<P> The Willis Tower was the first building to use Khan's bundled tube structure . This innovative design was structurally efficient and economic: at 1,450 feet, it provided more space and rose higher than the Empire State Building, yet cost much less per unit area . This structural system would prove highly influential in skyscraper construction . It has been used in most supertall buildings since then, including the world's tallest building, the Burj Khalifa . To honor Khan's contributions, the Structural Engineers Association of Illinois commissioned a sculpture of him for the lobby of the Willis Tower . </P> <P> Sears executives decided that the space they would immediately occupy should be efficiently designed to house their Merchandise Group, and that floor space for future growth would be rented out to smaller firms and businesses until Sears could retake it . The latter floor areas had to be designed to a smaller plate, with a high window - space to floor - space ratio, to be attractive and marketable to prospective lessees . Smaller floorplates required a taller structure to yield sufficient square footage . Skidmore architects proposed a tower with large 55,000 - square - foot (5,100 m) floors in the lower part of the building, and gradually tapered areas of floorplates in a series of setbacks, which would give the Sears Tower its distinctive look . </P> <P> As Sears continued to offer optimistic projections for growth, the tower's proposed floor count rapidly increased into the low hundreds, surpassing the height of New York's unfinished World Trade Center to become the world's tallest building . The height was restricted by a limit imposed by the Federal Aviation Administration (FAA) to protect air traffic . The financing of the tower was provided by the Sears company . It was topped with two antennas to permit local television and radio broadcasts . Sears and the City of Chicago approved the design, and the first steel was put in place in April 1971 . The structure was completed in May 1973 . The construction cost about US $150 million at the time, equivalent to $830 million in 2018 . By comparison, Taipei 101, built in 2004 in Taiwan, cost around the equivalent of US $2.21 billion in 2018 dollars . </P> <P> Black bands appear on the tower around the 29th--32nd, 64th--65th, 88th--89th, and 104th--108th floors . These are louvres that allow ventilation for service equipment and obscure the structure's belt trusses . Even though regulations did not require a fire sprinkler system, the building was equipped with one from the beginning . There are around 40,000 sprinkler heads in the building, installed at a cost of $4 million . </P>

How much did it cost to build the sears tower