Final answer:
The correct answer is C. 900°F (482°C). At temperatures above this, unprotected steel will expand and potentially deform. These temperatures are accounted for in engineering designs to ensure the structural integrity of steel constructions under different temperature conditions.
Step-by-step explanation:
When exposed to temperatures above 900°F (482°C), unprotected steel will expand and twist. This is because the thermal expansion of steel is a function of temperature. As per physics, when substances like steel are heated, their particles begin moving more rapidly, which in turn increases the space between particles, causing the material to expand.
Steel starts to lose its strength and stiffness around this temperature, and the risk of deformation becomes significant. This is very important to consider in structural engineering and design to prevent structural failures in the event of a fire. Railway tracks, for instance, are built with expansion gaps to accommodate for temperature-induced changes in length. The problem related to structural integrity under thermal stress is that as the steel expands, any constraints can introduce significant internal forces, which can lead to deformation or failure.
For example, the Eiffel Tower expands by nearly 6 inches (15 cm) in hot weather due to this exact effect of thermal expansion. Moreover, calculations such as the change in length for a steel railroad rail can be made using the equation for linear thermal expansion ΔL = αLΔT, where α is the coefficient of linear expansion, L is the original length, and ΔT is the change in temperature.