Tony Baines ran Rotherham-based Birkhull Engineering. He was beside the delivery truck in the city centre when the one tonne beam fell, killing him instantly.
Construction workers at the Fountain Precinct heard a loud bang and ran to help. One shocked builder told The Star newspaper the man was on his side with his arm flung out, and there was “a lot of blood”.
Mr Baines is believed to have been one of three Birkhull workers delivering girders to Fountain Precinct for use in refurbishment works.
They were transported from the firm’s base at Taylors Close, Parkgate, Rotherham, on the back of a small Nissan truck which was not fitted with a crane.
The accident happened as the first girder was being unloaded. A small four-wheeled trolley could be seen at the rear of the vehicle.
A Birkhull employee said: “I came in to work this morning and said ‘good morning’ to him as he was leaving to go on site. We are all in shock.”
A South Yorkshire Police spokeswoman said: “We are investigating a fatal incident, which occurred on Balm Green, Sheffield city centre, around 10am on Wednesday.
“It is believed a 50-year-old man from the Rotherham area was fatally injured whilst unloading metal girders from the back of a lorry.
“A joint enquiry with South Yorkshire Police and the Health and Safety Executive is underway into the circumstances surrounding the incident.”