CHICAGO (WLS) -- Four workers were injured when an I-beam collapsed at the River Point Tower construction site in Chicago's West Loop.Chicago Fire Department officials said crews were working on River Point Tower near Canal and Lake streets when the structural steel beam fell shortly after 9 a.m. Monday. Two workers rode the beam down 20-feet and a third worker was trapped 10-feet below grade before being rescued. He was hit by falling debris and had to be stabilized before being transported to the hospital."There was a structural steel collapse about 20 feet above. We're not exactly sure why it happened, but the structural steel fell," Assistant Deputy Fire Commissioner Mark Nielsen said. "One guy was a little below grade and we provided care. We wanted to make sure everything's right and spinal immobilization is done correctly. That was our main concern with this gentleman."None of their injuries is life-threatening, officials said. A fourth worker received minor injuries.All four of the injured workers are employed by Midwest Steel. They were transported to Stroger, Northwestern Memorial and Rush University hospitals.OSHA is investigating and a preliminary report shows: the workers were properly tied off on the I-beam; the beam had just been secured but the bolts were sheared off; and safety nets appear to have been present, but may have been broken by the falling beam.River Point broke ground one year ago. The 50-story office tower will have a plaza and river walk.