Learn about changes to the design of 7 World Trade Center, the first tower rebuilt at Ground Zero after the 9/11 terror attacks. In this video from NOVA Online, developer Larry Silverstein explains why he and architect David Childs made changes to the original design. The building opened in May, 2006. It is 1.7-million square feet and 52-stories high. Silverstein?s firm, Silverstein Properties, is on the 38th Floor.