Hello, I'm a materials science expert with a special interest in the properties of steel and its applications in construction. I'm here to provide you with a detailed and accurate response to your question regarding the temperature at which steel weakens.

Steel is an alloy primarily composed of iron and carbon, and it is widely used in construction due to its high strength, ductility, and toughness. The performance of steel at elevated temperatures is a critical consideration for safety and structural integrity, especially in high-rise buildings and other critical infrastructure.

It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C. This is a significant point in the material's performance characteristics, as it indicates a substantial reduction in the steel's ability to carry loads and resist deformation. The softening of steel at these temperatures is due to the reduction in the material's yield strength, which is the point at which the steel begins to undergo plastic deformation rather than elastic deformation.

However, it is important to note that steel does not suddenly lose all of its strength at a specific temperature. The loss of strength is a gradual process that occurs over a range of temperatures. The rate at which steel loses its strength depends on several factors, including the specific alloy composition, the presence of impurities, and the duration of exposure to high temperatures.

In the context of structural applications, such as in buildings, the design of steel structures typically accounts for the potential loss of strength at elevated temperatures. Engineers use various methods to ensure that structures are safe and can withstand the expected loads, even under extreme conditions. This includes the use of fire-resistant materials, structural redundancy, and other design features that enhance the resilience of the building.

Furthermore, the 50% loss of strength at 650°C, while significant, does not by itself explain complex structural failures, such as the collapse of a building. Structural failures are often the result of multiple factors, including the load path, the presence of other materials, and the overall design and construction of the structure. It is also crucial to consider the role of other structural elements and the interaction between different materials under extreme conditions.

In the case of the World Trade Center (WTC) collapse, it is widely accepted that the failure was not solely due to the weakening of steel at high temperatures. The collapse was a result of a combination of factors, including the impact of the aircraft, the subsequent fires, and the structural design of the buildings. The fires caused by the impact led to a complex set of conditions that affected the overall performance of the structure, and it is the interaction of these factors that ultimately led to the tragic events.

In conclusion, steel does weaken at elevated temperatures, with significant softening occurring around 425°C and a substantial loss of strength at 650°C. However, the behavior of steel in structural applications is complex and depends on a variety of factors. It is essential to consider the broader context of structural design, material properties, and environmental conditions when assessing the performance of steel at high temperatures.

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It is known that structural steel begins to soften around 425--C and loses about half of its strength at 650--C. This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse.read more >>