Willis Tower (previously called Sears Tower) is the tallest building in North America. The building is constructed of nine 75-foot-square tubes of welded steel that extend 50 and 108 stories high. Floors are suspended within the tubes. Engineer Fazlur R. Kahn devised this structural solution specifically for the project. The steel-frame building is clad in black aluminum and bronze-toned glass. Model created by Mason Thrall and Nathan King.