The newest art installation on Nicollet Mall arrived Saturday, with the help of a crane and a team of welders braving windchills in the teens. “Nimbus,” a massive 10-ton sculpture by artist Tristan Al-Haddad, was originally supposed to be installed last spring. But a shortage of workers to complete it and weather-related complications delayed its downtown Minneapolis debut. But on Saturday, the 45-foot-wide curved sculpture was hanging from a crane as it was lowered onto the site of the Theater in the Round in front of the Minneapolis Central Library. Once that step was completed, welders anchored it to an 80,000-pound concrete footing below the sidewalk. The completed work will feature colored light beamed through the sculpture.