Parliament proceedings had to be cancelled yesterday when the oak ceiling beam slipped out of its stainless-steel mounting and dangled down over the politicians.
It is the second year in a row that pieces have fallen from a Stirling prize-winning building. Last April a giant glass panel fell 28 storeys from Norman Foster's Swiss Re tower.
Ironically this latest incident also meant a scheduled debate about architecture had to be postponed.
The AJ understands the Health and Safety Executive has launched an investigation into the episode - the latest in a long line of embarrassments for RMJM and Enric Miralles' much-troubled £431 million project.
It has been rumoured the freezing weather could have been to blame for the incident, although all the parties, including structural engineer Arup, contractor Bovis and RMJM are keeping tight lipped about the possible causes.
However, Arup did admit it had a team on site to investigate the chamber roof. A spokesman said: 'The situation is being assessed and investigations are ongoing.'
The complicated chamber-roof structure features more than 60 beams, set in clusters of four, secured at both top and bottom.
Conservative MSP, John Scott, who boasts a civil engineering degree, said: 'There has been a serious structural failure in one of the trusses that supports the roof of the debating chamber.'
He added: 'In my view it is essential that for the structural integrity of the building to be maintained, it not only needs to be repaired but the whole of the remaining structure needs to be thoroughly checked and inspected before anyone reenters the chamber.'