Employees were standing on the "B" economizer, approximately 4 feet off the ground, attempting to plumb six tubing elements. The assembly sequence was optimized for the "B" economizer by building tubular elements approximately 20 feet wide by 5 feet tall by 2 feet and weighing 1,050 lbs. each and installing them into a fixture to prevent toppling. At the time of the incident, six elements were installed into the fixture and all were held in place with F-clamps at the top while the bottoms were welded to the header pipe. Employee 1 and Employee 2 were standing on top of the fixture, 4 feet off the ground. The elements were leaning away from the employees toward the South. Employee 1 and Employee 2 instructed another employee on the ground to remove the clamps that were restraining/holding the tops of the elements in an attempt to manually plumb and reblock the elements. Employee 1 leaned two elements into the vertical position and upon leaning the third element upright, all six elements toppled toward them. The employees were struck by and crushed by the falling elements and suffered broken bones including broken ribs and a punctured lung. Both employees were hospitalized.