All told, the design and structural innovations lowered the buildings’ weight, sped the pace of construction, dropped the cost of materials, and increased the anticipated return on what became a $1 billion investment, more than triple the initial cost estimates. Yet those and other advances came with an unwanted, largely overlooked price: they collectively made the Twin Towers of the World Trade Center more susceptible to fire, especially when compared with older buildings whose exteriors were clad in fire-resistant masonry, whose floors were divided into compartments like the hull of a ship,
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