"Until the World Trade Center was built, most skyscrapers were supported by simple steel or concrete frames."

"But that meant that interiors were interrupted by columns. For the Trade Center, architects and engineers, including Mr. Robertson, sought to create column-free expanses for commercial tenants. He did that by making the towers giant steel tubes, with about half of the weight borne by exterior columns. The rest of the weight was carried by the towers’ steel-and-concrete cores. Floors were supported by lightweight steel trusses linking the exterior columns to the cores, giving tenants column-free spaces measuring about three-quarters of an acre. According to Mr. Robertson, the buildings had been designed to withstand the impact of a Boeing 707, but the planes flown into the towers were heavier 767s. And his calculations had been based on the initial impact of the plane; they did not take into account the possibility of what he called a 'second event,' like a fire. When the planes struck the towers, they sliced through the steel frames, but the buildings remained standing. Many engineers concluded that conventionally framed buildings would have collapsed soon after impact. The twin towers stood long enough to allow thousands of people to escape."

From "Leslie Robertson, Who Engineered the World Trade Center, Dies at 92/He remained proud of the towers, which stood long enough for thousands to escape on 9/11, but carried with him 'a troubled heart'" (NYT).