Design and build joint venture Taylor Woodrow Alstom said the bridge over Fairham Brook in Clifton was under construction when a pre-cast concrete beam slid off a temporary bearing and dislodged another beam.
A spokesperson for the firm told the BBC: “We carry out strict procedures during all installations of this kind and I can confirm that no-one was in danger or injured as a result of the incident.
“An investigation is being carried out and until it is complete no further details will be available.”
Local residents said they heard a loud rumbling sound as the deck fell.
The £570m NET project involves building two new two routes – Clifton via Wilford, known as Line Two, and Chilwell via Queens Medical Centre and Beeston, known as Line Three.