The Willis Tower in Chicago is being recognized as the largest “green” building in the country. Building owner EQ Office, part of private-equity group Blackstone Group, says the Willis Tower has been designated as the largest U.S. building to earn the U.S. Green Building Council Leadership in Energy and Environmental Design’s “platinum” designation. The status was achieved through improvements to the tower’s heating and cooling system, installing energy-efficient LED lights and more. The tower is currently wrapping up a five-hundred-million-dollar expansion.