building's 150 stories rotated 2.4 degrees from the one below it for a total 360-degree rotation. In February 2008, prices for the condominiums were announced as ranging from $750,000 to US$40 million. For supplemental structural support, each floor was to be surrounded by cantilevered corners and four concave sides. Similar to the Willis Tower (formerly Sears Tower) and John Hancock Center observation decks, the Chicago Spire design included a community room on the top floor offering residents a view of four U.S. states. The design for the soaring four story lobby of the skyscraper included translucent glass walls framed by arching, steel-reinforced concrete vaults. The building has been described as a giant "drill bit" by the public and others in the media have likened it to a "tall twisting tree" and a "blade of grass". The curved design offered two major benefits to the structure of the building. First, curved designs, such as that found in Calatrava's Turning Torso in Malmö, Sweden, tend to add to the strength of a structure. A similar principle has been used in the past with curved stadium roofs. In addition to structural support, the curved face of the exterior would minimize wind forces. In rectangular buildings, a fluid wind flow puts pressure on the windward face of the building; while air moves around it, a suction is applied to the leeward face. This often causes a sway in tall buildings usually counteracted, at least partially, by stiffening the structure or by using a dynamic wind damper. Since the curved design of the Chicago Spire would not completely negate wind forces, a tapering concrete core and twelve shear walls radiating from it were planned to counteract the remaining wind load. Additionally, the Chicago Spire was designed with world-class sustainable engineering practices to meet Leadership in Energy and Environmental Design Gold recognition. Sustainable features included recycled rainwater, river water used for cooling, ornithologically-sensitive glass to protect migratory birds, intelligent building and management systems, waste storage and recycling management, and monitored outdoor air delivery. Construction Following the city approval, it was announced that construction of the Chicago Spire was to begin in the summer of 2007, with caisson work scheduled to begin as early as June 2007. DuSable Park was designated as a staging area for the construction of the tower. The sales center for the Chicago Spire opened on January 14, 2008. On September 19, 2008,