An employee was lifting a 12' x 8" diameter steel vent pipe with three coworkers. They were using a come-along/chain fall to lift the pipe in place while the other employees were guiding. The pipe was shifted to maneuver around a guard rail, causing the rigging beam to slide off the I-beams it was placed on. The entire rigging assembly and load fell. As the rigging fell, the chain whipped around and struck the employee in the right arm, fracturing the ulna and radius.