Nikkei opened sharply lower Tuesday on weak U.S. economic data that triggered concerns over the global economy. At 9:15 a.m., the 225-issue Nikkei Stock Average was down 234. 88 points, or 1.77 percent, from Monday at 13,040.78, after falling to as low as 13,004.46. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 21.91 points, or 1.93 percent, at 1, 112.08.