The accident in Brentford happened last week but the death has only now been confirmed as a construction fatality.
The man, who had been working for site logistics firm Wilson James, is understood to have been on a walkway when the girder hit him. The incident occurred on 5 June and he was pronounced dead at the scene.
A statement released by the contractor said: "We can confirm that there has been a tragic incident on one of our sites resulting in a fatality.
"Our immediate thoughts are with the family of the deceased and we are doing all we can to fully cooperate with the emergency services.
"We cannot comment further at this stage as the incident is now under investigation, we are cooperating fully with the HSE."