Khalifa Building Material - Assignment Example

Briefly discuss the main aspects of behaviour of plastics, steel, concrete and wood under normal conditions and under fire conditions. Plastics Plastics re made of long molecular chains of carbon compounds which gives them some special properties that lacks in other materials like metals. Plastics are light in weight, are resistant to corrosion and their fabrication is easy. Plastics will start being soft, contract and then melt on exposure to temperatures of 1000C or more. At high temperatures plastic material forms combustible gaseous products when the materials decompose after melting. There can be then ignition of the gases by a flame or sparks depending on how high the temperature is, the duration of exposure to heat and the level of ventilation in the vicinity of the material. Timber Timber will undergo various colour changes when exposed to high temperature. At 120 to 150 C the timber will turn brown at the temperature of 200 to 250 C it will turn to black while there will be emission of combustible vapours at temperatures of 400 to 450 C. Above the temperature of 400 there will be ignition of the surface of the timber and the timber will char at a steady rate. Timber is a widely used material in construction industry even though it is highly combustible. There are inherent features with timber that gives it good performance on exposure to most building fires. Timber is not easy to ignite and when it ignites it will form char on the outer surface which is a solid residue resulting from the decomposition of the and mostly made of carbon. The spread of fire clues can be obtained by analyzing the char pattern and relative depth and appearance. The rate at which char is formed will differ with the wood species. This is because timber of different species will vary in bulky density (Buchanan , 2002). The heat flux incident on the wood and the moisture content will also affect the rate of char formation. With timber having low thermal conductivity combined with formation of a protective char layer, heavy timber sections may be able to provide significantly high level of fire resistance. Use of a larger section of timber excess of what is actually needed to carry the design load where the excess will be sufficient to protect the member for a certain period is a design concept that has been adapted and is referred to as sacrificial timber concept. Steel In modern construction industry steel is the most important structural material where it is either used as structural steel or in reinforced concrete as reinforcing steel. Reinforcing steel can come in the form of reinforcing bars or as high tensile strength steel tendons applied in pre-stressed concrete. Structural steel has higher vulnerability to fire in comparison to reinforcing steel which where the steel is embedded in concrete which insulates the steel from high temperature and thus ensuring that the steel is protected from significant losses in strength. Steels have very high thermal conductivity and are usually used in thin sections and this makes to be predisposed to heating up in a short duration when exposed to fire when there is no insulation. It is due to this reason that it is a requirement by current design codes that main structural members to have insulation. The rate at which heating takes place is dependent upon thermal conductivity, the density and specific heat. The average density of steel is 7850 kg/m3 while thermal conductivity is approximated at 54 W/mK at room temperature which reduces to about 27 W/mK when the temperature rises to around at 8000C where simple calculation as involved the specific heat capacity of steel can be approximated to 600 J/kg_K (Drysdale, 1998). . When steel is exposed to fire its strength will decrease as well as its ability to withstand deformation. There is change to other properties when steel is exposed to fire for a prolonged period of exposure. Concrete The coefficient of thermal expansion for concrete is not very different from that of steel other for some class of concrete this may be higher. The stresses in the concrete also plays a major role in the level of concrete thermal expansion where the thermal expansion coefficient is considerably lowered where there is large compressive stresses and lower in unstressed concrete. This can be attributed to the fact creep effects become more pronounced in concrete at a temperature of about 4000C where there is considerable increase in strains for small change in temperature beyond this point ( Di Nenno,1995). This is what is referred to as transient thermal creep and the compressive strength can be so large to completely counter the thermal expansion effect with the possibility of even having contraction. There is also significant reduction in concrete stiffness at high temperatures and this also results in additional strains. With concrete having low thermal diffusivity its strength loss is becomes low. In concrete design it important to know the relationship between the strength of the concrete and temperature. Spalling is one of the most destructive effects of fire on concrete which constitutes of loss of material ranging from superficial damage large chunks of materials being blown out explosively. The degree of spalling is generally dependant of the type of aggregate used. In aggregates rich in silicates there is a sudden increase in volume at certain temperature levels resulting from crystal structure changes. Where the fires are severe the changes in volume may be very rapid that it result to very high internal stresses causing spalling of surface layers and as a result there will be exposure of the reinforcing steel and this may lead to lose of bearing capacity of the structure. Spalling involves having lines of striation and surface materials being lost a result of which is having chipped, cracked broken or cratered appearance. Since and kind of differential expansion may cause spalling it therefore follows that having rapid cooling may also result to spalling this being possible when water is used to extinguish the fire. Even though spalling usually occurs at very high temperatures it is also possible for spalling to be experienced in areas of concrete or masonry where there is very high compressive stress even when the temperatures are relatively low further away from the hottest regions. The spalled areas are likely to appear lighter in colour in comparison to areas adjacent by virtue of exposure of clean subsurface materials. Colour changes in concrete may be used in giving a rough estimate of temperatures reached and residual strength. • Grey: (Under 300_C) Minimal loss of strength • Pink, red or red/brown: (300-650_C) 10 to 60 % of the strength is lost . • Grey-white: (650-900_C) 60 to 100% loss of strength. • Buff: (Over 1000_C) Sintered. Identify and discuss the different types of failure modes that can occur within structures. Buckling In buckling there a sudden failure of a structural member that where the member is subjected to intense compressive stresses even though the actual compressive stress at the point where failure occurs is usually well below the ultimate compressive stresses which the material can withstand. Sometimes buckling is described as failure due to elastic instability Creep In creep we have solid materials having permanent deformation as a result of being influenced by stresses. It occurs when a member is exposed to stresses which are lower that the yield strength but for a considerably long period. Fatigue This is a progressive and localized structural damage which will happen when a member is subjected to a load that cyclic in nature. The maximum stresses in the member will be less than the ultimate tensile stress limit which may also have a lower value than the yield stress limit of the material. Brittle Fracture In this failure there is no apparent plastic deformation of the material before fracture taking place. From the case of crystalline material there can be occurrence of fracture through cleavage due to tensile stress acting perpendicular to crystallographic plane which have low bonding strength or the cleavage planes. For the case amorphous solids the absence of crystalline structure means a concordat fracture, with cracks proceeding normal to the applied tension. Impact In the field of mechanics, an impact is described as being a high force or shock applied over a very short time period when two or more bodies are involved in collision. Such a force or acceleration will usually have greater effect than a lower force applied over a proportionally longer time period of time. The effect of the impact will depend critically on the relative velocity of the bodies to one another. Discuss the signs of collapse and collapse hazards of different types of construction. A building having cracks and bulges in walls and hearing of unusual noise from the building is some of the manifestation that the building may be collapsing. The different types of collapse hazards include weak columns, walls and beams that lack adequate strength to support the structure. Freestanding walls and loose chimneys may easily fall due to lack of support or wind load. Overhead hazards include loose debris, sections of concrete that could be hanging precariously on reinforcing bars. References Akintunde E.O., 2008 Engineering Properties of Locally Manufactured Burnt Brick Pavers forAgrarian and Rural Earth Roads. American Journal of Applied Sciences 5 (10): 1348-1351, 2008 Available at: http://www.scipub.org/fulltext/ajas/ajas5101348-1351.pdf Accessed 17th , March 2015 Buchanan. A.H. (2002). Structural Design for Fire Safety. John Wiley and Sons Ltd, 2002. Consultation paper (2009) Removal of restrictions on the self-certification of the installation of combustion appliances: Di Nenno, P.J. (1995). SFPE Handbook of Fire Protection Engineering. Society of Fire Protection Engineers, Drysdale. D. (1998). An Introduction to Fire Dynamics. John Wiley and Sons, 2nd edition, 1998. Helmenstine, A. M., (2010). Portland cement. Cement and Concrete Basics Available at: http://www.cement.org/basics/concretebasics_aggregate.asp Accessed 17th , March 2015 Planning, building and the environment: Building Regulations Available at: http://www.communities.gov.uk/planningandbuilding/buildingregulations/ Accessed 17th , March 2015 Read More

