On April 26, 2006, the Port Authority of New York and New Jersey approved a conceptual framework that allowed foundation construction to begin. A formal agreement was drafted the following day, the 75th anniversary of the 1931 opening of the Empire State Building. Construction began in May; a formal groundbreaking ceremony took place when the first construction team arrived. The symbolic cornerstone of One World Trade Center was laid in a ceremony on July 4, 2004. The stone had an inscription supposedly written by Arthur J. Finkelstein. However, construction was delayed until 2006 due to disputes over money, security, and design. The last major issues were resolved on April 26, 2006, when a deal was made between developer Larry Silverstein and the Port Authority of New York and New Jersey, so the cornerstone was temporarily removed from the site on June 23, 2006. Soon after, explosives were detonated at the construction site for two months to clear bedrock for the building's foundation, onto which of concrete was poured by November 2007. In a December 18, 2006, ceremony held in nearby Battery Park City, members of the public were invited to sign the first steel beam installed onto the building's base. It was welded onto the building's base on December 19, 2006. Foundation and steel installation began shortly afterward, so the tower's footings and foundation were nearly complete within a year. In January 2008, two cranes were moved onto the site. Construction of the tower's concrete core, which began after the cranes arrived, reached street level by May 17. However, construction of the base was not finished until two years later, after which construction of the office floors began, and the first glass windows were subsequently installed; during 2010, floors were constructed at a rate of about one per week. An advanced "cocoon" scaffolding system was installed to protect workers from falling, and was the first such safety system installed on a steel structure in the city. The tower reached 52 floors and was over tall by December 2010. The tower's steel frame was halfway complete by then, but grew to 82 floors by the tenth anniversary of the September 11 attacks, at which time its concrete flooring had reached 72 floors and the glass cladding had reached 56 floors. In 2009, the Port Authority changed the official name of the building from "Freedom Tower" to "One World Trade Center", stating that this name was the "easiest for people to identify with." The change came after board members of the Port Authority voted to sign a 21-year lease deal with Vantone Industrial Co., a Chinese real estate company, which would become the building's first commercial tenant to sign a lease.