One World Trade Center was designed by SOM with David Childs as the lead designer. It was co-developed by the Port Authority of New York & New Jersey and the Durst Organization. The skyscraper contains 3.1 million square feet of leasable office space, is 104 stories high, and 1,776 feet tall, making it the tallest building in the western hemisphere. It is scheduled to open in 2014. The square base matches the dimensions of the original twin towers and contains a lobby, retail, and mechanical systems. The windowless base is made of reinforced concrete covered with a metal and glass prismatic cladding. As the form rises in plan, the curtainwall corners gently taper up to the parapet. These facets are intended to reflect the changing colors of the sky and reinforce the iconic building as a beacon of hope. The top houses two observation decks and a restaurant. The spire extends 408 feet above the parapet and will serve as a place to hold broadcasting equipment. The equipment antenna is covered by a sculptural series of densely woven cables that glow with light at night.
Additional Reading: http://time.com/world-trade-center/