Use of a larger section of timber excess of what is actually needed to carry the design load where the excess will be sufficient to protect the member for a certain period is a design concept that has been adapted and is referred to as sacrificial timber concept. Steel In modern construction industry steel is the most important structural material where it is either used as structural steel or in reinforced concrete as reinforcing steel. Reinforcing steel can come in the form of reinforcing bars or as high tensile strength steel tendons applied in pre-stressed concrete.

Structural steel has higher vulnerability to fire in comparison to reinforcing steel which where the steel is embedded in concrete which insulates the steel from high temperature and thus ensuring that the steel is protected from significant losses in strength. Steels have very high thermal conductivity and are usually used in thin sections and this makes to be predisposed to heating up in a short duration when exposed to fire when there is no insulation. It is due to this reason that it is a requirement by current design codes that main structural members to have insulation.

The rate at which heating takes place is dependent upon thermal conductivity, the density and specific heat. The average density of steel is 7850 kg/m3 while thermal conductivity is approximated at 54 W/mK at room temperature which reduces to about 27 W/mK when the temperature rises to around at 8000C where simple calculation as involved the specific heat capacity of steel can be approximated to 600 J/kg_K (Drysdale, 1998). . When steel is exposed to fire its strength will decrease as well as its ability to withstand deformation.

There is change to other properties when steel is exposed to fire for a prolonged period of exposure. Concrete The coefficient of thermal expansion for concrete is not very different from that of steel other for some class of concrete this may be higher. The stresses in the concrete also plays a major role in the level of concrete thermal expansion where the thermal expansion coefficient is considerably lowered where there is large compressive stresses and lower in unstressed concrete. This can be attributed to the fact creep effects become more pronounced in concrete at a temperature of about 4000C where there is considerable increase in strains for small change in temperature beyond this point ( Di Nenno,1995).

This is what is referred to as transient thermal creep and the compressive strength can be so large to completely counter the thermal expansion effect with the possibility of even having contraction. There is also significant reduction in concrete stiffness at high temperatures and this also results in additional strains. With concrete having low thermal diffusivity its strength loss is becomes low. In concrete design it important to know the relationship between the strength of the concrete and temperature.

Spalling is one of the most destructive effects of fire on concrete which constitutes of loss of material ranging from superficial damage large chunks of materials being blown out explosively. The degree of spalling is generally dependant of the type of aggregate used. In aggregates rich in silicates there is a sudden increase in volume at certain temperature levels resulting from crystal structure changes. Where the fires are severe the changes in volume may be very rapid that it result to very high internal stresses causing spalling of surface layers and as a result there will be exposure of the reinforcing steel and this may lead to lose of bearing capacity of the structure.

Spalling involves having lines of striation and surface materials being lost a result of which is having chipped, cracked broken or cratered appearance. Since and kind of differential expansion may cause spalling it therefore follows that having rapid cooling may also result to spalling this being possible when water is used to extinguish the fire. Even though spalling usually occurs at very high temperatures it is also possible for spalling to be experienced in areas of concrete or masonry where there is very high compressive stress even when the temperatures are relatively low further away from the hottest regions.

Read More