The Comcast Center is the second-tallest building built in America outside of New York City or Chicago since 1993. The Comcast Center has 58 floors, of which 56 are occupiable. The structure of the Comcast Center comprises a central concrete core with steel-framed floors. The building's exterior features a glass curtain wall made of lightly tinted, non-reflective low-emissivity glass. The tower tapers inward towards the top and features two cutouts near the top of the building on the north and south sides. To prevent the tower from swaying too much in the wind, the Comcast Center contains a double-chambered concrete tuned liquid column damper, the largest such damper in North America. Receiving a gold Leadership in Energy and Environmental Design (LEED) rating in April 2009, the Comcast Center is the tallest LEED building in Philadelphia. The building was designed to use 40 percent less water than a typical office building, and the plaza was designed to reduce heat-island effect from the pavement by 70 percent. Reducing air conditioning and lighting costs, the low-emissivity glass curtain wall blocks 60 percent of heat while allowing 70 percent of the Sun's light inside.