Two employees were fabricating a 50,000-gallon steel storage tank and were putting the last head on the tank. One employee was on top of the tank, jacking the head seam tight, when the lug where the jack's hook was attached tore free from the weld on the shell. The jack fell and struck the injured employee, who was on the ground, in the head causing his hard hat to come off. The injured employee became disoriented. Moments later, the chain, hook, and lug also fell and struck the injured employee. The injured employee was hospitalized with a skull fracture and head laceration.