The steel beam fell about 25m from a crane and smashed through three floors of Garside Waddingham estate agents in Preston last January.
Preston Magistrates’ Court heard that two women office workers were working in the ground floor shop when the 4m long girder crashed through the ceiling into their office.
The girder was one of 18 similar steel beams to be lifted by the crane that morning on a construction site for a new hotel on Fox Street in Preston.
Bridlington-based Pocklington Steel Structures had spent the morning lifting girders at a 45-degree angle to fit into the new hotel structure.
But HSE, which brought the prosecution, found the contractor had failed to ensure the girders could not slip out of the choke-hitched chains by attaching shackles in holes drilled through them.
The investigation concluded the beams should have been lifted when level and not at an angle.
Anthony Polec, the investigating inspector at HSE, said: “It must have been terrifying for the staff on duty in the estate agents that day when they suddenly heard and then saw a steel girder crashing down next to their desks, right where the public normally stand.
“It was only by chance that the incident happened on a Saturday when fewer staff were at work, and that no members of the public were in the building at the time.
“For such a high lift, near to people on the ground, it is simply not good enough to wrap chains around girders, in a so-called choke hitch, without securing them.
“Two office workers had a lucky escape but several people could easily have been killed. It’s vital construction companies make sure they use cranes safely to prevent similar incidents happening again.”
Pocklington Steel Structures admitted safety breaches that put the lives of workers and the public at risk. In addition to the £15,000 fine, it was also ordered to pay £6,706 in costs.
Over the past decade there have been more than 60 accidents involving cranes, including nine deaths and 25 serious injuries.