A construction worker who fell off scaffolding was saved by the plastic sheeting that was wrapped around the Brighton building where he was working, the Boston Fire Department said. The man, 32, fell at least 30 feet before his fall was stopped, officials said. He was on the scaffolding around 9:15 a.m. outside the five-story building at 1959 Commonwealth Ave. when he fell, crashing through planking on the scaffolding below him. As he plummeted, the man became ensnarled in the plastic sheeting that wrapped the building as a shield against weather, said Boston Fire Department spokesman Steve MacDonald. He said the man was taken to a Boston hospital with injuries that included a broken arm.