazards in partly collapsed, damaged and collapsed structures. They can be classified as falling hazards, collapse hazards and other hazards[FEM00]. Failing hazard is where part of a structure or its components is at risk of falling. Collapse hazard is in which the volume of the enclosed space acquired by the structure will be reduced, as a result of stability loss. Other hazards include asbestos, carbon monoxide, toxic gases, and other hazardous materials[FEM00]. The degree of hazard in collapse and falling hazards is strongly associated with mass and how more failure nay take place[FEM00]. The potential of a brittle, sudden failure should be recognized as compared to structures in which redundant configuration and material ductility may provide some signs of an additional collapse. Debris and small, non-structural components can be greater hazards than the whole structural stability, particularly in small aftershocks and wind gusts[FEM00]. Reference List daS05: , (da Silva, et al., 2005, p. 486), Gil10: , ( Gillie, 2010, p. 1), daS05: , (da Silva, et al., 2005, p. 490), Gil10: , ( Gillie, 2010, p. 8), Gil10: , ( Gillie, 2010, p. 9), daS05: , (da Silva, et al., 2005, p. 499), Gil10: , ( Gillie, 2010, p. 11), Gil10: , ( Gillie, 2010, p. 12), FEM09: , (FEMA US&R Response Sys & U.S. Army Corps of Engineers, 2009, p. 5), Cha05: , (Chaturvedi, 2005, p. 151), Cha05: , (Chaturvedi, 2005, p. 152), FEM09: , (FEMA US&R Response Sys & U.S. Army Corps of Engineers, 2009, p. 11), FEM00: , (FEMA National US&R Response System, 2000, pp. 2-11), FEM00: , (FEMA National US&R Response System, 2000, p. 4), FEM00: , (FEMA National US&R Response System, 2000, p. 7), FEM00: , (FEMA National US&R Response System, 2000, p. 22), FEM00: , (FEMA National US&R Response System, 2000, p. 24), Read More