For those who were alive at the time, the collapse of a span on the West Gate Bridge, at 11.50am on October 15, 1970, remains seared into their consciousness. It has become like the moon landing or the assassination of president Kennedy. People remember where they were when they heard the news, and what they were doing.

For all Australians, this was a profoundly shocking event. For those who lost loved ones, it was a tragedy the rest of us can barely imagine.

A photograph taken from the water shows the area missing from the bridge after the span crash.

The collapse of the bridge was the worst industrial accident in Australian history. Thirty-five workers were killed on that day, either from the 58-metre fall into the Yarra, or through being struck by fragments, or in the ensuing fire.

The fatal error that led to the collapse happened because a 114-millimetre gap between two half-girders on span 10-11 at the partly constructed bridge's western end had to be closed. It was decided that 10 eight-tonne concrete blocks should be used as ballast, to weigh down the higher of the two girders. That caused the span to buckle.