The incident was reported at about 2:20 p.m. at 350 Mission St., according to the fire chief's office.
The two workers were installing scaffolding at the site when it collapsed, sending one worker falling 40 feet and another worker falling about 30 feet, fire spokeswoman Mindy Talmadge said.
They were both taken to the hospital with life-threatening injuries, Talmadge said.
The building at 350 Mission St. is a 27-story office building currently under construction, according to the project's website.
