One World Trade Center (previously coined the "Freedom Tower" by Governor Pataki) is the centerpiece of Libeskind's design. The building rises to , the height of the original World Trade Center north tower, and its antenna rises to the symbolic height of 1,776 feet (541 m). This height refers to 1776, the year in which the United States Declaration of Independence was signed. The tower was a collaboration between Studio Daniel Libeskind and Skidmore, Owings & Merrill architect David Childs. Childs acted as the design architect and project manager for the tower, and Daniel Libeskind collaborated on the concept and schematic design. The design was finalized in 2004, but was revised extensively after the NYPD raised security concerns, which delayed the start of the construction by two years. In 2006, the Port Authority took over from Silverstein Properties. The project's developer Tishman Construction Corporation was the construction manager at the time. Construction began in April 2006. Digging the foundation and installing tower-foundation steel columns, concrete, and rebar took twice as long as it normally would due to the existence of the subway line under West Broadway nearby. The building reached grade level by 2010, progressed at a pace of one floor a week, topped out in August 2012, and was structurally completed in May 2013. The building opened on November 3, 2014, and the first 170 employees of anchor tenant Conde Nast began their work there.