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The iconic twin towers, standing tall against the backdrop of the night sky.
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- The World Trade Center incorporated 12 distinct steel grades, with yield strengths reaching up to 100ksi.
- The steel thickness in the towers' external walls varied depending on the height, with lower sections featuring steel as thick as 4 inches (10 centimeters) and upper floors using steel as thin as 0.25 inches (6 millimeters) to effectively manage varying stress levels.
- Although steel has a high melting point, the combination of the plane impacts and ensuing fires compromised the structural integrity of the World Trade Center, resulting in the towers' collapse. The removal of fireproofing materials left critical structural components vulnerable to extreme heat, leading to a devastating structural failure.
Given the immense scale of the original World Trade Center, it’s no surprise that a variety of steel grades—12 in total—were employed during its construction. These ranged from the widely used A36 steel, with a yield strength of 36ksi (kilopounds per square inch), to much stronger variants capable of withstanding up to 100ksi [source: FEMA].
The yield strength of steel refers to the maximum stress it can endure before permanently deforming. For instance, A36 steel, a blend of carbon and iron like all steels, can withstand 36,000 pounds per square inch (36 kilopounds) of pressure before losing its original shape.
The external walls of the World Trade Center's twin towers featured steel of varying thicknesses to accommodate differing pressure loads at various heights. Lower levels typically used steel up to 4 inches (10 centimeters) thick, while upper floors utilized steel as thin as 0.25 inch (0.64 centimeter) [source: FEMA].
For the floors, a combination of A36 and ASTM A 242 steel was utilized. ASTM A 242, a high-strength, low-alloy (HSLA) steel, provided exceptional strength while reducing the overall weight of the structure, as less material was needed.
In total, approximately 200,000 tons (181,436 metric tons) of steel were used in the construction of the World Trade Center, which, at the time, stood as the largest buildings on the planet [source: Ross].
Next, discover what became of all that steel on September 11, 2001.
September 11, 2001
A common argument among 9/11 conspiracy theorists is that the steel used in the World Trade Center couldn't melt at the temperatures generated by the fires after the jetliners struck the buildings. This claim is technically correct.
The melting point of steel is 2,750 degrees Fahrenheit (1510 Celsius). However, jet fuel burns at temperatures ranging from 800 to 1500 degrees Fahrenheit (426.7 to 815.5 Celsius) [source: Popular Mechanics]. So, what caused the towers to collapse on that September morning?
The steel deformed under the extreme conditions.
Although the steel girders of the twin towers wouldn't melt in the jet fuel fires, they would undoubtedly weaken under the intense heat. Estimates suggest that at 1,100 degrees Fahrenheit (593.3 Celsius), they could lose up to half their strength [source: Popular Mechanics]. Additionally, other flammable materials within the buildings could have ignited, potentially raising the overall temperature of the fires.
However, a comprehensive study by the National Institute of Standards and Technology (NIST) revealed that even the combined effects of the fires and the planes' impact weren't sufficient to cause the buildings' collapse. The report identified the dislodgement of fireproofing materials during the impact as the primary factor, exposing critical structural elements to extreme heat. As these components weakened, the floors sagged, pulling the compromised columns inward and triggering a cascading failure that led to the towers' eventual collapse [source: NIST].
