CMC UK, of Trident Industrial Park, Glass Avenue, Cardiff, pleaded guilty to breaching health and safety at work legislation.
It was fined £112,500 with £96,000 in costs.
Mark Walker, 37, suffered fatal injuries while moving a steel beam onto a conveyor on 24 June 2012.
Cardiff Crown Court heard Mr Walker was working alone and was to use a computer controlled saw for the first time.
He had to separate H-beam columns that were already stacked to be put on the conveyor that fed the saw.
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The stacked columns were 16 metres long and weighed over two tonnes each.
He lifted one end of a column with an overhead crane and put two wooden bearers in the middle while he got between the columns to pull the hoist chains through.
While doing this, the wooden bearers gave way and the top column fell on Mr Walker. He died at the scene.
A Health and Safety Executive (HSE) investigation found there were no instructions on how to split and lift the columns safely.
In addition, Mr Walker, an experienced warehouseman, had not been given training for the task.
HSE also found there was no safe system of work for splitting or separating columns.
HSE Inspector Dean Baker said: "Mark Walker tragically lost his life in this avoidable incident.
"The company did not adequately plan the work that Mark was tasked with and expected a number of employees to separate large steel columns at waist height.
"They should have identified the risk of these columns falling and ensured that the columns were separated at floor level or in a suitable rack. Had the lifting operation been properly planned and appropriately supervised and employees given adequate training, Mr Walker would still be alive."
(LM)
