Mechanical Properties of Steel and Concrete at Elevated Temperatures - Lab Report Example

PART 3: FAILURE MODES Student Course Date PART 3: FAILURE MODES At elevated temperatures, the mechanical properties of both steel and concrete vary from those at normal temperature conditions. Mechanical properties of steel at elevated temperatures For any steel structure, the mechanical properties of the steel are used to determine the design for fire resistance. For temperature rise and consequently distribution to take place in a steel member a number of factors such as the density, specific heat and the thermal conductivity need to be considered. The mechanical properties affected by the elevated temperatures are mainly the elasticity modulus, the strength, thermal expansion coefficient and lastly the material creep (Yuan and Wang, 2000, 33). The stress-strain relationship for structural steel is shown in figure 1. The relationship shows the curves obtained both at room and elevated temperatures. It can be seen that both the yield and ultimate strength decrease with increase in the temperature. The yield strength of the structural steel at 200C is 300N/mm2 , the strength reduces to 250, 200 and 120 at 2000C, 4000C and 6000C respectively. The strain on the other hand increases with the increase in temperature as can be seen from the figure 1. The ultimate strength of the structural steel also reduces considerably with the elevated temperature. The value of strength obtained at a temperature of 200C is double the ultimate strength achieved at a temperature of 6000C. Therefore, the load carried by the steel at elevated temperature is less. This reduction in the load carried due to the reduction in the strength of steel at elevated temperatures must be considered in the design of steel structures. Figure 1: stress-strain relationships for structural steel at varying temperature Figure 2 shows the relationship between the modulus of elasticity of both the structural steel and the reinforcing bars steel and temperature. The modulus of elasticity is important in the steel structures. The stress that can be sustained in the steel structure is a product of the strain and the steel elastic modulus. At room temperature the elastic modulus EO for common steels is about MPa. As the temperature increases, the strain increases. Since; Where; From the equation, as the strain increases therefore, the elastic modulus provided the stress remains constant. From the figure, as the temperature increases, the elastic modulus reduces at varying rates between the structural and the reinforcing bars. The reducing rate of the elastic modulus for the reinforcing bars is linear explained by their small cross-section which leads to a uniform strain throughout their cross-section. For the structural steel, the elastic modulus does not reduce in a linear manner due to the large cross-section; this therefore means that the strain is not uniform throughout the cross-section (Xiao and König, 2004, 100). In both the steels, it can be seen that there is a significant reduction of the elastic modulus with increase in temperature. The elevated temperatures therefore reduces the elastic modulus of the steel which affect the integrity of the steel. Figure 2: modulus of elasticity versus temperature In figure 3, the relationship between the thermal conductivity and the temperature is obtained. The thermal conductivity of steel reduces with the increase in the temperature. As the steel is heated, it expands resulting to a lower thermal conductivity. On the other hand, the specific heat of the steel increases with the increase in temperature. Elevated temperature therefore, reduces the thermal conductivity and increases the specific heat of the steel. Figure 3: Thermal conductivity versus temperature Mechanical properties of concrete at elevated temperatures Concrete is the most commonly used material in the construction industry. It is can very high stresses safely. Concrete is made up aggregates (coarse and fine aggregated), water and cement as the binder. The normal strength of the concrete used is 20-50 MPa. Concrete can be divided into normal-weight and lightweight concrete depending on the density of the concrete. At elevated temperatures, the mechanical properties of concrete are different from those at normal temperature. In figure 4, the relationship established between the stress and temperature is shown. The unsanded and sanded concrete is considered in coming up with this relationship. The compressive strength of the concrete at elevated temperature reduces (Malhotra, 1965, 382). The rate of the strength reduction varies depending on whether the concrete is sanded on unsanded. When the temperatures are too high, the strength of the concrete will be so low that it cannot withstand the load it was designed for. Figure 4: Stress versus temperature Figure 5 shows the relationship between the elastic modulus and temperature for concrete. The elastic modulus for concrete varies depending on the water-cement ratio in the concrete, the nature and amount of aggregates and the concrete age. The range of the elastic modulus is between MPa to MPa. The elastic modulus of the concrete decreases rapidly with increase in temperature in all the concretes as can be seen from the figure. The rate of decrease will majorly depend on the type of concrete involved. Generally, elevated temperatures result to lower elastic moduli of the concrete involved which ultimately affects the strength of the concrete. Figure 5: Elastic modulus versus temperature Figure 6 shows the relationship between the specific heat and temperature of both the normal-weight and lightweight concrete. The specific heat varies as the temperature increases although a clear relationship cannot be obtained. The rapid increase in the specific heat is experienced between 4000C and 6000C. However, the specific heat at 10000C is higher than that at 500C, this therefore indicates that elevated temperatures result to higher specific heat of the concrete. . Figure 6: Specific heat versus temperature At elevated temperatures spalling of concrete takes place. This is as a result of the heat that the concrete experiences as the temperature is elevated. This leads to the breaking of the concrete from the surface resulting to large explosions. If the spalling is extensive, the stability of the structure is compromised since the reinforcement of the concrete is exposed. A number of factors influence the extent of spalling such as intensity of fire, strength of concrete, the porosity of concrete mix, type of aggregate, concrete moisture content and intensity of load. The spalling is extremely high if the concrete has very high moisture content and is exposed to elevated temperatures. At elevated temperatures, the thermal conductivity and the coefficient of thermal expansion are also lowered in the concrete. Role of fire in inducing structural collapse Fire interferes with the structural integrity of a structure. The mechanical properties of both the concrete and steel elements that make up any structure at elevated temperature are lowered. The main mechanical components that are interfered with at elevated temperatures as a result of fire are the strength, elastic modulus, strain and thermal conductivity of the elements. An example of a structure in which fire induced its collapse was reported in New York City on 9th September 2011. The building was known as World Trade Center 7 (WTC 7). The building had 47 stories. The fires that led to the collapse of the WTC 7 building were uncontrolled; the fire heated the beams of the floor and floor girders that led to the failure of a critical support column. This led to a fire-induced building collapse. The collapse was progressive in nature. Figure 7 shows how the building collapsed due to thermal expansion. This was as a result of the expansion of the steel beams as a result of the fire’s heat. The expansion led to the damaging of the multiple floors framing system. Ultimately, a 13th floor girder lost its link/connection to the 79th (shown in figure 8) column which in this building was the critical column that led to the 13th floor collapse. As the subsequent floors failed, column 79 was left unsupported leading to its buckling that initiated the building global collapse. Figure 7: WTC 7 collapse due to thermal expansion Figure 8: WTC 7 typical floor showing the location of the columns Works Cited B. Wu, J. Yuan, and G. Y. Wang, 2000, 8–15. “Experimental Study on the Mechanical Properties of HSC after High Temperature,” Chinese J. Civil Engineering 33(2) H. L. Malhotra, 1965, 382–88. “The Effect of Temperature on the Compressive Strength of Concrete,” Mag. Corncr. Res. 8(2), Ingberg, S.H. 1928, 3-61. “Tests of the Severity of Fires,” Quartery NFPA. 22, J. F. Muir, 1977, 77-1467. “Response of Concrete Exposed to High Heat Flux on Surface,” Research Paper Sand, Sandia National Laboratories, Albuquerque, New Mexico, J. Xiao and G. König, 2004, 89–103. “Study of Concrete at High Temperature in China—An Overview,” Fire Safety Journal 39, Read More