Final answer:
The expansion of steel members is due to thermal expansion, which depends on the steel's coefficient of thermal expansion, the original length, and the change in temperature. While 4 inches of expansion suggests high temperatures, a definitive temperature cannot be identified without specific initial conditions.
Step-by-step explanation:
The steel structural members in question likely expand due to thermal expansion, a physical phenomenon where materials change in size and shape when exposed to changes in temperature. The coefficient of linear expansion for steel, typically around 12×10-6 /°C, is used along with the initial length of the material and the change in temperature to calculate the change in length using the formula ΔL = αLΔT, where ΔL is the change in length, α is the coefficient of linear expansion, L is the original length, and ΔT is the change in temperature.
Given that structural steel can expand by about 4 inches (100 mm) when heated, the question asks at which temperature this occurs. The specific temperature at which such an expansion takes place can be largely variable, depending on the length of the steel member and the exact coefficient used, but extreme expansions are associated with higher temperatures, easily exceeding 500°F (260°C). Without specific values for the starting length of the steel members and the temperature at which they are initially measured, we can only estimate that such an expansion likely occurs at the higher end of the given temperatures, suggesting answers C or D would be most probable. However, without the required initial conditions or additional context, a definitive answer cannot be provided.
In practical applications, engineers must take into account potential expansion and stress due to temperature changes to ensure the integrity and safety of structures, as illustrated in various examples provided in the question's background information.