At least three skyscrapers under construction that will surpass the height of the world's tallest building, Taipei 101 in Taiwan, are using the concrete-core technique (as well as a number of others under proposal in Russia and Korea). Indeed, the 1,776-foot high Freedom Tower, the anchor of the World Trade Center site, will in many ways simply be a larger version of the adjacent 7 WTC. The Chicago Spire, at 2,000 feet high, and Burj Dubai, soon to be the tallest building in the world at a staggering 2,700 feet, will also each have hollow concrete spines anchoring floors that will cascade and twist around them.
Making buildings with a concrete core isn't a new idea, but the cost of constructing them in the past has been prohibitive. "The main drawback at one time was that a steel frame was so much faster to build," says Mir Ali, professor of architecture at the University of Illinois at Urbana-Champaign. "It took you approximately three to four days to build a steel-frame floor. With concrete, it used to take 10 to 14 days a floor."
But with advances in construction techniques – better and cheaper concrete, more powerful pumps, easy-to-assemble slip and fly forms – crews can now put up a concrete core as fast as a steel frame. Moreover, very tall steel-frame buildings like the former World Trade Center towers and the Sears Tower often shimmy and sway in the wind. The top floor of a concrete-core high rise is as solid as a first-floor lobby.