Sitting on two city blocks and rising one quarter mile (1,454 feet) above the ground, the Tower's 110 stories comprise some 4.5 million gross square feet of office and commercial space. It was designed by the architectural firm of Skidmore, Owings and Merrill, and was constructed in a little less than two and half years. At 1450 ft, the tallest building in the United States. Until recently the world's tallest structure, and still has the highest occupied floor level in the world. 75-ft square tubes end at staggered levels to step back the overall massing.