英語時差：世貿(mào)大廈

Amid the shocking events that occurred on September 11th, one of the most startling was the sight of the collapse of the Twin Towers soon after hijacked planes hit them. In this Moment of Science we explain how and why the collapse occurred. Designed and built in the 1960s, each tower had a tube structure, meaning that the concrete-encased steel columns that held the building up and bore its weight were on the building's exterior. Inside, each Tower had a vertical steel core at its center that housed stairwells and elevators. Steel beams radiated outward from the core, supporting the floors and attaching to the exterior columns. Part of the reason that the buildings collapsed as they did is because aside from the floor reinforcements, there were no columns holding the building up from the inside.

When the planes hit they severely damaged the exterior columns. But the impact alone was not what caused the collapse; it was the intense heat generated by exploding airplane fuel. Each plane carried 24,000 gallons of fuel, enough to create fires hotter than 1472 degrees Fahrenheit. Exposed to such intense heat, both the exterior columns and floor beams began to weaken. Consequently, the upper floors caved in, collapsing the floors below. The accumulated weight was enough to destroy the entire building.

Even though they eventually gave way, the Towers' structure saved thousands of lives by withstanding the heat for as long as they did, allowing many of the buildings' occupants to reach the ground. Had they been designed differently, the Towers might have fallen over like giant trees, creating a disaster of even larger proportions.